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Runaway Social Selection in Human Evolution

Authors:
  • Baylor University University of Missouri

Abstract

Darwin posited that social competition among conspecifics could be a powerful selective pressure. Alexander proposed a model of human evolution involving a runaway process of social competition based on Darwin’s insight. Here we briefly review Alexander’s logic, and then expand upon his model by elucidating six core arenas of social selection that involve runaway, positive-feedback processes, and that were likely involved in the evolution of the remarkable combination of adaptations in humans. We discuss how these ideas fit with the hypothesis that a key life history innovation that opened the door to runaway social selection, and cumulative culture, during hominin evolution was increased cooperation among individuals in small fission-fusion groups.
fevo-10-894506 June 1, 2022 Time: 9:36 # 1
REVIEW
published: 02 June 2022
doi: 10.3389/fevo.2022.894506
Edited by:
Jaroslava Varella Valentova,
University of São Paulo, Brazil
Reviewed by:
Peter J. Richerson,
University of California, Davis,
United States
Annemie Ploeger,
University of Amsterdam, Netherlands
*Correspondence:
Mark V. Flinn
mark_flinn@baylor.edu
These authors have contributed
equally to this work and share first
authorship
Specialty section:
This article was submitted to
Behavioral and Evolutionary Ecology,
a section of the journal
Frontiers in Ecology and Evolution
Received: 11 March 2022
Accepted: 13 May 2022
Published: 02 June 2022
Citation:
Crespi BJ, Flinn MV and
Summers K (2022) Runaway Social
Selection in Human Evolution.
Front. Ecol. Evol. 10:894506.
doi: 10.3389/fevo.2022.894506
Runaway Social Selection in Human
Evolution
Bernard J. Crespi1, Mark V. Flinn2*and Kyle Summers3
1Department of Biology, Simon Fraser University, Burnaby, BC, Canada, 2Department of Anthropology, Baylor University,
Waco, TX, United States, 3Department of Biology, East Carolina University, Greenville, NC, United States
Darwin posited that social competition among conspecifics could be a powerful
selective pressure. Alexander proposed a model of human evolution involving a runaway
process of social competition based on Darwin’s insight. Here we briefly review
Alexander’s logic, and then expand upon his model by elucidating six core arenas of
social selection that involve runaway, positive-feedback processes, and that were likely
involved in the evolution of the remarkable combination of adaptations in humans. We
discuss how these ideas fit with the hypothesis that a key life history innovation that
opened the door to runaway social selection, and cumulative culture, during hominin
evolution was increased cooperation among individuals in small fission-fusion groups.
Keywords: social selection, human evolution, cooperation, runaway processes, cumulative culture
“There can be no doubt that a tribe including many members who, from possessing in a high degree the
spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to give aid to each other
and to sacrifice themselves for the common good, would be victorious over most other tribes; and this would
be natural selection”
Charles Darwin [1871: 166]
“Why are we all alone [in]. . . our tendency and ability to cooperate and compete in social groups of
millions?”
Richard Alexander (1990: 1)
INTRODUCTION
In 1871–2 Charles Darwin tackled two major challenges to his theory of evolution: costly displays
and weaponry used in courtship, and the descent of humans. He proposed that mating competition
among conspecifics — sexual selection — was a potent evolutionary force that could explain the
apparent enigmas of bright plumage, antlers, even the great horn of the Rhinoceros beetle. Sir
Ronald Fisher (1930) furthered Darwin’s concept of sexual selection by positing that intraspecific
competition could involve a process of “runaway” selection if choice was based on comparisons
favoring relative extremes. Crook (1972) and West-Eberhard (1979, 1983) expanded Darwin’s and
Fisher’s concepts to include selection from all aspects of social interaction among conspecifics,
termed “social selection.” Humphrey (1976) and later Dunbar (1993) and Tomasello (1999)
suggested that intelligence could evolve in the context of social competition and cooperation among
conspecifics: cleverness in the “social chess game.” Alexander (1990); (see also Alexander, 1974,
1979, 1989) integrated these ideas into a comprehensive model of “How humans evolved” involving
a process of “runaway social selection” where hominins “increasingly became their own principal
hostile force of nature,” with cooperation and coalitions posited as crucial and complementary
aspects of hominin social environments (Alexander, 1987, 2006;Wrangham, 1999).
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We use the term “runaway” selection here to refer broadly
to selection that involves either: (1) arms races within species,
whereby competition-based selection between individuals and
groups leads to reciprocal, escalating trait expression and
elaboration across evolutionary time, or (2) positive feedback
between the selection pressures and evolutionary changes in
one trait (e.g., a phenotype subject to choice), and selection
pressures and evolutionary changes in a second trait (e.g., choice
of the phenotype), such that evolutionary changes in both traits
become mutually reinforcing across generations (e.g., Nesse,
2007;Nakamaru and Dieckmann, 2009;Piantadosi and Kidd,
2016;Bailey and Kölliker, 2019). As such, runaway selection can
apply to a wide variety of sets of phenotypes, including aspects
of social interactions, in addition to those involved in female
choice and sexual selection (e.g., Bailey and Kölliker, 2019).
