Available via license: CC BY 4.0
Content may be subject to copyright.
RESEARCH ARTICLE
Targeted conspiratorial killing, human self-domestication
and the evolution of groupishness
Richard W. Wrangham*
Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA
*Corresponding author. E-mail: wrangham@fas.harvard.edu
Abstract
Groupishness is a set of tendencies to respond to group members with prosociality and cooperation in
ways that transcend apparent self-interest. Its evolution is puzzling because it gives the impression of
breaking the ordinary rules of natural selection. Boehm’s solution is that moral elements of groupishness
originated and evolved as a result of group members becoming efficient executioners of antisocial indivi-
duals, and he noted that self-domestication would have proceeded from the same dynamic. Self-domes-
tication is indicated first at ∼300,000 years ago and has probably gathered pace ever since, suggesting
selection for self-domestication and groupishness for at least 12,000 generations. Here I propose that a
specifically human style of violence, targeted conspiratorial killing, contributed importantly to both
self-domestication and to promoting groupishness. Targeted conspiratorial killing is unknown in chim-
panzees or any other vertebrate, and is significant because it permits coalitions to kill antisocial individuals
cheaply. The hypothesis that major elements of groupishness are due to targeted conspiratorial killing
helps explain why they are much more elaborated in humans than in other species.
Keywords: Execution hypothesis; morality; cooperation; chimpanzee
Social media summary: Planned killing of antisocial individuals occurs only in humans, and helps to
explain why we have prosocial tendencies.
Unlike other vertebrates, humans (Homo sapiens) have long been considered to be more cooperative
and prosocial than expected from theories of kin selection or mutualism. One kind of explanation
credits humans’elaborate cognitive abilities, because these permit more complex systems of reciprocal
exchange than other animals can manage, including social norms, contracts and laws (dos Santos &
West, 2018). In addition to such proximate pressures, however, evidence of prosociality in human
infants suggests that human tendencies to be more cooperative than other primates are evolved traits
(Graham et al., 2013; Tomasello, 2016). Furthermore human prosociality has genetic and neuroana-
tomical correlates (Raghanti, 2019; Tiihonen et al., 2020). Following Haidt (2012) I call such evolved
propensities ‘groupishness’, characterised as a tendency to cooperate and be prosocial in ways that
appear to transcend genetic self-interest. Groupishness in humans includes spontaneously helping
unrelated group members, having a social conscience, accepting and enforcing a moral code, conform-
ing to group norms, sharing resources, and being concerned about fairness and reputation. In this
paper I suggest that the evolution of human groupishness was strongly influenced by a unique
human ability, targeted conspiratorial killing.
Groupishness is a conundrum because individuals are expected to be self-interested except when
they can sufficiently benefit kin. Darwin (1871) grappled with this problem when trying to explain
the evolution of morality. He assumed that individuals who aid non-kin experience a cost relative
© The Author(s), 2021. Published by Cambridge University Press on behalf of Evolutionary Human Sciences. This is an Open Access article,
distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unre-
stricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Evolutionary Human Sciences (2021), 3, e26, page 1 of 21
doi:10.1017/ehs.2021.20
to those who are less prosocial, and therefore concluded that such behaviour was not explicable in
terms of natural selection theory acting within groups. Most subsequent investigators have reached
the same impasse, leading some to invoke human-specific applications of group selection or multilevel
selection theory (e.g. Darwin, 1871; Choi & Bowles, 2007; Wilson &Wilson, 2007). Such efforts are
generally considered unsuccessful (West et al., 2011).
An alternative approach challenges the core assumption that agents suffer costs by being groupish.
Instead, groupishness benefits agents by protecting them from punishment. The argument depends on
a special feature of human society. Within groups, individuals with a reputation for being antisocial
can be punished by coalitions of others. If such punishments are sufficiently systematic and costly,
groupishness is less costly than selfishness. According to this perspective groupishness is equivalent
to a self-imposed tax. The tax protects the agent from the long-term costs of acting against the inter-
ests of the punishing alliance (Boehm, 2012; Wrangham, 2019b).
Darwin (1871) noted that selection acts against aggressiveness when violent men are executed or
imprisoned, but he did not pursue the implications of this observation. More than a century later
Boehm (1999, 2012, 2014, 2017, 2018; Gintis et al., 2015) argued explicitly that groupishness benefits
individuals because in hunter–gatherer societies that represent an environment of evolutionary adapt-
edness, the costs of antisocial behaviour can be very high. Boehm’s focus, like Darwin’s, was on the
morality of fairness, a major component of groupishness.
Moral feelings associated with fairness were once thought to be present in non-humans such as
capuchins (Cebus apella) and chimpanzees (Pan troglodytes) (Brosnan & de Waal, 2003).
Experiments show, however, that only humans have a tendency to sacrifice personal gain for the
sake of equality, whereas non-humans’apparent concern for fairness reflects other motivations
such as efforts to manipulate an experimenter (Engelmann et al., 2017; McAuliffe & Santos, 2018).
Accordingly traits associated with fairness such as senses of responsibility, obligation, duty, guilt
and shame appear to be restricted to humans, making their evolution a particularly interesting puzzle
(Tomasello, 2016). In contrast moral emotions concerned with sympathy, such as compassion, con-
cern and benevolence, are evidenced in non-humans (de Waal, 2006).
According to Boehm a mid-Pleistocene phase of distinctive moral evolution began in Homo as a
result of alliances of bullied males predictably killing unremittingly aggressive alpha males within
their own group. The fact that such an alliance could safely dispatch the most physically intimidating
member of the group meant that it could equally well kill any other group member. Accordingly a
wide set of antisocial behaviours became intensely risky for group members, such that a reputation
not merely for being a violent bully but also for being a trouble-maker, competitor, bringer of bad
luck or consistently selfish could lead to an individual being killed. This novel threat of the severest
punishment for antisocial behaviour created a strong incentive for following norms for the sake of self-
protection, provided that the costs of doing so were not too high. The long-term result was selection
against antisocial behaviour and in favour of prosocial behaviour, cooperation and conformism, a
dynamic that ultimately favoured the moral senses and other components of groupishness. In
short, groupishness was favoured when the evolution of capital punishment meant that selfish behav-
iour became much more costly than previously (Cofnas, 2018; Wrangham, 2019b).
Boehm’s‘execution hypothesis’has several merits. It fits the historical record of every kind of soci-
ety, from small-scale to state, because executions have been a conventional mechanism of controlling
antisocial and amoral behaviour worldwide (Woodburn, 1982; Otterbein, 1986; Boehm, 1999,2017). It
provides a logical explanation for how despotic alpha-male behaviour, which is typical of group-living
primates, was controlled and selected against in H. sapiens (Boehm, 1999,2012; Wrangham, 2019a). It
accounts for males in small-scale societies having egaIitarian social relationships in the form of a
reverse dominance hierarchy (Boehm, 1993; Erdal & Whiten, 1994). It fits the inference that
Pleistocene Homo were skilled killers of large animals, suggesting that well-planned proactive kills
of group members would have been low risk. The idea that Pleistocene Homo were capable of
within-group killing is also supported by the fact that coalitions of chimpanzees are likewise known
to kill adults within their own groups, reportedly including especially aggressive individuals
2 Richard W. Wrangham
(Boehm, 2012; Wilson et al., 2014). The hypothesis that a violent form of social selection could have
been responsible for promoting the evolution of groupishness thus fits data on both humans and
chimpanzees.
The execution hypothesis suffers, on the other hand, from at least three kinds of problem. First,
theoretical models have not identified the conditions under which subordinates benefit by proactively
killing the alpha. This omission is problematic because in some circumstances subordinates could in
theory benefit from the presence of an alpha that contributes positively to collective actions (Gavrilets
& Fortunato, 2014; Gavrilets, 2015). Here, in contrast to that idea, I assume that if subordinates’cog-
nitive ability allows them to kill a selfish alpha cheaply, and so proactively that the alpha has no oppor-
tunity to form a counter-coalition, subordinates always benefit from killing him or her. Although this
idea is derived from the ethnographic record (Boehm, 2012), and in spite of some relevant theorising
(Gavrilets et al., 2008; Gavrilets, 2012; Ihara, 2020), the conditions for it to be true do not appear to
have been formally modelled (Mesterton-Gibbons et al., 2011; Bissonnette et al., 2015).
Second, the execution hypothesis is difficult to test because the frequency and patterning of
Pleistocene executions is unknown. When and how rapidly groupishness was influenced by the pro-
posed selection pressure therefore cannot be estimated directly. The evolution of groupishness is logic-
ally linked to the evolution of self-domestication, however, by the claim that both outcomes result
from the same selection pressure against highly aggressive behaviour, namely targeted killing
(Boehm, 2012,2017; Hare, 2017; Wrangham, 2019b; see Figure 1). This inferred linkage is useful
because unlike groupishness, self-domestication has anatomical and genetic correlates that indicate
the pattern of its evolution. Inferrable features of the evolution of self-domestication can therefore
in theory be used to inform an understanding of the evolution of groupishness.
The third weakness to be discussed is the problem of why within-group killing had different evo-
lutionary results in chimpanzees from H. sapiens despite occurring in supposedly similar ways. The
ideal solution will explain not only why groupishness occurs in humans, but also why it is found
barely or not at all in chimpanzees or other non-humans. I consider four suggestions.
The first is morality. Boehm (2017, p. 761) proposed that morality made within-group killing more
effective in humans than in chimpanzees: ‘a human group consumed with moral outrage can become a
still more efficient killing machine’. While this idea may well be correct, it does not fit Boehm’s core
argument that within-group killing was responsible for the evolution of human-style morality. If
human-style morality was produced by within-group killing, something else must have given
within-group killing its initial special power.
Figure 1. Hypothesised effects of targeted conspiratorial killing (TCK). The upper and lower pathways diagram the evolution of
self-domestication and groupishness, respectively. Targeted conspiratorial killing is cheap because it can be conducted at low
risk to the killers. It was putatively enabled by language becoming sufficiently sophisticated to foster conspiratorial behaviour.
A positive feedback loop followed as a result of increases in features such as social tolerance, linguistic skills and tendencies
for conformity, which made targeted conspiratorial killing increasingly easy to organise.
