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A link between altruism and sexual selection:
Genetic influence on altruistic behaviour and
mate preference towards it
Tim Phillips
1
*, Eamonn Ferguson
2
and Fruhling Rijsdijk
3
1
School of Biology, University of Nottingham, UK
2
School of Psychology, University of Nottingham, UK
3
Social, Genetic and Developmental Psychiatry Research Centre, Institute of
Psychiatry, London, UK
Altruistic behaviour raises major questions for psychology and biology. One hypothesis
proposes that human altruistic behaviour evolved as a result of sexual selection.
Mechanisms that seek to explain how sexual selection works suggest genetic influence
acting on both the mate preference for the trait and the preferred trait itself. We used a
twin study to estimate whether genetic effects influenced responses to psychometric
scales measuring mate preference towards altruistic traits (MPAT) and the preferred
trait (i.e., ‘altruistic personality’). As predicted, we found significant genetic effects
influencing variation in both. We also predicted that individuals expressing stronger
MPATand ‘altruistic personality’ would have mated at a greater frequency in ancestral
populations. We found evidence for this in that 67% of the covariance in the phenotypic
correlation between the two scales was associated with significant genetic effects. Both
sets of findings are thus consistent with the hypothesized link between sexual selection
and human altruism towards non-kin. We discuss how this study contributes to our
understanding of altruistic behaviour and how further work might extend this
understanding.
Altruistic or selfless behaviour has been defined as any act that increases the survival
chances and reproductive success of another individual at the expense of the altruist
(Ridley & Dawkins, 1981). Yet evolutionary theory predicts competition between
organisms in the ‘struggle for existence’ (Darwin, 1859) and so altruistic behaviour
appears at first sight to be at odds with natural selection. Much effort has gone into
attempting to resolve this evolutionary puzzle. Altruism has been seen as based on the
fitness of genes shared by close kin (Hamilton, 1963), as linked to reciprocal altruism
(Trivers, 1971) and promoted by the reputation of altruists through indirect reciprocity
* Correspondence should be addressed to Tim Phillips, 3 Banton Close, New Oscott, Birmingham B23 5YT, UK (The work was
conducted while the first author was based at the Behaviour and Ecology Research Group, School of Biology, University of
Nottingham, Nottingham, UK) (e-mail: Ptjp2749@aol.com).
The
British
Psychological
Society
1
British Journal of Psychology (2010), in press
q2010 The British Psychological Society
www.bpsjournals.co.uk
DOI:10.1348/000712610X493494
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(Leimar & Hammerstein, 2001). Other approaches involve gene/culture co-evolution
(Lumsden & Wilson, 1981), ‘new’ group selection (Wilson & Sober, 1994), and
‘benevolence’ (Ferguson, Farrell, & Lawrence, 2008) (see Bshary & Bergmu
¨ller, 2008;
Lehmann & Keller, 2006, for extensive theoretical reviews).
This paper focuses on a relatively unexplored approach to this puzzle – one that links
sexual selection (Darwin, 1859, 1871) with human altruism towards non-kin (Iredale,
Van Vugt, & Dunbar, 2008; Kelly & Dunbar, 2001; Miller, 2000; Phillips, Barnard,
Ferguson, & Reader, 2008; Roberts, 1998; Zahavi, 1977). Darwin (1871) proposed two
forms of sexual selection. The first, since termed intrasexual selection, involves
competition (typically between males) to drive off or kill rivals, with the females
remaining passive throughout. The second, since called intersexual selection, involves
competition to attract mates with, usually, females playing an active role in mate choice.
It is the second form of sexual selection that is considered here.