Alexander focused on a process of “runaway social selection”
involving arms-race competition among individuals for “social
cleverness” (including language abilities, social skills, aptitudes
for cultural information, coalition building, and multiple other
types of intelligence) that became increasingly important in
human evolution (Alexander, 1990;Flinn and Alexander, 2007).
There are other plausible models of human evolution involving
social competition (e.g., Hrdy, 2009;van Schaik and Burkart,
2010;Wrangham, 2019); our main objective here is to describe,
extend and expand upon Alexander’s runaway social selection
model, which connects to both Darwin’s (1871) ideas about
selection associated with competition among conspecifics and his
model of human descent. As such, we consider runaway social
selection involving both competitive arms races, and positive
feedbacks driven by mutually reinforcing selection during choice-
trait coevolution.
DARWIN AND THE DESCENT OF
HUMANS
Darwin (1871) suggested an evolutionary scenario for humans
involving a positive feedback loop between tool use and
intelligence. Initially a “smart ape” began to use tools; this
advantage led to further selection for intelligence and more
sophisticated tools, and eventually to upright bipedal locomotion,
precision hand control, reduced dentition, social cooperation,
morality, and other human traits. Evidence from hominin
paleontology and archeology over the past 150 years has
not supported Darwin’s tool use model as he presented it.
Fossils indicate that Australopithecines and perhaps even earlier
hominins were habitual bipeds for >2 MY prior to significant
changes in brain evolution (McBrearty and Brooks, 2000;Antón
et al., 2014;Almécija et al., 2021). Tool use also predates
increases in cranial capacity by at least 1 MY (McPherron
et al., 2010), it is not restricted to hominins and it is not
subserved by specialized neurobiological mechanisms (Geary and
Huffman, 2002;Geary, 2005;Sherwood et al., 2008;Bruner,
2021). Although technology is clearly a significant part of the
human evolutionary story (Osiurak and Reynaud, 2020), it
apparently does not account for our extraordinary social mental
aptitudes including such traits as empathy, language, mental
time-travel, consciousness, and mind-reading (Herrmann et al.,
2007;Haber and Corriveau, 2020), or for the uniqueness of the
hominin evolutionary trajectory.
Darwin recognized that culture — socially transmitted
information and materials — was also a key selective pressure
in human evolution. Indeed, he noted (1871, pp. 78–79) that
“the formation of different languages and of distinct species, and
the proofs that both have been developed through a gradual
process, are curiously parallel.” Aptitudes for language, learning,
and the sociality underpinned acquisition of information were
increasingly important for hominin survival and reproduction,
eventually resulting in the extraordinary Anthropocene niche
that we inhabit today. Why humans are “the uniquely unique
species” (Alexander, 1990) who developed such extraordinary
cognitive and cultural abilities remains an elusive evolutionary
puzzle (Tomasello, 1999;Henrich and McElreath, 2003;Laland
and Seed, 2021). The problem is further complicated by inherent
biases of humans trying to understand themselves (Alexander,
1987;Varella, 2018).
ALEXANDER AND RUNAWAY SOCIAL
SELECTION
An important universal trait of mammals is maternal care
of altricial (helpless) offspring. Many mammalian species,
including most primates, also have varying levels of alloparental
support and protection by relatives. Beyond these shared
features, however, humans exhibit a suite of highly unusual
traits (Alexander, 1990;Chapais, 2009), many of which appear
adaptively responsive to variable conditions. Humans are the
only species characterized by the combination of stable breeding
bonds; flexible and extensive alloparenting and considerable male
parental effort within multi-male groups; lengthy childhoods;
cryptic ovulation; extended bilateral, multi-generational, and
affinal kin recognition; grandparenting; influence of relatives over
mate choice; language; variable group composition and inter-
group relationships; and a suite of other human-elaborated traits.
Alexander’s model of how hominins evolved this combination
of traits is based on the concept that hominin evolution became
an increasingly autonomous and self-reinforcing, runaway
process. A key selective pressure on hominins was thus
interactions with other hominins, particularly with regard to its
selective effects on brain evolution (Alexander, 1990; see also
Flinn et al., 2005). Concomitant with the increased importance of
competition and cooperation among conspecifics was an increase
in “ecological dominance,” whereby predation and competition
from other species became weaker and weaker selective forces on
hominin phenotypes.
We describe a set of revisions and extensions to Alexander’s
model, in the light of recent work on human social and sexual
evolution and behavior. First, we discuss an explanation for the
initial split in the evolutionary trajectories of hominins and Pan,
and how it underpinned the divergence of the two lineages.
Second, we operationalize Alexander’s model of runaway social
evolution by explicitly describing the relevant arenas of social
selection and social competition, what traits were selected for
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and how, and how different arenas and forms of social selection
contributed to runaway effects.