Evolutionary Human Sciences 3
Second, Boehm (2014, p. 170) alluded to the possession of lethal weapons as a reason why ‘we have
taken our own evolutionary course’compared with chimpanzees. The idea that the use of weapons
transformed human social life has been suggested often since Woodburn (1982) pointed out that well-
armed men such as hunter–gatherers can easily kill other members of their groups (Bingham, 2000;
Okada and Bingham, 2008; Phillips et al., 2014; Chapais, 2015; Gintis et al., 2015). For weaponry to
have been critical, the origin of a newly efficient type of lethal weapon should have coincided with, or
immediately pre-dated, the purported origin of self-domestication and elaboration of human group-
ishness at ∼300,000 years ago or shortly after. This means that the type of weaponry must have
been an advance on spears, because spears were already sophisticated by 400,000 years ago
(Thieme, 1997). Lithic culture appears to have become more complex around 300,000 years ago
(Shea, 2017; Brooks et al., 2018), but complex projectile weapons are not known until 60,000–
70,000 years ago (arrows, Backwell et al., 2017). The apparent lack of any significant improvement
in weaponry coinciding with the earliest evidence of self-domestication undermines the idea of its
importance in the evolution of groupishness.
Third, Boehm (2012) argued that climate change in the middle Pleistocene was a critical factor. His
idea was that unfavourable weather made food production, and especially meat acquisition, unpredict-
ably hazardous. In these circumstances, he suggested, food-sharing became necessary, and despots
who refused to share meat were killed so that everyone could survive. In support of this proposal,
Boehm (2014) cited archaeological evidence that between 400,000 and 250,000 years ago patterns
of hunting and meat-sharing changed to reflect a greater importance of cooperation. The climate
explanation treats the cognitive ability of mid-Pleistocene Homo and chimpanzees as being equally
well adapted for within-group killing, that is, climate variation merely elicited a ‘potential [that] has
been continuously present in the human, chimpanzee, and bonobo lines for five to seven million
years’(Boehm, 2018, p. 696). Evidence reviewed in this paper will challenge that notion however,
because chimpanzees appear to be incapable of organising the killings of despots.
Fourth, Wrangham (2019a, b) proposed that the style of within-group killing is critically different
between humans and chimpanzees because only humans have enough language ability to plan execu-
tions to be cheap and safe for the killers.
Below, I investigate why within-group gang attacks have favoured groupishness in humans but not
in chimpanzees. I begin by considering the relationship between groupishness and self-domestication.
Self-domestication
Scholars have claimed for more than 2000 years that humans are behaviourally similar to domesticated
animals (Wrangham 2019b). Until Darwin, the assumption was that humans had always been domes-
ticates, but when evolutionary theory replaced natural theology the implication changed. Similarities
between humans and domesticated animals then became attributed to humans having evolved from a
less domesticated ancestor (Bagehot, 1872).
Boas (1911) found the idea irresistible and was the first to call humans ‘self-domesticated’. He sug-
gested that self-domestication began in the early Quaternary, at the start of the Pleistocene, and was
further developed after fire was controlled (Boas, 1911, p. 76). The self-domestication idea duly gained
traction (review by Brüne, 2007; e.g. Fischer, 1914; Lorenz, 1940; Mead, 1954; Gould, 1977; Belyaev,
1984). However, against the notion, the criteria for deciding whether humans qualified as domesticates
were vague, and no mechanism was proposed for why self-domestication might have occurred. The
idea did not appear to be testable, and the concept of self-domestication seemed weirdly particular
because it was applied only to humans and not to any other species. For such reasons the idea was
often dismissed (Darwin, 1871; Haldane, 1956; Dobzhansky, 1962). In the last 20 years, however, self-
domestication theory has overcome many of these problems.
The meaning of self-domestication is an important issue that requires consideration of the meaning
of ‘domestication’also. Traditional definitions of domestication often include criteria such as one spe-
cies taking care of another, one species benefiting from the relationship with another species, or the
4 Richard W. Wrangham
domesticate living as a pet or on a farm (Leach, 2003; Zeder, 2015). To include such criteria in a def-
inition of domestication would make the concept of self-domestication meaningless.
Theorists of human self-domestication therefore define both domestication and self-domestication
in terms of their biological consequences (Thomas & Kirby, 2018; Wrangham, 2019b). All of the many
definitions of domestication invoke the resulting relationship between domesticator and domesticate
(Zeder, 2015). Tameness is the sine qua non of this relationship, and is the only domestication-related
trait known to occur in all domesticated mammals and birds (Lord et al., 2020; Sánchez-Villagra & van
Schaik, 2019). Much evidence indicates that selection for tameness is primarily responsible for domes-
ticated traits in general (Arbuckle, 2005; Wilkins et al., 2014; Trut et al., 1991, 2020; Zeder, 2020). For
those reasons domestication and self-domestication are conveniently defined purely in terms of tame-
ness, equivalent to docility or a reduced propensity for reactive aggression (Wrangham, 2018).
Here therefore I define both domestication and self-domestication as a reduction in a species’pro-
pensity for reactive aggression. Farmyard domesticates tend to exhibit reduced reactive aggression
towards both humans and conspecifics. In self-domesticated species, in contrast, the reduction in
the propensity for aggression occurs without any other species being actively engaged, and the aggres-
sion being reduced is primarily towards conspecifics (Hare et al., 2012; Hare, 2017; Theofanopolou
et al., 2017; Wrangham, 2019b). However self-domestication can in theory reduce aggression towards
humans, as suggested for urban animals (Hare and Woods, 2020; Parsons et al., 2020).
Domestication and self-domestication are thus different processes, but the self-domestication
hypothesis proposes that they share important underlying commonalities in biological mechanisms.
Most importantly, selection against reactive aggression is predicted to generate a similar syndrome
of traits in each case. This concept of a domestication or self-domestication syndrome does not
mean that all traits in the syndrome invariably co-occur: they do not (Lord et al., 2020). Instead, it
means that they occur with statistical regularity (Wilkins et al., 2014; Wilkins, 2017; Trut et al.,
2020; Zeder, 2020), regardless of whether the selective pressure comes from human agency or from
any of a variety of mechanisms in the wild that favours reduced reactive aggression (Hare et al.,
2012; Hare, 2017; Wrangham, 2018,2019b).
Note that a definition of self-domestication in terms of selection for reduced aggressiveness means
that self-domestication must have happened often in the history of life. Yet the first example of self-
domestication in a wild animal was not proposed until 2012 (bonobos Pan paniscus; Hare et al., 2012).
Two reasons could explain why self-domestication has been detected rarely.
First, the ability to recognise it depends on comparison between a species that has experienced
selection against aggression and a closely related species that has not. However, the required compar-
and will often not exist. In the absence of an informative relative of a giraffe, we have no way to infer
the evolutionary history of giraffes’aggressiveness.
Second, as expected with antagonistic pleiotropy, signals of self-domestication that occurred long
ago are expected to be masked by subsequent evolutionary changes (Stearns & Medzhitov, 2015).
Losses would include non-adaptive traits originally produced as part of a self-domestication syndrome.
The best opportunities for recognising that a species self-domesticated will therefore be when the
target species has a close relative or a palaeontological record that helps to model the unselected ances-
tor, and when self-domestication has been recent. The frequency of such conditions has not been esti-
mated. Here I briefly consider self-domestication in non-humans to support the claim that selection
against reactive aggression in the wild can have parallel effects to those found in captivity.
Self-domestication in non-humans
The most closely examined case for self-domestication in a wild species is for bonobos (Hare et al.,
2012). Bonobos and chimpanzees are sister species separated by the Congo River ever since a dry per-
iod allowed their common ancestor to cross from the right bank to the left, where bonobos now live
(Takemoto et al., 2015; de Manuel et al., 2016). Separation occurred between 0.87 (Won & Hey, 2005)
and 2.1 million years ago (de Manuel et al., 2016). Gorillas (Gorilla gorilla) are the closest out-group to
Evolutionary Human Sciences 5
Pan, and chimpanzees show more similarities to gorillas than bonobos do, including in the behaviour-
ally significant cranio-facial region. This indicates that chimpanzees have changed less from the com-
mon ancestor than bonobos have (Pilbeam & Lieberman, 2017).
The argument for self-domestication comes from comparing phenotypes. Relative to chimpanzees,
bonobos have much less aggressive males and they exhibit many anatomical, behavioural and cognitive
traits that are characteristic of domesticated animals, including short faces, smaller teeth, smaller
brains, reduced sexual dimorphism in teeth, reduced body mass, increased play, increased gregarious-
ness, increased tolerance, delayed cognitive development and neotenous crania (Hare et al., 2012;
Hare, 2017; Rosati, 2019; Wrangham, 2019b). The many convergences with domesticated animals
and the inferred polarity of evolutionary change suggest that the mechanisms responsible for reduced
aggression in bonobos are similar to those in domesticated animals. Initial genetic tests support this
hypothesis (Kovalaskas et al., 2020).
Why bonobos evolved a reduced propensity for aggression compared with chimpanzees is not
known. An ecological hypothesis points to functional consequences of gorillas being absent in the
bonobo range, whereas most chimpanzees that live in similar habitats to bonobos co-occur with gor-
illas (Hare et al., 2012). According to this idea, the absence of proto-gorillas in the proto-bonobo habi-
tat allowed proto-bonobos to utilise ‘gorilla foods’more and therefore to be more gregarious than their
chimpanzee-like ancestors (cf. Toda & Furuichi, 2020). Increased gregariousness allowed female bono-
bos to form self-protective mutual coalitions in response to male aggression and to increase their abil-
ity to choose preferred males as mates; the fitness of the most aggressive males was therefore reduced.
This scenario is based on the principle that the present is a key to the past, given that female bonobos
are routinely seen to collaborate in defeating aggressive males (Tokuyama & Furuichi, 2016). While no
tests have been devised for this ecological scenario, it illustrates the theoretical point that self-
domestication is expected to occur via diverse mechanisms.