It has been proposed that altruism towards non-kin evolved as a ‘handicap’ that, if
successfully overcome, gives a reliable indicator of phenotypic and genetic quality to
others, including potential mates (Zahavi, 1995; Zahavi & Zahavi, 1997). However, the
‘handicap principle’ (Zahavi & Zahavi, 1997) is one of a number of mechanisms that
seek to explain how sexual selection operates (Andersson, 1994; Andersson &
Simmons, 2006), including, for example, the ‘runaway’ mechanism (Fisher, 1958), and
any one of these or any combination might also have favoured the evolution of altruistic
traits (Phillips et al., 2008). A key feature of these mechanisms is that they rely on a mate
preference expressed in one sex towards a preferred trait expressed in the other. The
mate preference is seen as being subject to genetic influence (Andersson & Simmons,
2006) while much modelling of sexual selection assumes genetic influence acting on the
preferred trait (e.g., Lande, 1981).
Twin studies offer a well-established methodology for measuring whether there is
latent genetic influence acting on variation in a trait (e.g., Neale & Cardon, 1992). The
study reported here uses responses obtained from twins to two psychometric scales
designed to measure respectively mate preference towards altruistic traits (MPAT) and
‘altruistic personality’ (i.e., the preferred trait). Intersexual selection is commonly seen
as being based on genes associated with both the mate preference and the preferred
trait of interest (e.g., Lande, 1981). If genetic effects were found to influence variation in
responses to both psychometric scales this would be consistent with sexual selection
having acted on the evolution of altruism towards non-kin in humans.
To date, research has focused on genetic influence on altruistic traits alone but not
on mate preference towards altruism. Within this literature, there is evidence of genetic
influence acting on altruistic traits (Rushton & Bons, 2005; Rushton, Fulker, Neale, Nias,
& Eysenck, 1986) and related constructs such as ‘social responsibility’ (Rushton, 2004)
and ‘prosocial behaviour’ (Gregory, Light-Haeusermann, Rijsdijk, & Eley, 2008; Hur &
Rushton, 2007), all assessed psychometrically. There is also evidence of genetic
influence on reciprocating behaviour measured by trust and ultimatum games (Cesarini,
Dawes, Fowler, Johannesson, & Lichtenstein, 2008; Wallace, Cesarini, Lichtenstein, &
Johannesson, 2007). While other studies have failed to find any evidence of genetic
influence on self-assessed altruism (Bouchard & Loehlin, 2001; Krueger, Hicks, &
McGue, 2001), the balance of evidence suggests a genetic effect. In this study, we
therefore assessed the following hypothesis:
(1) Psychometrically assessed altruism and preference for altruistic traits will both
be subject to genetic influence.
2Tim Phillips et al.
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A further prediction can be made, based on the sexual selection hypothesis, as to how
human altruism towards non-kin evolved. In ancestral populations, individuals
expressing a stronger MPAT can be expected to have mated more frequently with
those expressing more prominent altruistic traits. Their offspring would thus have
inherited genes coding for a stronger mate preference and preferred trait at a greater
frequency than average and evidence for this should be reflected in modern
populations. Thus, a link between intersexual selection and human altruism can be
tested not only through evidence of genetic influence acting on mate preference and
preferred trait but also by finding a genetic correlation between the two (as measured
here by responses to the two psychometric scales). In this study, we therefore also
assessed the following hypothesis:
(2) That a genetic correlation will be found between MPAT and altruistic traits.
Aims of the study
This study is the first to test the sexual selection hypothesis for the evolution of human
altruism by measuring genetic effects related to both mate preference for altruistic traits
and the expression of altruism as a trait. We test two hypotheses central to the sexual
selection hypothesis. Firstly, we test whether measures of both mate preference for
altruism and the expression of altruistic behaviour are subject to significant genetic
effects. Secondly, we test whether a significant genetic correlation between mate
preference for altruistic traits and the expression of altruistic behaviour can be found.
This latter process would also be consistent with sexual selection having influenced the
evolution of altruism towards non-kin.
Methods
Overview
We employed a classic twin study design to estimate latent genetic and environmental
effects on variation in responses to psychometric scales that measure MPAT and
‘altruistic personality’. We also estimated the genetic and environmental overlap
between the two variables.