HOMININ ORIGINS
Hominins and Panins last shared a common ancestor about
6.5–9 MYA (Andrews, 2020;Almécija et al., 2021). From
an orthograde last common ancestor, Hominins and Panins
initially diverged into distinct niches, with associated changes
in locomotion leading to upright bipedalism in hominins, and
knuckle-walking in Pan, similar to Gorillas. Arguably, the shift
to bipedalism implies a more terrestrial niche, with different
foraging opportunities and predation pressures (Harcourt-Smith,
2007;Almécija et al., 2021). Early hominins likely developed
a fluid sociality similar to that characteristic of contemporary
hunter/gatherer societies, involving tolerance of, and interaction
with, individuals from other small, local, low-density groups
(in contrast to chimpanzees and gorillas; more similar in some
but not all aspects to bonobos as discussed below), eventually
encompassing flexible alliances and coalitions in their fission-
fusion context (Walker et al., 2011;Apicella et al., 2012;Macfarlan
et al., 2014;Migliano et al., 2017, 2020). Such alliances may have
been beneficial, at least initially, in terms of cooperative foraging,
food-sharing, and protection from predators (Allen-Arave et al.,
2008;Smith et al., 2016). This initial difference, possibly linked
to the gradual shift to a more open and mobile, terrestrial niche,
would have enabled a series of subsequent evolutionary changes:
(1) a flexible, distinctive pattern of extended family relationships
that supported longer periods of child development (Washburn
and Lancaster, 1968;Lovejoy, 1981;Hrdy, 2009, 2014;Hawkes,
2020); (2) critical aspects of the fluid and complex coalitional
sociality posited above (Gavrilets et al., 2008;Chapais, 2009, 2011;
Hawkes et al., 2018); and (3) an environment in which cultural
innovations were increasingly important for foraging, defense
against predators, and success in cooperation and competition
with conspecifics (Hill et al., 2011;Lotem et al., 2017;Flinn, 2021;
Garg et al., 2021).
Early hominins also diverged from Pan in which other
sets of individuals were most important to them. For female
hominins, relationships with mothers, sisters, daughters, aunts,
and grandmothers were of increasing importance. But so
too were fathers, mates, brothers, and sons. From the male
hominin perspective, relationships with paternal relatives —
fathers, brothers, and sons — were of increasing importance
for cooperative defense and foraging. As posited above, female
relatives — wives, mothers, sisters —, and children benefited from
this crucial support from males.
Hence the conundrum, analogous to the “matrilineal puzzle”
proposed by Richards (1950); (see also Irons, 1983;Macfarlan
et al., 2014;Dyble et al., 2015) emerges. How can males and
females be with kin who reside in different places? How to
help both your sister and your wife? And how to effectively
avoid inbreeding problems if male and female relatives — father-
daughter, brother-sister — co-reside? The solution is found in
most hunter-gatherer foraging-band societies: flexible, fluid camp
residence and social networks. Individuals can choose to stay or
visit with whomever is most useful to them at a particular time.
With inter-camp group tolerance and cooperation, hominins
got the best of both worlds; help from maternal and paternal
kin, mates and affines. This pattern of “exploded fission-fusion”
sociality (Marlowe, 2004;Foley and Gamble, 2009;Macfarlan
et al., 2014) stands in stark contrast to that of all other hominids.
A key consequence and benefit of this fluid interactive
social system was an open door for cumulative culture and
language. Socially transmitted information could move easily
and rapidly across the hominin social landscape (Hill et al.,
2011, 2014;Walker et al., 2011;Gowlett et al., 2012). A good
idea (“meme”) would spread fast and far (go “viral”). “Good
ideas” were not limited to tools, engineering, and technology,
but include social tactics and strategies (e.g., Coward and Grove,
2011). And, as described below, such a complex matrix of
self-selected interactions provides excellent opportunities for
social selection to exert its runaway effects, by the various
mechanisms, in the various contexts, that typify human
cognition, behavior and culture.
PSYCHOLOGICAL MECHANISMS OF
SOCIAL SELECTION
Selective arenas represent specific contexts within which
social selection mediates variation in inclusive fitness among
individuals. The primary psychological mechanisms of social
selection, within these contexts, are cognitive and emotional.
Thus, an individual will benefit the most in inclusive fitness under
social selection if they can:
(1) Individually recognize all of the persons in their group;
(2) Discern the relationships of kinship, friendship, and
sexuality among all interacting individuals in their group;
(3) Figure out, consciously or unconsciously, how each
individual person who they interact with could best be
manipulated, cooperated with, or competed with, in what
way, to maximally increase their own lifetime inclusive
fitness;
(4) Discern and infer, consciously or unconsciously, how any
other individual would be expected to respond to these
possible actions toward them, from being able to take their
mental perspective regarding their strategies and abilities to
maximize their own inclusive fitness.
Each other person in a group thus has some potential inclusive
fitness value to a focal individual, that could be maximized by
success in providing benefits, imposing costs, or taking control
of behavior away. Ability to achieve this potential will be some
function of asymmetries in information, physical and intellectual
power, alliances, and leverage (control of a resource or service
that cannot be taken by force; Strassmann and Queller, 2010;
Watts, 2010;Bissonnette et al., 2015). An individual would
also benefit tremendously from knowing their own abilities and
best strategies for increasing inclusive fitness, in this complex
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multidimensional web of social interactions and their mental
representations.
The cognitive challenges of being able to most-effectively
maximize inclusive fitness via the four steps described above
are open-ended and almost unimaginably complex, for any
extended human group of reasonable size, such as 50 to
150. As a result, social selection and responses to the
selection, in the context of evolving human social-cognitive-
emotional abilities, can proceed virtually without limit, being
constrained only by human neural computing power, manifest
in brain size and modular specializations, and being driven
by multiple forms of runaway social selection, as described
in detail below.