A more speculative case of self-domestication in the genus Pan is suggested by research on western
chimpanzees, P. troglodytes verus. Yamakoshi (2004) and Pruetz et al. (2017) noted that western chim-
panzees are less aggressive, more tolerant and more gregarious than eastern chimpanzees, P. troglo-
dytes schweinfurthii. Shea and Coolidge (1988) measured whole skulls and found that the length of
Figure 2. Distribution of chimpanzees and bonobos. 1, Western chimpanzee, P. troglodytes verus; 2, Nigerian–Cameroonian chim-
panzee, P. troglodytes ellioti; 3, central chimpanzee, P. troglodytes troglodytes; 4, eastern chimpanzee, P. troglodytes schweinfurthii;
5, bonobo, P. paniscus. P. troglodytes verus is separated from P. troglodytes ellioti by the Dahomey Gap, a region too dry to support
forest. P. paniscus is separated from P. troglodytes schweinfurthii by the Congo River. Map is from Prüfer et al. (2012).
6 Richard W. Wrangham
the cranium as a whole, as well as the combined length of the maxillary molar row, was significantly
reduced in western compared with eastern chimpanzees, as expected if they have self-domesticated.
Behavioural and anatomical similarities between western chimpanzees and bonobos would appear
to be homoplasies, because bonobos became separated from the common ancestor in east or central
Africa long before the evolution of the western subspecies (Figure 3; Prado-Martinez et al., 2013;
de Manuel et al., 2016).
The possibility that three cases of self-domestication can be found in the Hominidae, a family with
few living species, suggests that a wider search for cases of self-domestication will be revealing.
Suggestive evidence has been found in urban vs. rural populations of red foxes, Vulpes vulpes
(Parsons et al., 2020), and in wild marmosets, Callithrix jacchus (Ghazanfar et al., 2020). The evolution
of dogs from wolves, Canis lupus, is widely assumed to have begun with self-domestication (Coppinger
& Coppinger, 2000). Other possible taxa that have been speculated to be self-domesticated based on
low levels of within-species aggression include some island forms compared with continental ancestors
(Hare et al., 2012; Hare, 2017; Wrangham, 2019b).
Self-domestication in H. sapiens
In the case of human evolution, self-domestication has been proposed to have happened in the
Pliocene with Australopithecines, or in the early Pleistocene with Homo erectus (Table 1). Such claims
may be correct but are hard to assess, so I restrict discussion to H. sapiens.
The traditional idea that humans are an unusually unaggressive species by comparison with typical
wild animals has rarely been examined. In one study, the frequency of dyadic fighting among humans
was two to three orders of magnitude lower than in chimpanzees or bonobos (Wrangham et al., 2006;
Wrangham, 2019b). However the important comparison for understanding human self-domestication
is how frequently reactive aggression occurs in H. sapiens compared with ancestral Homo, not with
apes.
Injuries indicated by fossils can in theory indicate rates of fighting. Beier et al. (2018) compared
fossils of H. sapiens with H. neanderthalensis, a species that has contributed around 2% of genes to
living humans (Gokcumen, 2020) and which offers a helpful model of pre-sapiens ancestors
(Williams, 2013). Based on 21 specimens that had at least one traumatic lesion in the cranium, the
two species showed no difference in frequency of injury (∼5% in each case), although H. neandertha-
lensis tended to be injured and die when younger. Such studies are promising, but it is difficult to dis-
tinguish injuries sustained in fighting from those incurred while hunting. Furthermore the lighter skull
and skeleton of H. sapiens might mean that they are relatively more vulnerable to trauma. Direct evi-
dence on the evolution of aggressiveness is therefore currently inconclusive.
Instead of direct evidence of fighting rates, the argument for self-domestication in H. sapiens comes
from anatomical changes during evolution, comparisons with living domesticates and genetic compar-
isons with other Homo species.
First, for most of the Pleistocene, Homo species showed few signs of a self-domestication syndrome.
Starting ∼315,000 years ago, however, when the earliest fossils attributable to H. sapiens were formed
(Hublin et al., 2017), informative trends include multiple features found in domesticated animal spe-
cies. Four are used by archaeologists to recognise domestic animals in the fossil record: reduced sexual
dimorphism in various bones and teeth, shorter faces and smaller molars, lighter bodies and (in the
last 30,000 years) reduced cranial capacity (Leach, 2003; Cieri et al., 2014; Hublin et al., 2017). Brow
ridges have also been reduced and faces have become relatively narrow. Since narrower faces nowadays
are associated with reduced aggressiveness, the narrowing trend strongly suggests that H. sapiens has
become increasingly docile (Cieri et al., 2014; Haselhuhn et al., 2015; Deska et al., 2018). In short, gra-
cilisation tendencies similar to those found in domestication are found throughout the evolution of H.
sapiens, and are putatively responses to selection against reactive aggression (Hare, 2017; Wrangham,
2019b). This idea contrasts with traditional views that treat each gracilised trait as responding
Evolutionary Human Sciences 7
Figure 3. Inferred population history of Pan and Homo, showing possible self-domestication events (starred). Ape phylogeny is from Prado-Martinez et al. (2013), who estimated the times of popu-
lation splits. Homo phylogeny is from Schlebusch et al. (2017). No fossil data are available to help estimate the times of self-domestication in P. paniscus and P. troglodytes verus.
8 Richard W. Wrangham
independently to different selection pressures (e.g. Brace et al., 1987; Ruff et al., 1993,1997; Pearson,
2000; Lieberman et al., 2002).
Second, comparisons of traits in H. sapiens with those in living domesticates have focused espe-
cially on dogs, Canis familiaris, and foxes (Hare, 2017). Compared with their wolf ancestors, dogs
exhibit low reactive aggression, high play, increased cranial neoteny, high tolerance, high prosociality,
low responsiveness of the HPA axis, low trabecular bone fraction, high oxytocin activity, a long juven-
ile period and relatively cooperative patterns of communication. All of these features are found in
humans, suggesting that their occurrence is owed to a process of domestication similar to the evolution
of dogs from wolves (Hare, 2017; Raghanti, 2019; Thomas & Kirby, 2018; Progovac & Benítez-Burraco,
2019; Wrangham, 2019b; Chirchir, 2021). Belyaev (1984) was inspired by these kinds of change in his
experimentally domesticated foxes to suggest that humans had self-domesticated.
Third, genetic comparisons are in their infancy but are already promising thanks to a growing
understanding of the mechanisms underlying domestication. A leading candidate for explaining
why domesticated animals from different lineages show convergent phenotypes is a mild neurocristo-
pathy, that is, a reduction in the rate of migration and/or total number of neural crest cells (Wilkins
et al., 2014). Effects of a neurocristopathy are well established in the production of unpigmented hair at
terminal sites of melanoblast migration (tips of feet, tail and forehead), a common feature of domes-
ticated animals (San-Jose & Roulin, 2020). In other cases of domestication-linked traits, such as floppy
ears, short face, reduced brain size and increased tameness itself, the putative effects of neurocristo-
pathy are plausible but unproven (Wilkins et al., 2014; Wilkins, 2017).
The neurocristopathy hypothesis is sufficiently suggestive, however, that researchers have looked for
genomic evidence to test it. Supportive evidence has been found in cats, Felis silvestris catus (Montague
et al., 2014), rabbits, Oryctolagus cuniculus (Carneiro et al., 2014), horses, Equus caballus
(Librado et al., 2017), dogs, Canis familiaris (Pendleton et al., 2018), rats, Rattus norvegicus (Singh
et al., 2017), buffalo, Bubalus bubalus (Luo et al., 2020), and camels, Camelus dromedarius and C. bac-
trianus (Fitak et al., 2020). These studies suggest that domestication is often achieved at least partly by
the evolution of a mild neurocristopathy.
Evidence for neurocristopathy has therefore been looked for in H. sapiens compared with H. nean-
dertalensis and H. denisova, two close relatives that exhibit no anatomical signals of self-domestication.
In support of the self-domestication hypothesis, neural crest-related changes are found in H. sapiens
compared with its congeners (Theofanopolou et al., 2017; Zanella et al., 2019). Functional tests by
Zanella et al. (2019) showed that reduced activity of the regulatory gene BAZ1B in H. sapiens causes
down-regulation of multiple genes regulating neural crest migration and maintenance. As a result
neural crest cells are formed and migrate more slowly in H. sapiens than they would have done in
H. neandertalensis and H. denisova, with changes in the craniofacial structures and temperament of
H. sapiens that are as expected from self-domestication.
Table 1. Proposed occurrences of self-domestication in the human lineage. The case discussed in this paper is in the
bottom row
Time Evidence Reference
4–7 ma Bipedality, reduced canines Raghanti (2019)
1–2 ma Appearance of Homo erectus Hare (2017)
0.1–1 ma Brain increase Hare (2017)
50–500 ka Material culture Hare (2017)
0–300 ka Cranio-facial feminisation, cranial
globularisation, skeletal gracilisation
Leach (2003), Cieri et al. (2014), Progovac &
Benítez-Burraco (2019), Wrangham
(2019b)
ma, Million years ago; ka, thousand years ago.
Evolutionary Human Sciences 9
Such studies should eventually include other mechanisms suspected to contribute to domestication,
including changes to the thyroid hormone system and the glutamate signalling system. The thyroid
hormone hypothesis proposes that tameness is essentially a juvenile characteristic that has been
retained into adulthood (Crockford, 2002). Because thyroid hormones regulate growth rate, they are
thought to be master regulators of domestication. The glutamate receptor hypothesis focuses on
how changes in neurotransmission might contribute to a reduction in the propensity for reactive
aggression, such as through attenuation of the HPA stress response and/or release of oxytocin and
vasopressin (O’Rourke & Boeckx, 2020). Both hypotheses predict that domesticated and self-
domesticated species will show changes in relevant genes compared with wild ancestors or sister spe-
cies that have not been tamed (Wilkins, 2017).
These mechanisms, and possibly others, mean that wherever suitable comparands occur, hypoth-
eses of self-domestication are testable using a priori predictions of specific genetic changes. In the case
of H. sapiens, the split from H. neandertalensis and H. denisova is estimated to have occurred between
400,000 and 700,000 years ago (Prüfer et al., 2014; Stringer, 2016). As expected, comparisons of H.
sapiens with those sister species indicate selection for genes related to self-domestication around
300,000 to 500,000 years (Zanella et al., 2019; Andirkó et al., 2021).