Participants
The sample comprised 80 identical or monozygotic (MZ) twin pairs (70 females) and
97 non-identical or dizygotic (DZ) twin pairs (87 females) registered with the
Department of Twin Research and Genetic Epidemiology at King’s College, London
(www.twinsuk.ac.uk). Due to the small number of male twin pairs, we present the
results of the female-only subsample (mean age ¼55:2 years; SD ¼13:21). However, a
model where both age and sex were partialled out produced equivalent results.
Instruments
Participants completed the following self-report measures:
.MPAT Scale (Phillips et al., 2008). Participants rated the desirability of nine items of
altruistic behaviour in a potential mate (e.g., ‘Once dived into a river to save
Altruism and sexual selection 3
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someone from drowning’) on a five-point scale ranging from undesirable (0) to very
desirable (4). These items were included at random with other items describing
behaviour in a potential mate unrelated to altruistic behaviour. The MPAT Scale was
reliable for the total sample (a¼:84) and the MZ (a¼:84) and DZ (a¼:82)
subsamples.
.Self-Report Altruism (SRA) Scale (Rushton, Chrisjohn, & Fekken, 1981). This
assesses ‘altruistic personality’ by asking participants to rate on a five-point scale
(1 ¼Never to 5 ¼Very often) the frequency with which they had performed 20
altruistic acts (e.g., ‘I have done voluntary work for a charity’). The validity of the
scale has been confirmed by significant positive correlations being found with peer
ratings and various measures of comparable traits (Rushton et al., 1981) and the
scale is now widely used as an index of altruism (e.g., Bressan, Colarerlli, & Cavalieri,
2009; Steele et al., 2008). In this study, the SRA Scale was reliable for the total sample
(a¼:85) and the MZ (a¼:84) and DZ (a¼:85) subsamples.
Procedure
Questionnaire data were obtained during six ‘twin days’ held between August 2005 and
April 2006 as part of a range of other scientific studies.
Data analysis
Standard biometrical genetic model-fitting methods (Neale, 1997) allow total variance in
each variable to be decomposed into additive genetic (A) effects, dominance genetic (D)
effects (reflecting interactions between alleles), environmental (E) effects unique to an
individual twin, and common (C) environmental effects shared between twins due to
their family environment. As MZ twins are genetically identical, correlations between
them for additive and dominance genetic effects will both be 1.0. As DZ twins have a
50% chance of sharing any given gene correlations for additive and dominance genetic
effects will be .5 and .25, respectively. The unique environment of each twin cannot be
expected to result in any correlation between twin pairs of either type while, in
contrast, both MZ and DZ twin pairs experience a common family environment, which
therefore entails a correlation of 1.0.
Where a sample consists of only MZ and DZ twin pairs reared together dominance
genetic (D) and common (C) environmental effects become confounded and so cannot
be used together in one model. Fortunately, it is possible to infer whether D or C are
more important in a data set on the basis of a simple criterion. Where DZ twin pair
correlations are less than half those of the MZ twin pair correlations then this tends to be
the result of dominance (D) genetic effects. This was found to be the case with both
MPAT (rMZ ¼:60; rDZ ¼:10) and SRA (rMZ ¼:48; rDZ ¼:21) scale responses and so we
fitted an ADE rather than an ACE model to the data.
Mx (Neale, 1997) was used to conduct structural equation modelling and estimation
of twin correlations. Mx works by fitting various models to the observed data and using
statistical methods to infer which model fits most accurately. To investigate the
influence of additive (A) and dominance (D) genetic effects, the goodness of fit between
the full ADE model and the observed data was compared with that of submodels in
which these two parameters were dropped (i.e., producing AE, DE, and E submodels).
As unique environment (E) also includes an estimate of measurement error, which has to
be retained, this parameter is never dropped. Goodness of fit was measured by
4Tim Phillips et al.