The psychological mechanisms of social selection can be
applied in a wide variety of specific contexts, or arenas,
whereby pairs and larger sets of humans interact. These
arenas can, in turn, delineate the different forms of runaway
social selection that lead to accelerated human evolution for
cognitive and cultural traits, and the remarkable suites of
adaptations that result.
ARENAS OF SOCIAL SELECTION
Arenas of human social selection exemplify the different contexts
of human interactions that contribute to runaway human
social evolution under the broad umbrella of Alexander’s
(1989; 1990) model. These arenas have been discussed
before, but not integrated together, and they have not been
considered in the framework of how humans evolved since
their divergence from a shared ancestor with the genus
Pan. The “runaway” component of runaway selection is
especially important because it can help to account, via positive
feedbacks, for the extraordinary rapidity of human cognitive and
social evolution.
Arenas of social selection help to indicate the mechanisms of
social interactions that can lead to the enhanced brains and more-
complex social cognition and emotion that characterize humans.
They represent pairs or larger sets of human interactions,
ordered by sex and age and number and nature of groupings,
that have been postulated to involve runaway effects. The
key question in particular is how runaway social selection is
expected to work in fluid populations of early humans, in
terms of how increased social abilities can translate into higher
inclusive fitness of individuals, and enhanced survival and
proliferation of groups, and in terms of evolutionary dynamics
across generations.
The First Arena: Arms Races
Most generally, runaway social selection effects within
generations can be driven by social competition, social
cooperation or social choice. The first arena of social selection
described here is direct, symmetric and asymmetric arms-race
competition within a group, whereby two or more individuals are
engaged in some fitness-related conflict where the individual with
better social skills (social “weaponry”) wins. Such competition
represents a classic arms race, where the selective pressure is
autocatalytic across generations, because the selective cause is
persistent and self-reinforcing. Arms races are normally thought
of in physical terms, where they become limited, across many
generations, by the costs of armament and tradeoffs with other
components of fitness. This limitation may apply to brain
size, over the long term, due to the high costs of neural tissue.
However, psychological arms races per se should be subject to no
such constraints, since they are governed by “software” — neural
organization — that can, in principle, complexify indefinitely,
and involve cumulative learning and culture. In hominins,
brains concomitantly evolved to become both larger and more
“socialized” (specialized for social cognition), with material
culture lagging behind (Geary, 2005;Gowlett et al., 2012;
Rilling, 2014), as expected under autocatalytic models driven by
social selection.
Possible examples of psychological arms races would be
enhanced abilities to read emotions and intentions in others,
levels in metacognition and theory of mind (“I think that they
think that I think,” etc.), ability to gain status (and recruit
partners in reciprocity, as discussed below) through displays
of cognitive abilities, and skills involved in strategically “out-
thinking” adversaries in conflicts (e.g., Byrne and Whiten, 1988;
Dunbar, 2014). The arsenal of social weapons would also include
a broad range of cognitive and emotional phenotypes whose
expression reciprocally selects on each other within and across
generations, mainly in the general context of motivating other
individuals to behave more, and more often, in the inclusive-
fitness interests of the actor. A number of studies have examined
coevolutionary arms races in the context of social selection and
intelligence (e.g., McNally et al., 2012;dos Santos and West,
2018;Coen, 2019), and have supported the conclusion that such
arms races can drive higher levels of intelligence, cooperation,
and social complexity. Indeed, as noted by Darwin (1871, p. 97)
“natural selection, arising from the competition of tribe with
tribe, would, under favorable conditions, have sufficed to raise
man to his high position.”
Among males, psychological arms races should be most
prominent where males are evenly matched physically,
such that simple muscular dominance cannot determine
competitive outcomes. Among females, it may commonly
involve indirect forms of (non-physical) aggression,
such as manipulations of social status and abilities
in competition for allies (friends) and social support.
Competition for useful allies is also likely to have been very
important to males.
Mental arms races are the psychosocial equivalent of
Darwin’s (1871) sexual selection by male-male competition.
Such simple, one-on-one psychological arms races should
ramify easily into one-on-multiple and multiple-on-multiple
interactions, given the fluidity of human groups and kin-
structured and reciprocity-structured organizations. Indeed, the
high fluidity and organizational complexity of human groups
are likely, in part, end products of such arms races as well.
The “multiples” of these interactions are presumably allies of
some sort, who can join psychological and physical forces to
better increase their inclusive fitness at the expense of others.
Once a competitive dyad expands, however, the dynamics
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necessarily change. Indeed, a pair of competing individuals
may themselves be allies in some domains and adversaries
in others. How they became so is a matter for the next
selective arena.