Comparison of within-group killing in humans and chimpanzees
Evidence that the most recent evolutionary phase of human self-domestication started with H. sapiens,
whereas earlier Pleistocene Homo species showed no signs of a self-domestication syndrome, suggests
that the period shortly before 300,000 years ago marked the beginnings of intensified selection against
reactive aggression. Subsequent acceleration of gracilisation trends from archaic to modern H. sapiens
suggests that self-domestication has continued to the present at an increasing rate. A critical question
therefore is: what happened before 300,000 years ago that would explain why reactive aggression was
selected against much more strongly than previously?
Based on Boehm’s execution hypothesis, the answer might seem to be that this was the first time
that human ancestors developed an ability to safely kill alpha males. While I will argue that this answer
is correct, it is also inadequate because other species are also known to kill adults in their own group.
The problem is particularly severe for chimpanzees because, according to Boehm (2018), chimpanzees
have a human-like capacity to remove a disliked despotic male by killing him. Boehm reported that in
chimpanzees and humans ‘bullies …are singled out for lethal attacks’[by coalitions] (Boehm, 2018,
p. 693). Therefore, ‘It is very likely that our chimpanzee-like ape ancestors ganged up on disliked indi-
viduals to temporarily or permanently eliminate them from the group’(Boehm, 2017, p. 763).
Those claims might be taken to suggest that there is no important difference between the propen-
sities of chimpanzees and humans to kill an alpha male. Yet domineering behaviour is an invariable
feature of alpha male chimpanzees, so it has not been reduced in that species as it has in humans. This
suggested to Boehm (2017, p. 769) that there is a feature of human coalitions that makes them into ‘a
special, social selection force’. The critical question can therefore be reformulated: do differences in the
nature of within-group killing of adults explain why, by 300,000 years ago, reactive aggression was
selected against in humans but not in chimpanzees?
The sole data available for answering this question come from Boehm’s(2017,2018) assembly of 13
relatively severe within-group gang attacks by chimpanzees on adult males (Table 2). Three of these
offer very little behavioural data because they were described only from the aftermath (former
alpha males Foudouko and Ntologi (in 1995), and young male Zesta). Furthermore it is not even cer-
tain that Ntologi’s 1995 death was caused by chimpanzees (Nakamura & Itoh, 2015). In seven cases
the victim died immediately or within days. In the other six cases the victim remained in the group
with a reduced dominance rank, sometimes after a prolonged period of being solitary.
Boehm was impressed by the similarities between these within-group gang attacks and the better-
known between-group gang attacks. For example he referred to ‘chimpanzees ganging up on a high-
ranking community member and attacking him as strangers are routinely attacked’(Boehm, 2018,
10 Richard W. Wrangham
Table 2. Violent within-group gang attacks among chimpanzees
Victim and date
Victim
social
rank Seen How attack started
Who
joined
who
Delay
before
attack
(min)
No. male
attackers in
first fight
No. male
attackers total
Reports of V
being
helped?
Reports of
attackers
wounded?
Victim’s
fate
First attacker’s
fate
Study
site,
group,
ref.
Pimu,
2 October
2011
Alpha All Dyadic fight attracted
attackers
A 0 4 (incl. Alofu) 5 Yes Yes Dead. Fight
took >2
hours
Alofu became
alpha
1
Vincent,
22 December
2004
Ex alpha All V, badly injured, joined two
meat-eating males
? 0 1 (Rudi,
alpha)
2 ? No Dead Rudi stayed
alpha
2
Grappelli,
29 October
1995
Low rank All but
start
Screams attracted alpha BT
and others. Some did not
attack
V? ≥6 ? 7 main attackers.
Alpha BT
briefly
involved at
start
No (but
some
embraces)
No Died ≥7
hours
later
First attacker not
known
3
Hatari,
17 February
2011
Low rank All but
start
? ? ? 1 or 2,
including
alpha Brutus
2 No No Died 10
days
later
First attacker not
known but
Brutus stayed
alpha
4
Foudouko,
15 June
2013
Ex alpha Body
only
? ? ? ? 5 No 1 male had
superficial
wounds
Dead First attacker not
known
5
Zesta,
4 November
1998
Low rank Body
only
V found prone 50 minutes
after screams heard
?? ?≥3, + alpha
Duane who
ate ZE’s flesh
No 3 males had
superficial
wounds
Died ∼2
hours
later
First attacker not
known
6
Ntologi, 14
November
1995
Ex alpha Body
only
? ? ? ? Many? No No Dead First attacker not
known
7
Goblin,
25 October
1989
Ex alpha All V, in poor health, joins
group for first time in 27
days
V≥11 1 (alpha,
Wilkie)
≥6 No No Exile Wilkie stayed
alpha
8
Jilba,
7 October
1991
Mid-rank All V joins meat-eating group
(6 M, 2 F). Alpha Kalunde
chased V
V≥2 1 (alpha,
Kalunde)
8 No No Exile 50
days
Kalunde stayed
alpha
9
Sheldon, early
December
2004
Ex alpha All V, after absence, was
attacked and briefly ‘pinned
to the ground’by ≥3 M incl.
alpha Kris
? ? ? 3 No No Exile First attacker not
known but
Kris stayed
alpha
10
(Continued)
Evolutionary Human Sciences 11
Table 2. (Continued.)
Victim and date
Victim
social
rank Seen How attack started
Who
joined
who
Delay
before
attack
(min)
No. male
attackers in
first fight
No. male
attackers total
Reports of V
being
helped?
Reports of
attackers
wounded?
Victim’s
fate
First attacker’s
fate
Study
site,
group,
ref.
Ntologi,
20 August
1991
Ex alpha All Chased by new alpha
Kalunde and others
? ? 1 or more,
incl. alpha
Kalunde
6? No No Exile Stayed alpha 11
Ferdinand,
1 October
2016
Alpha All V, after long absence,
arrived in good health
V After V
charged
1 (Fudge) 3. Other males
watched and
displayed
No No Lost rank to
Fudge
Became alpha 12
Frodo,
27 January
2003
Alpha All V, in poor health, arrived in
group. He was charged and
attacked, mostly by beta
male Sheldon
V ? ? 10? One M
(brother) did
not attack
Yes, F
(mother)
No Lost alpha
rank to
Sheldon
First attacker not
known
13
Attacks are taken from Table 20.1 in Boehm (2017) and Table 1 in Boehm (2018). Some details have been added, and corrections have been made in the light of new information. Cases are ordered by whether
victim died, was exiled or remained in the group. Within those categories, better observed cases are listed first.
‘Seen’:‘All’, entire interaction observed; ‘All but start’, the attack was seen except for how it began; ‘Body only’, the attack was not seen, but the victim’s body was informative.
‘How attack started’: V, victim; ?, attack not seen; F, female; M, male.
‘Who joined who’: A, aggressor joined victim; V, victim joined aggressor; T, aggressor and victim were socialising in party before the attack.
‘Delay before attack’shows how many minutes elapsed between the antagonists being in the same subgroup and the attack starting.
‘No. male attackers in first fight’refers to the attack that became coalitionary.
‘No. male attackers total’, Number of males recorded as participating in the attack at some point, not necessarily all at once.
‘Reports of V being helped?’: V, victim; yes, victim was defended by at least one male (M) and/or at least one female (F); no, no report of victim being defended.
‘Reports of attackers wounded?’:‘No’, no reports of attackers being wounded.
‘Study site, group, ref.’: 1, Mahale, M-group (Kaburu et al., 2013); 2, Gombe, Mitumba (Wilson et al., 2005; Mjungu, 2010); 3, Kibale, Ngogo (Watts, 2004); 4, Kyambura, only group (Nicole Simmons, personal
communication by email); 5, Fongoli, only group (Pruetz et al., 2017); 6, Budongo, Sonso (Fawcett and Muhumuza, 2000); 7, Mahale, M-group (Nishida, 1996; Nakamura & Itoh, 2015); 8, Gombe, Kasekela (Goodall,
1992; Boehm, 2017); 9, Mahale, M-group (Nishida et al., 1995); 10, Gombe, Kasekela (Wilson et al., 2004); 11, Mahale, M-group (Nishida et al., 1995); 12, Gombe, Kasekela (Mjungu & Collins, 2016); 13, Gombe,
Kasekela (Fallow, 2003).
12 Richard W. Wrangham
p. 689). Within-group gang attacks indeed resemble between-group attacks in that victims are ren-
dered helpless by the coordinated aggression of two or more males. However, several important dif-
ferences suggest that, whereas between-group gang attacks can involve collective proactive aggression
aimed at damaging a random stranger, within-group gang attacks involve mostly reactive aggression
used opportunistically by an individual male to compete for status, aided by a supporting set of sub-
ordinates. The differences are crucial.
First, in within-group attacks no hunting behaviour has been reported. When chimpanzees raid
into neighbouring territories, in contrast, they tend to show various kinds of hunting behaviour,
including sniffing the ground, stopping and listening in the direction of the neighbouring group,
maintaining high vigilance, and when a potential victim is detected, silently stalking before making
a sudden violent attack (Wrangham, 1999; Watts, 2004).
Second, when within-group attacks were seen from their beginning, in four of the five cases the
victim joined the aggressor(s) (Table 2). In between-group attacks, in contrast, the overwhelming pat-
tern is for aggressors to arrive at the victim. The difference indicates that within-group attacks were
less proactive than between-group attacks.
The exceptional incident, when a coalition arrived at the victim and immediately attacked him, was
the death of Pimu (Table 2). Even this case shows little evidence of proactivity, because the attack was
considered to be ‘best explained as an opportunistic challenge for social dominance’(Kaburu et al.,
2013, p. 794). It began in a relaxed social setting. Alpha-male Pimu was grooming with a rival,
Primus, when for no obvious reason Pimu bit Primus on his hand. Primus responded by biting
Pimu’s face. The exchange of bites precipitated an intense and noisy one-on-one fight which two
males tried to quell by attempting to separate the antagonists. Nine minutes after the fight began
Primus had left, but unfortunately for Pimu, who by now was wounded, four males arrived and imme-
diately attacked him. Male Alofu led these attacks, which continued intermittently for more than two
hours. Despite being defended to some extent by two males, Pimu died and Alofu became the new
alpha male. Apparently Alofu had heard Pimu fighting and arrived with the intention of taking advan-
tage of his temporary weakness. This case is one of only two in which an attack started as soon as a
coalition met the victim (the other being Rudi’s attack on Vincent; M. Wilson, pers. comm.).
Third, within-group attacks sometimes began several minutes after the antagonists met. The timing
was reported in five cases (Table 2). In three (Jilba, Grappelli and Goblin) the attack started from at
least 2 to at least 11 minutes after the victim and aggressors came together. In contrast, between-group
attacks invariably begin as soon as a victim is reached (Wrangham, 1999; Watts et al., 2006).
Fourth, the size of the gang reported in within-group attacks has been smaller than that in
between-group attacks. In two of the seven lethal attacks listed in Table 2 there were only two aggres-
sors, and of the total of 11 gang attacks, more than half had five attackers or fewer. In contrast the
smallest number of males recorded in 10 lethal between-group attacks on adult males listed by
Wilson et al. (2014) was three, which was the only case in which fewer than six attackers were involved.
This indicates that in within-group interactions, the power imbalance was relatively less, suggesting
that attacks were riskier and more costly.
Fifth, in two within-group attacks the victim was intermittently defended: Pimu was supported by
allied males, and Frodo was supported by his mother, who received some superficial wounds as a
result. Injuries to the aggressors were also reported in three cases (Table 2). In between-group attacks,
in contrast, victims are undefended and immobilised too completely to effectively fight back.
Sixth, within-group attacks could continue intermittently for much longer than the typical
between-group attack (up to more than 2 hours, compared with 10–20 minutes for between-group
attacks; Kaburu et al., 2013). The aggressors could afford to be relaxed because they were in familiar
territory where there was little chance of being surprised by members of other communities, unlike the
typical context of between-group encounters.
Seventh, within-group attacks showed a strong tendency to be related to within-group tensions
incurred by competition for the alpha rank. In at least three cases the fight began with the alpha male
attacking on his own (victims Vincent, Goblin and Jilba). Three times out of a total of 10 cases the
Evolutionary Human Sciences 13
alpha lost: he was killed once (Pimu) and lost his alpha rank twice (Ferdinand, Frodo). In those events the
attacks were led by the male who then became alpha. Seven times the alpha won: the alpha was a main
aggressor in three fights in which the victim died (Vincent, Hatari and Zesta), and in four further cases in
which the victim went into exile (Goblin, Jilba, Sheldon and Ntologi, August 1991). Between-group
attacks, in contrast, have not been reported to have any implications for the alpha male’s status.
In chimpanzees, the male whose bullying is most intense is invariably the alpha male: he forces all
others to give frequent signals of submission (Goodall, 1986). If bullies were being ‘singled out for
lethal attacks’, therefore, alphas should have been the most frequent victims. That alphas were attackers
more often (six times) than they were victims (three times) clearly undermines that account. Alpha
males are under frequent pressure to defend their rank by defeating challengers in physical fights.
Equally, challengers are constantly looking for opportunities to confront the alpha in an advantageous
context. Notably, Table 2 shows that the male who became or continued as alpha after the interaction
was invariably the individual who led the gang attacks. Thus chimpanzee gang attacks favoured the
‘bullies’, whether they were aspiring or incumbent alphas. This pattern is opposite to the reversed
dominance hierarchy described after human within-group killings.
In sum, compared with the premeditated nature of between-group gang attacks, to date
within-group attacks show less or no evidence of planning, less immediacy, less one-sidedness,
fewer attackers, higher risk of being wounded and longer duration. Overall within-group attacks
appear to be less efficient and less organised than between-group attacks. Current evidence does
not support bullies being ‘singled out for lethal attacks’.
Chimpanzee within-group attacks are also less efficient and less planned than human within-group
killings. Hunter–gatherer styles of within-group killing are well known. Most often an executioner, typ-
ically a kinsman of the victim, is delegated in advance and kills by ambush. Alternatively a group unites
to make a coordinated physical attack, or a killer’s action are approved by the community after he takes
sole responsibility for killing a disliked individual. The kills are cheap and effective not only because
lethal weapons are used, but also because the community agrees that a specific victim should be killed
or deserves to die (Boehm, 1999,2017). Without such agreement an executioner risks being regarded as
a danger to other group members, and therefore becoming vulnerable to being killed himself. Planning is
thus more evident in human executions than in chimpanzee within-group gang attacks.
Taken together, the limited available data show critical similarities and differences in within-group gang
attacks among chimpanzees and humans. The two species are similar in using coalitions that can assemble
immense power to kill or overcome a victim. Chimpanzee within-group gang attacks differ from human
executions, however, by showing little or no sign of being planned, and no consistent ability to kill the
alpha male. Instead, they appear to develop spontaneously as reactions to various contexts including
aggression by an alpha male towards a rival, aggression by a rival towards an alpha male and a resented
aggressor being physically weak and/or in the process of being defeated. Most important, chimpanzee
within-group gang attacks differ from human executions because they are not levelling mechanisms.
These conclusions suggest that differences in the features of within-group gang attacks between
humans and chimpanzees can indeed explain why reactive aggression is selected against in humans
but not in chimpanzees. In both species subordinate males would appear to benefit from escaping the
domination of an alpha male. Human subordinates can achieve that goal, because they can create coor-
dinated plans to safely kill even the most individually intimidating member of their group. In contrast
chimpanzees cannot create such plans. Being unable to predictably eliminate a despotic rival, their behav-
iour does not create a selection pressure against the bullying behaviour characteristic of alpha males.
Discussion
Why within-group gang attacks have not favoured groupishness in chimpanzees
A main aim of this review has been to investigate why, if the execution hypothesis is correct, self-domestication
and groupishness have not been selected by within-group killing among chimpanzees. Two differences
between human executions and chimpanzee within-group gang attacks appear to be critical.
14 Richard W. Wrangham
First, the coalitionary aggression used by chimpanzees in within-group gang attacks is constrained
to being largely reactive, whereas human executions are mostly proactive. Proactive gang aggression by
chimpanzees is possible only against members of other groups, when all out-group members are can-
didate victims for all raiders. In those circumstances no discussion is needed to determine who sides
with whom, or who the target is. Within groups, in contrast, a shared plan would be necessary to iden-
tify a potential victim and assemble a coalition, but the limited ability of chimpanzees to share inten-
tions means that no such plan is possible. This means that, unlike humans, chimpanzees cannot
systematically victimise antisocial individuals.
Second, chimpanzee within-group killings maintain the alpha-male role, whereas human
within-group killings eliminate it. Chimpanzee attacks are typically led by individuals who are defend-
ing their alpha status or attempting to acquire it. Other members of a coalition conform to a ‘winner-
support’strategy (Nishida et al., 1995). In humans living in small-scale, acephalous societies, in con-
trast, executions are levelling mechanisms: they are used to stop anyone from behaving despotically.
Admittedly large-scale human societies have leaders, but those leaders are not alphas in the animal
style. Unlike animals, human leaders depend for their power on the strength of their coalitions rather
than their ability to defeat rivals in one-on-one fights. The apparent similarities in within-group kill-
ings among chimpanzees and humans are thus deceptive. Chimpanzees are limited to reactively tar-
geting rivals for alpha male status, whereas humans can proactively eliminate alpha males.
The importance of targeted conspiratorial killing
Targeted conspiratorial killing is importantly different from less coordinated styles because, by allow-
ing a plan to be made, it can make the costs of killing exceptionally low even when the victim is an
individually intimidating fighter. Advanced weaponry is expected to contribute to reducing the costs
of lethal aggression but it is not vital, as shown by the kills made in between-group attacks by chim-
panzees, wolves and other species (Wrangham, 1999). The vital factor enabling proactive killing within
groups is the ability to conspire in a way that permits coalition partners to identify a victim, develop an
efficient plan and carry it out so as to give the killers a massive tactical advantage. After that ability had
evolved, humans became subject to a new form of social selection that no previous vertebrate had
experienced, namely to be less aggressive, less antisocial, more conformist and more punitive of non-
conformism than before, lest they be deliberately killed.
A focus on targeted conspiratorial killing thus builds on Boehm’s execution hypothesis by propos-
ing that a uniquely human ability was a necessary condition for developing human groupishness.
Self-domestication, according to Boehm’s hypothesis, was the first consequence of targeted conspira-
torial killing, when subordinate males collaborated to kill the alpha. The coalition of males who thus
held bullies in check then became a power over the group, able to impose a collective will on all types
of non-conformism and thereby inadvertently selecting for multiple psychological traits that underlie
different facets of human groupishness.
The role of language
Human self-domestication and language have been proposed to be related to each other by a positive
feedback loop. Self-domestication leads to less fighting and more tolerance. The increase in inter-
individual tolerance allows more effective use of language, which leads to coordination becoming
more skilled. Groups can then better suppress bullies, whether through more effective verbal aggres-
sion (Progovac & Benítez-Burraco, 2019; Del Savio & Mameli, 2020), social ostracism (Boehm, 2012;
Del Savio & Mameli, 2020) or killing (Boehm, 2012; Wrangham, 2019b). Equivalent suggestions have
been made for the relationship between groupishness and language (e.g. Boehm, 2012). Such ideas are
not controversial, although they have not been much elaborated.
The relationship between self-domestication and language is more contested with regard to origins.
One kind of proposal holds that the evolution of language depended crucially on self-domestication
Evolutionary Human Sciences 15
(Thomas & Kirby, 2018; Progovac & Benítez-Burraco, 2019). If so, targeted conspiratorial killing could
not have been responsible for initiating self-domestication. Alternatively, as implied in this paper, lan-
guage became sufficiently sophisticated before the origin of self-domestication that it launched the
process by making targeted conspiratorial killing possible (Wrangham, 2019a, b). The difficulty
about that idea is that it requires an explanation for how language became sophisticated before self-
domestication kicked in. Such an explanation would need to include why the ancestors of H. sapiens
developed a more sophisticated language than other Homo species (especially H. neanderthalensis and
H. denisova) that do not show anatomical signs of self-domestication. No such explanations have
apparently been proposed.