The Second Arena: Partner Choice
The second arena of social selection is partner choice in the
context of cooperative traits, which can lead to runaway choice-
trait coevolution under a variety of conceptual and mathematical
models (e.g., Nesse, 2007, 2010;Debove et al., 2015). Here,
“partner” refers to social partners, with whom one preferentially
interacts, over a relatively long period, due to various benefits
that accrue especially via mutualism and reciprocity, sometimes
combined with kinship. In the same way that classical sexually
selected mate choice is driven by attractive displays by one
individual to others, social partner choice is driven by attractive
social displays, usually involving demonstrations of prosocial
traits such as honesty, reliability, cooperative cultural and
religious beliefs, and generosity through social, informational
and material assistance in times of relative need or potential
for benefit (Nesse, 2007, 2010). There are some interesting
parallels here between social selection and classical Fisherian
runaway selection (typically involving choice of a few “top-
ranked” individuals) and “complementary” mate choice (such
as for immune system gene compatibility). Thus, for example,
social selection may entail tradeoffs between choosing the “best”
partner (i.e., individuals that are highly socially intelligent or
skilled in some area), versus choosing individuals who will
remain committed to, and focus on, the relationship, even when
mutualistic reciprocal relationships demand consistent effort and
attention, limiting the ability to engage effectively in many of
them. The latter type of relationship should also be promoted by
complementarity of different social (and other) abilities between
members of a dyad, which increases the benefits accruing to both.
Social partners are, of course, conventionally regarded as
“friends,” who in evolutionary terms represent allies who both
gain inclusive fitness benefits, over the longer term, from their
continued association. The partnerships can be of any dyadic
combination of the two sexes, or can involve larger groups
united through multiple partner choice events, merging into
and overlapping with other such groups in complex social
networks. Choice of partners in various contexts can also generate
“markets” for partners, with complex dynamics that can enhance
the competitive nature of the processes involved (Barclay, 2016;
Eisenbruch and Roney, 2017). Smith and Apicella (2020) describe
how partner choice, for traits that include generosity and foraging
ability, mediates campmate preferences among Hazda hunter-
gatherers.
The long duration of the human lifespan makes social
partnerships, in principle, highly beneficial to inclusive fitness,
especially if they involve complementary abilities, knowledge
or resources (Nesse, 2007, 2010). In humans, choices regarding
memberships in coalitional groups, based on the traits of the
group and its leader, should also be notably important (Boyd
and Richerson, 2009), and could themselves synergize with group
against group arms races, as discussed in more detail below.
Partner choice, like arms races, can result in the runaway
evolution of socially selected traits. By this process, the expression
of the chosen trait, and the choice of the trait, come to
be positively genetically associated with one another across
generations (Sachs et al., 2004), as they both increase rapidly
in frequency. Cultural analogs of this process can also lead
to culturally inherited patterns of association that do not
require genetic change, although such changes can themselves
impose selection for genetic change and gene-culture coevolution
(Richerson and Boyd, 2005;Lotem et al., 2017).
Nesse (2007, 2010) described how runaway partner choice
may have promoted a whole suite of uniquely human or
elaborated-in-humans traits, including theory of mind, extreme
forms of cooperation, capacities for morality, the importance of
building and protecting one’s reputation, and self-domestication
of the recent human species as a whole. Most generally, partner
choice, and choice of leaders and groups, should select for finer
and finer abilities to discriminate the socially salient qualities
of other individuals and groups, in terms of if and how much
interacting reciprocally with them, compared to alternatives, will
result in gains to inclusive fitness. In humans (and possibly
dolphins) the complexity of cooperation (and the intelligence
required to negotiate alliances) increased dramatically in the
context of triadic interactions among groups in nested hierarchies
(Connor, 2007;Gerber et al., 2021). The risk/reward ratio and
the number of options (and potential outcomes) were probably
critical with regard to selection for enhanced intelligence, and
these increased dramatically with expansion in the number of
levels in a nested hierarchy of interacting entities (individuals,
groups, groups within groups, etc.) and associated potential
alliances. As such, this arena of social selection should result
in notably enhanced abilities to judge character, truthfulness,
morality and social abilities, as well as the ability to display and
communicate these sorts of traits, even if they sometimes conflict
with one’s ability to maximize inclusive fitness by alternative,
relatively selfish, and self-serving means.
The Third Arena: Mate Choice
The third arena of social selection, human mate choice, was, of
course, originally formalized by Darwin (1871). It represents a
subset of partner choice that is sufficiently special and distinct
to warrant its own domain. Classical Fisherian mate choice by
sexual selection (Fisher, 1930;Lande, 1981;Kirkpatrick, 1982)
involves a process whereby choice by one sex (in animals, usually
females) for one or more fitness-related traits in the other sex
(usually males) results across generations in a positive genetic
correlation between stronger choice for the trait (typically in
females) and higher level of trait expression (typically in males) —
a runaway process that stops only when the trait is so highly
developed that it incurs strong costs in terms of some other
component of fitness, typically survival. This dynamic appears
responsible, at least in part, for the rapid evolution and high
diversity of sexually selected, mate-choice-related traits among
many non-human animal groups (Arnold, 1983).
Human mating systems have diverged substantially from the
Fisherian paradigm, in that (a) females, as well as males, exhibit
forms of sexually selected “beauty,” that may be chosen by the
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opposite sex; (b) mate choice is commonly more or less joint and
reciprocal, with both sexes engaging in choice of a partner based
on some criteria (though often with social constraints on choice);
and (c) mate choice engenders relatively long-term pair-bonding,
with mutual contributions to the rearing of offspring (e.g., Miller,
2000;Buss and Schmitt, 2019;Geary, 2021).