Objections to the putative role of targeted conspiratorial killing in favouring self-domestication and
groupishness
One kind of objection to the execution hypothesis is that it is not needed because alternative ideas can
explain the evolution of self-domestication and/or exceptional groupishness. Such alternatives include
two major classes.
The first comprises hypotheses that explain how the benefits of groupish behaviour could have been
favoured. For instance Tomasello et al.’s(2012) interdependence hypothesis suggested that selection
favoured individuals who cooperated to solve newly intense ecological problems. Boyd and
Richerson (2005) argued that selection among cultural systems led to the most cooperative groups suc-
ceeding. Sober and Wilson (1998) argued that prosocial moral systems evolved by group selection.
The second comprises hypotheses that focus on the controlof aggression, at least as an initial step.Hare
(2017) proposed that as brain size increased, there was a coincident increase in self-control that permitted
inhibition of aggressive tendencies (cf. Shilton et al., 2020). Del Savio and Mameli (2020)suggestedthat
increasing linguistic skills allowed group members to ostracise antisocial individuals so effectively that
aggressive tendencies would be selected against. Gleeson and Kushnick (2018) argued that females could
favour reduced aggression in males by choosing less aggressive males as mating partners.
A challenge for both types of explanation is to understand how selection would have caused alpha
males to have reduced fitness (Wrangham, 2019a). By definition, an alpha male in a small group can
outcompete others for access to resources. Hypotheses therefore need to explain not only why benefits
accrue to the less aggressive, but also why despotic males could not obtain those benefits by physical
force. Some such explanations have been advocated. Del Savio and Mameli (2020) argued that dom-
ineering individuals could be excluded from resources by being socially ostracised rather than killed.
Whether social ostracism or similar lesser punishments can be an effective force in reducing fitness
without being backed up by an ultimate threat of execution, however, has to my knowledge not
been demonstrated. Furthermore killing a bully may be less costly than non-lethal punishment,
since killing reduces the risk of the victim fighting back, whether immediately or in the future.
Gavrilets (2012) modelled a related idea, which was that alpha males predictably lost resources to coa-
litions that used reactive rather than proactive aggression. His model assumed that helpers supported
victims despite such fights being relatively costly. Application of such models to proactive lethal attacks
is desirable.
The proposal that targeted conspiratorial killing had a major influence in promoting human group-
ishness does not rule out the possibility that important elements of groupishness had already been
present in pre-sapiens species of Homo. Such ancestral elements could have facilitated food-sharing
among non-kin, a sexual division of labour, and the development of an initial form of language,
for example. According to the execution hypothesis as presented here, however, the development of
targeted conspiratorial killing represented a critical advance because it alone explains the replacement
of an ordinary alpha-male hierarchy with a reverse dominance hierarchy, with its many consequences
for prosociality (Boehm, 1999; Wrangham, 2019a).
Acknowledgements. I thank Christopher Boehm, Sergey Gavrilets, Brian Hare, Anne McGuire and Mike Wilson for valu-
able comments.
16 Richard W. Wrangham
Author contributions. RWW wrote the paper.
Financial support. This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Conflicts of interest. RWW declares none.
Data availability statement. No original data are presented.
References
Andirkó, A., Moriano, J., Vitriolo, A., Kuhlwilm, M., Testa, G., & Boeckx, C. (2021). Fine-grained temporal mapping of
derived high-frequency variants supports the mosaic nature of the evolution of Homo sapiens. bioRxiv
2021.01.22.427608. doi.org/10.1101/2021.01.22.427608
Arbuckle, B. S. (2005). Experimental animal domestication and its application to the study of animal exploitation in
Prehistory. In J.-D. Vigne, J. Peters, & D. Helmer (Eds.), First steps of animal domestication: New archaeozoological
approaches (pp. 18–33). Oxbow Books.
Backwell, L., Bradfield, J., Carlson, K. J., Jashashvili, T., Wadley, L., & d’Errico, F. (2017). The antiquity of bow-and-arrow
technology: Evidence from Middle Stone Age layers at Sibudu Cave. Antiquity,92, 289–303. doi:10.15184/aqy.2018.11
Bagehot, W. (1872). Physics and politics: Or thoughts on the application of the principles of ‘natural selection’and ‘inher-
itance’to political society. London. [In N. St John-Stevas (Ed.) The Collected Works of Walter Bagehot, VII. London, 1974.]
Beier, J., Anthes, N., Wahl, J., & Harvati, K. (2018). Similar cranial trauma prevalence among Neanderthals and Upper
Palaeolithic modern humans. Nature,563, 686–690.
Belyaev, D. K. (1984). Genetics, society and personality. In V. Chopra (Ed.), Genetics: New frontiers. Proceedings of the XVth
international congress on genetics (pp. 379–386). Oxford University Press.
Bingham, P. M. (2000). Human evolution and human history: A complete theory. Evolutionary Anthropology,9, 248–257.
Bissonnette, A., Perry, S., Barrett, L., Mitani, J. C., Flinn, M., Gavrilets, S., & de Waal, F. B. M. (2015). Coalitions in theory and
reality: A review of pertinent variables and processes. Behaviour,152,1–56. doi:10.1163/1568539X-00003241
Boas, F. (1911). The mind of primitive man. Macmillan.
Boehm, C. (1993). Egalitarian behavior and reverse dominance hierarchy. Current Anthropology,34(3), 227–240.
Boehm, C. (1999). Hierarchy in the forest: The evolution of egalitarian behavior. Harvard University Press.
Boehm, C. (2012). Moral origins: The evolution of virtue, altruism, and shame. Basic Books.
Boehm, C. (2014). The moral consequences of social selection. Behaviour,151, 167–183.
Boehm, C. (2017). Ancestral precursors, social control, and social selection in the evolution of morals. In M. N. Muller, R.
W. Wrangham, & D. P. Pilbeam (Eds.), Chimpanzees and human evolution. Harvard University Press.
Boehm, C. (2018). Collective intentionality: A basic and early component of moral evolution. Philosophical Psychology,31(5),
680–702. doi: 10.1080/09515089.2018.1486607
Boyd, R., & Richerson, P. (2005). Not by genes alone: How culture transformed human evolution. University of Chicago Press.
Brace, C. L., Rosenberg, K. R., & Hunt, K. D. (1987). Gradual change in human tooth size in the late Pleistocene and
post-Pleistocene. Evolution,41, 705–720.
Brooks, A. S., Yellen, J. E., Potts, R., Behrensmeyer, A. K., Deino, A. L., Leslie, D. E., …Clark, J. B. (2018). Long-distance
stone transport and pigment use in the earliest Middle Stone Age. Science,360(6384), 90–94. doi:10.1126/science.aao2646
Brosnan, S. F., & de Waal, F. (2003). Monkeys reject unequal pay. Nature,425, 297–299.
Brüne, M. (2007). On human self-domestication, psychiatry, and eugenics. Philosophy, Ethics, and Humanities in Medicine,2
(21), 1–9. doi:10.1186/1747-5341-2-21
Carneiro, M., Rubin, C.-J., Di Palma, F., Albert, F. W., Alföldi, J., Barrio, A. M., …Andersson, L. (2014). Rabbit genome
analysis reveals a polygenic basis for phenotypic change during domestication. Science,345(6200), 1074–1079.
Chapais, B. (2015). Competence and the evolutionary origins of status and power in humans. Human Nature,26, 161–183.
doi 10.1007/s12110-015-9227-6
Chirchir, H. (2021). Trabecular bone in domestic dogs and wolves: Implications for understanding human self-domestication.
Anatomical Record,304,31–41. doi:10.1002/ar.24510
Choi, J.-K., & Bowles, S. (2007). The coevolution of parochial altruism and war. Science,318, 636–640.
Cieri, R. L., Churchill, S. E., Franciscus, R. G., Tan, J., & Hare, B. (2014). Craniofacial feminization, social tolerance, and the
origins of behavioral modernity. Current Anthropology,55, 419–443.
Cofnas, N. (2018). Power in cultural evolution and the spread of prosocial norms. Quarterly Review of Biology,93(4),
297–318.
Coppinger, R., & Coppinger, L. (2000). Dogs: A startling new understanding of canine origin, behavior, and evolution.
Scribner.
Crockford, S. J. (2002). Animal domestication and heterochronic speciation: the role of thyroid hormone. In
N. Minugh-Purvis & K. McNamara (Eds.), Human evolution through developmental change (pp. 122–153). Johns
Hopkins University Press.
Evolutionary Human Sciences 17
Darwin, C. (1871). The descent of man and selection in relation to sex. J. Murray.
Del Savio, L., & Mameli, M. (2020). Human domestication and the roles of human agency in human evolution. History and
Philosophy of the Life Sciences,42(21), 1–25. doi:https://doi.org/10.1007/s40656-020-00315-0
de Manuel, M., Kuhlwilm, M., Frandsen, P., Sousa, V. C., Desai, T., Prado-Martinez, J., ... Marques-Bonet, T. (2016).
Chimpanzee genomic diversity reveals ancient admixture with bonobos. Science,354, 477–481.
Deska, J. C., Lloyd, E. P., & Hugenberg, K. (2018). Facing humanness: Facial width-to-height ratio predicts ascriptions of
humanity. Journal of Personality and Social Psychology,114(1), 75–94.
de Waal, F. B. M. (2006). Primates and philosophers: How morality evolved. Princeton University Press.
Dobzhansky, T. (1962). Mankind evolving. Yale University Press.
dos Santos, M., & West, S. A. (2018). The coevolution of cooperation and cognition in humans. Proceedings of the Royal
Society, B,285(20180723), 1–8. doi.org/10.1098/rspb.2018.0723
Engelmann, J. M., Clift, J. B., Herrmann, E., & Tomasello, M. (2017). Social disappointment explains chimpanzees’behaviour
in the inequity aversion task. Proceedings of the Royal Society B,284, 20171502. doi.org/10.1098/rspb.2017.1502
Erdal, D., & Whiten, A. (1994). On human egalitarianism: an evolutionary product of Machiavellian status escalation?
Current Anthropology,35(2), 175–178.
Fallow, A. (2003). Frodo, the alpha male. National Geographic Society.
Fawcett, K., & Muhumuza, G. (2000). Death of a wild chimpanzee community member: Possible outcome of intense sexual
competition. American Journal of Primatology,51, 243–247.
Fischer, E. (1914). Die Rassenmerkmale des Menschen als Domesticationserscheinungen. Zeitschrift für Morphologie und
Anthropologie 18, 479–524.
Fitak, R. R., Mohandesan, E., Corander, J., Yadamsuren, A., Chuluunbat, B., Abdelhadi, O., …Burge, P. A. (2020). Genomic
signatures of domestication in Old World camels. Communications Biology,3,1–10. doi:https://doi.org/10.1038/s42003-
020-1039-5
Gavrilets, S. (2012). On the evolutionary origins of the egalitarian syndrome. Proceedings of the National Academy of Sciences,
109, 14069–14074. doi:10.1073/pnas.1201718109
Gavrilets, S. (2015). Collective action problem in heterogeneous groups. Philosophical Transactions of the Royal Society B,370
(20150016), 1–17. doi:http://dx.doi.org/10.1098/rstb.2015.0016
Gavrilets, S., Duenez-Guzman, E. A., & Vose, M. D. (2008). Dynamics of alliance formation and the egalitarian revolution.
PLoS ONE,3(10), e3293.
Gavrilets, S., & Fortunato, L. (2014). A solution to the collective action problem in between-group conflict with within-group
inequality. Nature Communications,5(3256), 1–11.
Ghazanfar, A. A., Kelly, L. M., Takahashi, D. Y., Winters, S., Terrett, R., & Higham, J. P. (2020). Domestication phenotype
linked to vocal behavior in marmoset monkeys. Current Biology,30,1–17. doi:https://doi.org/10.1016/j.cub.2020.09.049
Gintis, H., van Schaik, C., & Boehm, C. (2015). Zoon Politikon: The evolutionary origins of human political systems. Current
Anthropology,56(3), 327–353.
Gleeson, B. T., & Kushnick, G. (2018). Female status, food security, and stature sexual dimorphism: Testing mate choice as a
mechanism in human self-domestication. American Journal of Physical Anthropology,167,458–469. doi:10.1002/ajpa.23642
Gokcumen, O. (2020). Archaic hominin introgression into modern human genomes. Yearbook of Physical Anthropology,171,
60–73. doi:10.1002/ajpa.23951
Goodall, J. (1986). The chimpanzees of Gombe: Patterns of behavior. Harvard University Press.
Goodall, J. (1992). Unusual violence in the overthrow of an alpha male chimpanzee at Gombe. In T. Nishida, W. C. McGrew,
P. Marler, M. Pickford, & F. B. M. de Waal (Eds.), Topics in primatology, Volume 1: Human origins (Vol. 1, pp. 131–142).
University of Tokyo Press.
Gould, S. J. (1977). Ontogeny and phylogeny. Harvard University Press.
Graham, J., Haidt, J., Koleva, S., Motyl, M., Iyer, R., Wojcik, S. P., & Ditto, P. H. (2013). Moral Foundations Theory: The
pragmatic validity of moral pluralism. Advances in Experimental Social Psychology,47,55–130.
Haidt, J. (2012). The righteous mind: Why good people are divided by politics and religion. Pantheon.
Haldane, J. B. S. (1956). The argument from animals to men: an examination of its validity for anthropology. Journal of the
Royal Anthropological Institute of Great Britain and Ireland,86,1–14.
Hare, B. (2017). Survival of the friendliest: Homo sapiens evolved via selection for prosociality. Annual Review of Psychology,
68, 155–186.
Hare, B., Wobber, V., & Wrangham, R. W. (2012). The self-domestication hypothesis: Bonobos evolved due to selection
against male aggression. Animal Behavior,83, 573–585.
Hare, B., & Woods, V. (2020). Survival of the friendliest: Understanding our origins and rediscovering our common humanity.
Random House.
Haselhuhn, M. P., Ormiston, M. E., & Wong, E. M. (2015). Men’s facial width-to-height ratio predicts aggression: a
meta-analysis. PLoS ONE,10(4), e0122637.
Hublin, J.-J., Ben-Ncer, A., Bailey, S. E., Freidline, S. E., Neubauer, S., Skinner, M. M., …Gunz, P. (2017). New fossils from
Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens.Nature,546, 289–292.
18 Richard W. Wrangham
Ihara, Y. (2020). A mathematical model of social selection favoring reduced aggression. Behavioral Ecology and Sociobiology,
74(91), 1–13. doi:10.1007/s00265-020-02875-4
Kaburu, S. S. K., Inoue, S., & Newton-Fisher, N. E. (2013). Death of the alpha: Within-community lethal violence among
chimpanzees of the Mahale Mountains National Park. American Journal of Primatology,75, 789–797.
Kovalaskas, S., Rilling, J. K., & Lindo, J. (2020). Comparative analyses of the Pan lineage reveal selection on gene pathways
associated with diet and sociality in bonobos. Genes, Brain and Behavior,e12715,1–11. doi.org/10.1111/gbb.12715
Leach, H. (2003). Human domestication reconsidered. Current Anthropology,44(3), 349–368.
Librado, P., Gamba, C., Gaunitz, C., Der Sarkissian, C., Pruvost, M., Albrechtsen, A., …Orlando, L. (2017). Ancient genomic
changes associated with domestication of the horse. Science,356, 442–445.
Lieberman, D. E., McBratney, B. M., & Krovitz, G. (2002). The evolution and development of cranial form in Homo sapiens.
Proceedings of the National Academy of Sciences,99, 1134–1139.
Lord, K., Larson, G., Coppinger, R. P., & Karlsson, E. K. (2020). The history of farm foxes undermines the animal domes-
tication syndrome. Trends in Ecology and Evolution,35(2), 125–136. doi:10.1016/j.tree.2019.10.011
Lorenz, K. Z. (1940). Durch Domestikation verursachte Störungen arteigener Verhalten. Zeitschrift für angewandte
Psychologie und Charakterkunde,59,1–81.
Luo, X., Zhou, Y., Zhang, B., Zhang, Y., Wang, X., Feng, T., …Qingyou, L. (2020). Understanding divergent domestication
traits from the whole-genome sequencing of swamp- and river-buffalo populations. National Science Review,7, 686–701.
doi: 10.1093/nsr/nwaa024
McAuliffe, K., & Santos, L. R. (2018). Do animals have a sense of fairness? In K. Gray & J. Graham (Eds.), Atlas of moral
psychology (pp. 393–400). Guilford Press.
Mead, M. (1954). Some theoretical considerations on the problem of mother–child separation. American Journal of
Orthopsychiatry,24, 471–483.
Mesterton-Gibbons, M., Gavrilets, S., Gravner, J., & Akçay, E. (2011). Models of coalition or alliance formation. Journal of
Theoretical Biology,274, 187–204. doi:10.1016/j.jtbi.2010.12.031
Mjungu, D. C. (2010). Dynamics of intergroup competition in two neighboring chimpanzee communities. PhD thesis,
University of Minnesota.
Mjungu, D. C., & Collins, A. (2016). Gombe gets a new Alpha –The fall of Ferdinand. https://janegoodall.ca/our-stories/
gombe-gets-new-alpha-fall-ferdinand-2.
Montague, M. J., Li, G., Gandolfi, B., Khan, R., Aken, B. L., Searle, S. M. J., …Warren, W. C. (2014). Comparative analysis of
the domestic cat genome reveals genetic signatures underlying feline biology and domestication. Proceedings of the
National Academy of Sciences,111(48), 17230–17325.
Nakamura, M., & Itoh, N. (2015). Conspecific killings. In M. Nakamura, K. Hosaka, N. Itoh, & K. Zamma (Eds.), Mahale
chimpanzees: 50 years of research (pp. 372–384). Cambridge University Press.
Nishida, T. (1996). The death of Ntologi, the unparalleled leader of M group. Pan Africa News,3,4.
Nishida, T., Hosaka, K., Nakamura, M., & Hamai, M. (1995). A within-group gang attack on a young adult male chimpanzee:
Ostracism of an ill-mannered member? Primates,36, 207–211.
Okada, D., & Bingham, P. M. (2008). Human uniqueness –Self-interest and social cooperation. Journal of Theoretical Biology,
253, 261–270.
O’Rourke, T., & Boeckx, C. (2020). Glutamate receptors in domestication and modern human evolution. Neuroscience &
Biobehavioral Reviews,108, 341–357. doi: https://doi.org/10.1101/439869
Otterbein, K. F. (1986). The ultimate coercive sanction: A cross-cultural study of capital punishment. HRAF Press.
Parsons, K. J., Rigg, A., Conith, A. J., Kitchener, A. C., Harris, S., & Zhu, H. (2020). Skull morphology diverges between urban
and rural populations of red foxes mirroring patterns of domestication and macroevolution. Proceedings of the Royal
Society B,287(20200763), 1–10. doi:http://dx.doi.org/10.1098/rspb.2020.0763
Pearson, O. M. (2000). Activity, climate, and postcranial robusticity: implications for modern human origins and scenarios of
adaptive change. Current Anthropology,41(4), 569–589.
Pendleton, A. L., Shen, F., Taravella, A. M., Emery, S., Veeramah, K. R., Boyko, A. R., & Kidd, J. M. (2018). Comparison of
village dog and wolf genomes highlights the role of the neural crest in dog domestication. BMC Biology,16(64), 1–21.
doi.org/10.1186/s12915-018-0535-2
Phillips, T., Li, J., & Kendall, G. (2014). The effects of extra-somatic weapons on the evolution of human cooperation towards
non-kin. PLoS ONE,9(5), e95742.
Pilbeam, D. R., & Lieberman, D. E. (2017). Reconstructing the Last Common Ancestor of chimpanzees and humans. In M.
N. Muller, D. R. Pilbeam, & R. W. Wrangham (Eds.), Chimpanzees and human evolution (pp. 22–141). Harvard
University Press.
Prado-Martinez, J., Sudmant, P. H., Kidd, J. M., Li, H., Kelley, J. L., Lorente-Galdos, B., ... Marques-Bonet, T. (2013). Great
ape genetic diversity and population history. Nature,499(7459), 471–475.