For human mate choice, the main considerations in the
second arena apply, specifically, to the situation where males
and females each choose one individual of the other sex by
some criteria. Individuals are thus under selection to display
socially selected traits (e.g., intelligence, cleverness, humor,
conversational ability, kindness, a variety of social skills), and to
choose some overlapping constellation of such traits in others
(Etcoff, 1999;Miller, 2000). Choice of a good opposite-sex partner
for mating, reproducing, providing, and parenting is probably
a much more challenging task than making and maintaining a
same-sex friend, and thereby should represent a stronger socially
selective filter. Pair-bonded males and females are thus selected
to be able to successfully navigate the psyche of their mate, in a
much more intimate, cognitively complex, and fitness-salient way
than for friends. Indeed, among animals, comparative analyses
by Dunbar and Shultz (2007) have demonstrated that among
carnivores, artiodactyls and bats, larger relative brain size is
associated with pair-bonding, and that in primates it is linked
with complex, enduring social relationships even more broadly
(as well as with larger group size); they argue that these findings
reflect “the cognitive demands of the behavioral coordination
and synchrony that is necessary to maintain stable pair-bonded
relationships.” Humans appear to represent an extreme of social
selection and bonding effects on relative brain size and behavioral
coordination, especially given the partially divergent optimal
mating and parenting strategies of the two sexes, and the complex
mixtures of confluence and conflicts of inclusive-fitness interest
that ensue.
From an evolutionary perspective, mutual mate choice in
humans becomes subject to runaway dynamics due to genetic
correlations of socially selected pair-bond related traits with
choice of these traits; for example, females choose kind, caring
males, males with genes for these traits are selected for, and the
genes for the choice and the traits become associated and rise
in frequencies across generations. For this type of sexual-social
selection, it remains unclear if there are evolutionary brakes on
the process equivalent to those operative during natural selection
by predation against, say, too-large of a train in peacocks.
Possibilities for such “brakes” might be over-expression of choice
(such that no or few individuals are deemed suitable for a mate),
or expression of prosocial, altruistic, or parenting-related traits
to such a degree that they became maladaptive in the context of
maximizing inclusive fitness as a whole.
The Fourth Arena: Caregiver-Offspring
Choice and Signaling
Runaway coevolution between social signals and their choice
includes not just cooperation partners, and female-male pairs,
but also offspring interacting with their caregivers, specifically
mothers, alloparents, and fathers (West-Eberhard, 2003,
p. 467; Hrdy, 2013). By this mechanism, offspring benefit
from producing signals, such as high levels of subcutaneous
fat, vigorous crying, smiling, eye contact, and other social
interactions with caregivers (“other-regarding” in Hrdy’s term),
that represent indicators of their phenotypic and genetic
“quality” (inclusive fitness value) and that prompt increases
in feeding and engagements that enhance social-emotional
cognition and learning. Such signals are expected to be
predominantly honest indicators of offspring value, but may
include manipulative elements (West-Eberhard, 2003), as in
other models of signal-receiver interaction, that could reinforce
increased discriminability of cues by receivers. Social selection
and evolution should thus increase maternal, alloparental,
and paternal sensitivities to offspring cues (to better reward
higher-value offspring, invest less in lower-value ones, and
tell honest from dishonest signals), and increase offspring
aptitudes and success at solicitation. As for the other forms
of signal-choice system, the result is genetic and/or cultural
correlations and coevolution by a self-reinforcing runaway
process (West-Eberhard, 2003).
Runaway social selection between caregivers and offspring
represents an integral component of the human life history
evolving toward increased alloparental and paternal care, shorter
interbirth intervals and higher reproductive rates, larger-brained
offspring (which are more expensive to produce and rear),
and neural precocity and plasticity combined with physical
altriciality (Alexander, 1990;Hrdy, 2009;Piantadosi and Kidd,
2016;Sherwood and Gómez-Robles, 2017). Neural precocity, in
turn, forms part and parcel of social precocity and the evolution
of greatly enhanced human social and emotional cognition, the
acquisition of which is inherently developmental and centers
around the elongated human childhood and adolescence (Bogin,
1990;Flinn et al., 2011;Ponzi et al., 2020). This selective arena
is especially important given that infant mortality has long
represented such a substantial component of variation in fitness
among humans, and that such mortality can be reduced in
a variety of socially salient ways, including effective offspring
solicitation, alloparental contributions to maternal nutrition and
infant care, paternal augmentation of food supplies, and broader
social support in the group for mothers who warrant or earn
it. For example, in many human groups, there is evidence of
strong positive associations of lower child mortality with higher
cognitive abilities of the mothers (e.g., Sandiford et al., 1997;
Piantadosi and Kidd, 2016).
The Fifth Arena: Cultural Traits and
Social-Cultural Learning
A final arena of within-group social selection is culture, the
human-created material and information-based aspects of the
environment that underpin tools, customs, religion, arts, and
beliefs (Flinn and Alexander, 1982). All human phenotypes
derive from interactions of genes with environments —
especially cultural and social ones — but culture is special
because it can be transmitted both vertically (like genes, or
language as noted by Darwin, 1871) and horizontally (as
memes), with horizontal transmission potentially proceeding
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at a very rapid pace. As such, human traits can evolve due
to “gene-culture coevolution” (interactions of genetically based
human phenotypes with cultural aspects of environments;
Laland and Seed, 2021), commonly due to differential human
adoption and perpetuation of different cultural phenotypes and
culture acting as a causal agent for selection (Whiten et al., 2017;
Richerson et al., 2021).
Cultural change can proceed under a runaway process,
whereby increases in cultural complexity and sophistication
(the “traits”) generate environments that select for enhanced
social-cultural learning and more-effective adoption of cultural
behaviors (the “choices”), especially by young individuals, leading
to runaway coevolution (e.g., Alexander, 1979; Flinn and
Alexander, 2007;Boyd and Richerson, 2009;Rendell et al., 2011;
Legare and Nielsen, 2015;Legare, 2017;Lotem et al., 2017;
Muthukrishna et al., 2018;Markov and Markov, 2020). This
process, coupled with “arms race” elements of cultural change,
may have been especially effective in driving the recent and
accelerating human cultural change that has so complexified
human social environments. As such, and given the cumulative
nature of human cultural change, this arena of social selection
should be exerting stronger and stronger effects on human
evolution as time proceeds, relative to the other four (Birch and
Heyes, 2021; cf. Wadley, 2021).
The Final Arena: Between-Group
Competition
The five arenas of runaway social selection described above all
operate within human groups, where groups may be delineated
by various terms including families, bands, villages, tribes, or
ethnic, linguistic, or cultural groups, of any sizes, that each
has some conception of “us” in relation to “them.” Whereas
social selection and competition occur extensively within such
groups, their evolution should be constrained by demographic,
ecological and anti-cooperative effects that weaken the group in
the context of their competitive interactions with other groups
(Lahti and Weinstein, 2005). As such, a final, higher-level arena of
social selection operates between groups, as described by Darwin
(1871) in terms of competition between human “tribes,” and
by Alexander specifically in terms of runaway imbalances of
power. In particular, Alexander posits, following from Darwin’s
(1871) views that conflict between tribes selects for within-tribe
cooperation and morality, that human social evolution has been
driven, in large part, by group against group competition that
selects for enhanced within-group cooperation as a means to
counter external threats (Alexander, 1979, 1987, 1990, 2006).
Alexander’s balance/imbalance of power model represents a form
of arms race, with both cognition and culture as weaponry,
that selects for larger and larger, more and more cooperative
groups, with better and better ways to compete, though larger
group size may also exacerbate within-group variation and
conflicts and dilute the benefits of winning. Such between-group
conflicts would have originated in hominins on small scales
(indeed, presumably reminiscent of the “warfare” of common
chimpanzees; Mitani and Watts, 2001), as represented now
in some extant human societies (e.g., Berndt, 1964;Chagnon,
1977;Macfarlan et al., 2014), but escalating as populations
increase in size.
Perhaps the most telling evidence in support of Alexander’s
model is the observation that human history is, in considerable
part, the history of human warfare based on groups defined by
culture, language, and ultimately, genes (Bowles, 2009;Turchin
et al., 2013;Bauer et al., 2016;Turchin, 2016). Warfare may,
however, represent only the most extreme, obvious and effective
form of between-group human competition, since humans
compete, and cooperate to compete, in fluid, dynamic groups
at all levels from families to nations, and based on biological
kinship, ethnic markers of diffuse long-term ancestry, and
cultural differences represented by kinship that can be mainly
or purely psychological (Jones, 2003). Groups may also form
on the basis of complementary skill sets or interests. In this
context, the fluidity of human groupings, with shifting of alliances
across time and space as a universality rather than exception,
may connect the early evolution of hominids, lost in prehistory,
with the recent evolution of modern, historic humans — and all
points in between.
The six arenas of social selection described here each
generates, given any degree of heritability, social evolution of
sets of psychological traits and abilities that have collectively
“made us human” (Table 1). What is striking about these
sets of phenotypes is that they encompass a tremendous range
of human-elaborated psychological and social traits, many
of which are expected to be reinforcing across arenas (e.g.,
honesty and morality in partner choice and among-group
arms races), transferrable across domains (e.g., finer-scale social
discrimination in caregiver choice, partner choice, mate choice),
or complementary (e.g., abilities to compromise with, lead,
persuade, or control other individuals). The effects of these
interacting arenas of social selection echo the emphasis of
Laland and Seed (2021) on “dynamical feedbacks between
mutually reinforcing aspects of cognition,” with human cognitive
uniqueness arising from “trait interactions and feedbacks,” with
the salient traits evolving squarely in the context of complex
sociocultural landscapes (see also Dean et al., 2013;Whiten,
2018;Lombard and Högberg, 2021;Spikins et al., 2021). As
such, runaway social selection and evolution appear to exhibit
the breadth, power and scope to help explain, in principle,
how modern humans evolved psychologically from chimp-
human ancestors.