Progovac, L., & Benítez-Burraco, A. (2019). From physical aggression to verbal behavior: Language evolution and self-
domestication feedback loop. Frontiers in Psychology,10(2807), 1–19. doi: 10.3389/fpsyg.2019.02807
Evolutionary Human Sciences 19
Pruetz, J. D., Ontl, K. B., Cleaveland, E., Lindshield, S., Marshack, J., & Wessling, E. G. (2017). Intragroup lethal aggression in
West African chimpanzees (Pan troglodytes verus): Inferred killing of a former alpha male at Fongoli, Senegal.
International Journal of Primatology,38,31–57.
Prüfer, K., Munch, K., Hellman, I., Akagi, K., Miller, J. R., Walenz, B., ... Pääbo, S. (2012). The bonobo genome compared
with the chimpanzee and human genomes. Nature,486, 527–531.
Prüfer, K., Racimo, F., Patterson, N., Jay, F., Sankararaman, S., Sawyer, S., ... Pääbo, S. (2014). The complete genome sequence
of a Neanderthal from the Altai Mountains. Nature,505,43–49.
Raghanti, M. A. (2019). Domesticated species: It takes one to know one. PNAS,116, 14401–14403.
Rosati, A. (2019). Heterochrony in chimpanzee and bonobo spatial memory development. American Journal of Physical
Anthropology,169(2), 302–321. doi:10.1002/ajpa.23833
Ruff, C. B., Trinkaus, E., & Holliday, T. W. (1997). Body mass and encephalization in Pleistocene Homo.Nature,387, 173–
176.
Ruff, C. B., Trinkaus, E., Walker, A., & Larsen, C. S. (1993). Postcranial robusticity in Homo. I: Temporal trends and mech-
anical interpretation. American Journal of Physical Anthropology,91,21–53.
San-Jose, L. M., & Roulin, A. (2020). On the potential role of the neural crest cells in integrating pigmentation into behavioral
and physiological syndromes. Frontiers in Ecology and Evolution,8(278), 1–9.
Sánchez-Villagra, M. R., & van Schaik, C. P. (2019). Evaluating the self-domestication hypothesis of human evolution.
Current Anthropology,28, 133–143. doi:10.1002/evan.21777
Schlebusch, C. M., Malmström, H., Günther, T., Sjödin, P., Coutinho, A., Edlund, H., …Jakobsson, M. (2017). Southern
African ancient genomes estimate modern human divergence to 350,000 to 260,000 years ago. Science,358, 652–655.
Shea, J. J. (2017). Occasional, obligatory, and habitual stone tool use in hominin evolution. Evolutionary Anthropology,26,
200–217.
Shea, B. T., & Coolidge, H. J. (1988). Craniometric differentiation and systematics in the genus Pan. Journal of Human
Evolution,17, 671–685.
Shilton, D., Breski, M., Dor, D., & Jablonka, E. (2020). Human social evolution: Self-domestication or self-control? Frontiers
in Psychology,11(134), 1–22. doi: 10.3389/fpsyg.2020.00134
Singh, N., Albert, F. W., Plyusnina, I., Trut, L., Pääbo, S., & Harvati, K. (2017). Facial shape differences between rats selected
for tame and aggressive behaviors. PLoS ONE,12(4), e0175043.
Sober, E., & Wilson, D. S. (1998). Unto others: The evolution and psychology of unselfish behavior. Harvard University Press.
Stearns, S. C., & Medzhitov, R. (2015). Evolutionary medicine. Oxford University Press.
Stringer, C. (2016). The origin and evolution of Homo sapiens.Philosophical Transactions of the Royal Society B,371,
20150237.
Takemoto, H., Kawamoto, Y., & Furuichi, T. (2015). How did bonobos come to range south of the Congo river?
Reconsideration of the divergence of Pan paniscus from other Pan populations. Evolutionary Anthropology,24, 170–184.
Theofanopoulou, C., Gastaldon, S., O’Rourke, T., Samuels, B. D., Martins, P. T., Delogu, F., …Boeckx, C. (2017).
Self-domestication in Homo sapiens: Insights from comparative genomics. PLoS ONE,12(10), e0185306. doi:https://doi.
org/10.1371/journal.pone.0185306
Thieme, H. (1997). Lower Paleolithic hunting spears from Germany. Nature,385, 807–810.
Thomas, J., & Kirby, S. (2018). Self domestication and the evolution of language. Biology and Philosophy,33(9). doi:10.1007/
s10539-018-9612-8
Tiihonen, J., Koskuvi, M., Lähteenvuo, M., Virtanen, P. L. J., Ojansuu, I., Vaurio, O., …Lehtonen, Š. (2020). Neurobiological
roots of psychopathy. Molecular Psychiatry,25(12), 3432–3441. doi:https://doi.org/10.1038/s41380-019-0488-z
Toda, K., & Furuichi, T. (2020). Low resistance of senior resident females toward female immigration in bonobos (Pan panis-
cus) at Wamba, Democratic Republic of the Congo. International Journal of Primatology,41, 415–427. doi:10.1007/
s10764-019-00126-6
Tokuyama, N., & Furuichi, T. (2016). Do friends help each other? Patterns of female coalition formation in wild bonobos at
Wamba. Animal Behaviour,119,27–35.
Tomasello, M. (2016). A natural history of human morality. Harvard University Press.
Tomasello, M., Melis, A. P., Tennie, C., Wyman, E., & Herrmann, E. (2012). Two key steps in the evolution of human cooper-
ation: The interdependence hypothesis. Current Anthropology,53, 673–686. doi:10.1086/668207
Trut, L. N., Dzerzhinskii, F. Y., & Nikol’skii, V. S. (1991). Intracranial allometry and craniological changes during domesti-
cation of silver foxes. Genetika,27(9), 1605–1611.
Trut, L. N., Kharlamova, A. V., & Herbeck, Y. E. (2020). Belyaev’s and PEI’s foxes: A far cry. Trends in Ecology and Evolution,
35(8), 649–651.
Watts, D. P. (2004). Intracommunity coalitionary killing of an adult male chimpanzee at Ngogo, Kibale National Park,
Uganda. International Journal of Primatology,25(3), 507–521.
Watts, D. P., Muller, M. N., Amsler, S. A., Mbabazi, G., & Mitani, J. C. (2006). Lethal inter-group aggression by chimpanzees
in Kibale National Park, Uganda. American Journal of Primatology,68, 161–180.
20 Richard W. Wrangham
West, S. A., El Mouden, C., & Gardner, A. (2011). Sixteen common misconceptions about the evolution of cooperation in
humans. Evolution and Human Behavior,32, 231–262. doi:10.1016/j.evolhumbehav.2010.08.001
Wilkins, A. S. (2017). Revisiting two hypotheses on the ‘domestication syndrome’in light of genomic data. Vavilov J Genet
Breed,21(4), 435–442.
Wilkins, A. S., Wrangham, R. W., & Fitch, W. T. (2014). The ‘domestication syndrome’in mammals: A unified explanation
based on neural crest cell behavior and genetics. Genetics,197, 795–808.
Williams, F. L. (2013). Neandertal craniofacial growth and development and its relevance for modern human origins. In F.
H. Smith & J. C. M. Ahern (Eds.), The origins of modern humans: Biology reconsidered ( pp. 253–284). John Wiley.
Wilson, M. L., Boesch, C., Fruth, B., Furuichi, T., Gilby, I. C., Hashimoto, C., …Wrangham, R. W. (2014). Lethal aggression
in Pan is better explained by adaptive strategies than human impacts. Nature,513, 414–417.
Wilson, M. L., Collins, D. A., Wallauer, W. R., & Kamenya, S. (2005). Gombe Stream Research Centre annual report 2005.
Kigoma, Tanzania.
Wilson, M. L., Kamenya, S., Collins, D. A., & Wallauer, W. R. (2004). Gombe Stream Research Centre annual report 2004.
Kigoma, Tanzania.
Wilson, D. S., & Wilson, E. O. (2007). Rethinking the theoretical foundation of sociobiology. Quarterly Review of Biology,82
(4), 327–348.
Won, Y., & Hey, J. (2005). Divergence population genetics of chimpanzees. Molecular Biology and Evolution,22, 297–307.
Woodburn, J. (1982). Egalitarian societies. Man,17(3), 431–451.
Wrangham, R. W. (2019a). Hypotheses for the evolution of reduced reactive aggression in the context of human self-
domestication. Frontiers in Psychology,10(1914), 1–11. doi:10.3389/fpsyg.2019.01914
Wrangham, R. W. (1999). Evolution of coalitionary killing. Yearbook of Physical Anthropology,42,1–39.
Wrangham, R. W. (2018). Two types of aggression in human evolution. Proceedings of the National Academy of Sciences,115
(2), 245–253. doi/10.1073/pnas.1713611115
Wrangham, R. W. (2019b). The goodness paradox: The strange relationship between virtue and violence in human evolution.
Alfred A. Knopf.
Wrangham, R. W., Wilson, M. L., & Muller, M. N. (2006). Comparative rates of aggression in chimpanzees and humans.
Primates,47,14–26.
Yamakoshi, G. (2004). Food seasonality and socioecology in Pan: Are West African chimpanzees another bonobo? African
Study Monographs,25(1), 45–60.
Zanella, M., Vitriolo, A., Andirko, A., Martins, P. T., Sturm, S., O’Rourke, T., …Testa, G. (2019). Dosage analysis of the
7q11.23 Williams region 1 identifies BAZ1B as a major human gene patterning the modern human face and underlying
self-domestication. Science Advances,5(eaaw7908).
Zeder, M. (2015). Core questions in domestication research. Proceedings of the National Academy of Sciences,112(11), 3191–
3198. doi/10.1073/pnas.1501711112
Zeder, M. (2020). Straw foxes: domestication syndrome evaluation comes up short. Trends in Ecology and Evolution,35(8),
647–649.
Cite this article: Wrangham RW (2021). Targeted conspiratorial killing, human self-domestication and the evolution of
groupishness. Evolutionary Human Sciences 3, e26, 1–21. https://doi.org/10.1017/ehs.2021.20
Evolutionary Human Sciences 21