DISCUSSION
Uncovering the selective pressures that gave rise to the first
hominins, and to modern humans, has been a perpetual challenge
ever since Darwin drafted the first clear hypotheses of human
origins in 1871. From a broad perspective, it makes sense
that the most exceptional human features, large social brains,
complex cooperative and competitive interactions, and elaborate
culture, should themselves reflect the selective pressures that
guided their evolution. The logic of runaway social selection
suggests that humans generated and became their own primary
selective pressures, through diverse forms of arms races within
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Crespi et al. Social Selection in Human Evolution
TABLE 1 | Phenotypes and abilities postulated to be selected for, in six different arenas of human social selection and evolution.
Arms races within
groups
Partner choice Mate choice Caregiver-offspring
relationship
Cultural traits –
social learning
Arms races among
groups
Joint attention;
Scenario-building;
Theory of mind;
Leadership;
Manipulation of status;
Social creativity;
Persuasion; Coercion
Loyalty; Honesty;
Reputation; Social
status; Sustained
reciprocity; Fine social
discrimination abilities
Fine discrimination of
cognitive, emotional,
physical traits and
abilities; Ability to
navigate psyche and
behavior of mate
Enhanced physical,
reproductive and
survival abilities
Infant, child eye
contact, smiling,
laughing, early
language development,
secure attachment;
Social development,
maturation in
childhood, adolescence
Fine discrimination of
offspring traits
Enhanced
sophistication and
efficiency of social
learning about cultural
norms, including
aspects of language,
religion, customs, arts,
rules and laws,
institutions, technology,
and resource
acquisition methods
Parochial altruism;
Ingroup morality;
Heroism; Xenophobia;
Complex within-group
cooperation;
Dehumanization of
other groups
FIGURE 1 | The six arenas (five within groups, one between groups), of human runaway social selection. See Table 1 for further details on the specific phenotypes
selected for in each arena.
and between groups, and through choice-trait coevolutionary-
dynamic interactions involving allies, mates, and offspring with
caregivers. By the hypotheses presented here, each of these arenas
of social selection drove the evolution of different, interacting
dimensions of human sociality and culture, that merged to
create the humans inhabiting our world today (Figure 1). This
hypothesis is by no means incompatible with those based on
other selective pressures postulated to be important in human
evolution, such as alloparental care (Hrdy, 2009;van Schaik and
Burkart, 2010) and self-domestication (Wrangham, 2019), but it
stresses the importance of runaway social selection as a potential
key factor in how and why modern humans evolved.
Tracing the selective history of humans relies on one part
evolutionary logic, one part ecology, one part psychology and
neuroscience, one part anthropology, and all parts grounded
in phylogeny and evidence from archeological remains. An
enduring part of the puzzle, the initial divergence of the
eventual Homo and Pan lineages, is addressed here with the
hypothesis that the divergent evolution of hominins was “kick-
started,” in an ape with a small (i.e., 400 cc) brain living at
relatively low densities, by ecological conditions that favored
increased fluidity, connectivity, tolerance, and especially local
cooperation within and between small social groups. At first,
such cooperation need not be sophisticated or complex, and
need not involve larger brains. But when conditions eventually
arose that allowed the evolution of larger and more complex
brains (e.g., cooking of foods, and use of more energy-
dense foods), and the first manifestations of culture, such
early humans would have been poised to enter a socio-
ecological niche characterized by increased population densities,
larger brain sizes, enhanced competition and cooperation,
cumulative culture, and strategic social choices, that collectively
encompassed the multiple mechanisms of runaway social
selection described above.
Increases in understanding of Darwin’s (1871) “insensible
grading” from an apelike form to humans requires clear and
specific hypotheses that make testable predictions. Indeed, a
primary criticism of the runaway social selection model is
that it lacks concrete empirical support, in terms of the
specific processes involved. The mechanisms that underpin the
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Crespi et al. Social Selection in Human Evolution
hypotheses described here, especially those relating to choice-
trait coevolution among allies, mates, and offspring-caregiver
interactions, can, however, be evaluated in extant human groups,
and ecological benefits from relatively simple primate alliances
can be evaluated in field populations using the most relevant taxa.
In particular, empirical evaluation of the hypotheses described
here regarding social selection in human evolution will require
testing for evidence of the operation of each proposed process
and link in the causal positive-feedback cycles, especially in small-
scale human societies. The hypotheses would thus be falsified
by robust findings that the within-generation processes that
underpin runaway social selection, in any given arena, do not
occur in human societies or, if they occur, do not impact upon
variation in inclusive fitness. Neurology and neuroendocrinology
may also provide salient evidence of mechanisms for social
competition (Dunbar and Shultz, 2007;Rilling, 2014;Shultz
and Dunbar, 2014), and the evolutionary transitions of brain
evolution (Sherwood et al., 2008;Sherwood and Gómez-Robles,
2017;Stout and Hecht, 2017;Bruner, 2021). Although the
challenges inherent in all analyses of the broad scope of human
evolution are daunting, the intellectual rewards remain profound,
in better comprehension of just what we as a species are, and how
we came to be.
AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct, and intellectual
contribution to the work, and approved it for publication.
FUNDING
This work received funding from Baylor University Department
of Anthropology and the Natural Science and Engineering
Research Council of Canada, Discovery Grant 2019–04208.
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