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likelihood ratio chi-square tests (see Table 1: 22LL) with best-fitting models being
chosen on the basis of parsimony (i.e., denoted by non-significant changes in the
chi-square). The Akaike Information Criterion (AIC) was used as a parsimony fit index
with the submodel having the lowest value of the AIC index chosen as the best fitting.
As we sought to measure genetic correlations between MPAT and SRA Scale responses,
we employed a bivariate model which allows the covariance between variables to be
decomposed into additive genetic (A) effects, dominance genetic (D) effects, and
environmental (E) effects (see Figure 1). The Note to Figure 1 explains the paths between
the parameters and how the phenotypic correlation (r
ph
) between MPAT and SRA Scale
responses is calculated. On this basis, the best fitting bivariate model will be presented.
MPAT SRA
E1
A1D2
A2
E2
r
a
r
e
D1
d
2
d
2
a
2
e
2
e
2
a
2
r
d
Figure 1. The correlated-factors solution of the bivariate ADE model. Note. The correlations between
additive (A
1
,A
2
), dominance (D
1
,D
2
), and environmental (E
1
,E
2
) factors in relation to MPAT and SRA
Scale responses are denoted by r
a
,r
d
, and r
e
, respectively. Using additive genetic effects for illustrative
purposes, the paths from A
1
to MPAT and from A
2
to SRA are the square roots of their standardized
additive genetic effects. The phenotypic correlation (r
ph
) due to additive genetic effects is calculated
by (pa2
MPAT £ra£pa2
SRA), to D effects by (pd2
MPAT £rd£pd2
SRA), and to environmental effects
by (pe2
MPAT £re£pe2
SRA).
Table 1. Model-fitting results of the bivariate ADE model for the MPAT and SRA scale responses
Goodness of fit
Model 22LL df x
2
df p-value AIC Dx
2
Ddf p-value
Saturated 4,144.29 579
ADE 4,157.85 596 13.56 17 .70 220.44
AE
a
4,163.12 599 18.83 20 .53 221.17 5.27 3 .15
E 4,209.64 602 65.35 23 ,.001 19.35 51.79 6 ,.001
Note. Dx
2
(Ddf ) are the chi-square difference tests between the submodels (AE, E) and the full ADE
model.
a
AE is the best-fitting model. Results are based on the female-only subsample.
Altruism and sexual selection 5
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Results
The MZ and DZ twin correlations for the MPAT and SRA Scale responses are reported in
Table 2, as are the cross-twin, cross-trait correlations (i.e., correlating twin 1 MPAT with
twin 2 SRA Scale responses and vice versa). The fact that all three statistics related to MZ
twins were significant indicated that genetic effects might be influencing these data
(see Table 2).
The genetic bivariate ADE model fitted the data well (x2
ðdf ;17Þ¼13:56, p¼:70; see
Table 1), indicating that the assumptions of the genetic model (i.e., equal means and
variances for the MPAT and SRA Scale responses across twins) were consistent with
the data. The AE submodel showed the best fit to the data, since dropping D resulted in
a non-significant worsening of fit (Dx2
ðDdf ;3Þ¼5:27, p¼:15) and the lowest AIC value
(Table 1). The overall influence of genetic effects is shown in the E submodel by the
significant drop in fit (Dx2
ðDdf ;6Þ¼51:79, p,:001) when A and D are dropped.
The standardized parameter estimates of the full ADE and the best-fitting AE
submodel are reported in Table 3. In the AE submodel, variation in both MPAT and SRA
Scale responses showed significant additive genetic effects of 57% and 48%, respectively
(see Table 3).
Focusing on the best-fitting bivariate AE submodel (see Table 4), a significant
phenotypic correlation was found between MPAT and SRA Scale responses (rph ¼:30:
95% CIs .18, .41) while the additive genetic correlation between MPAT and SRA Scale
responses was also significant (ra¼:38: 95% CIs .08, .62). Using the calculation detailed
Table 2. Maximum likelihood estimates (with 95% CIs) of MZ and DZ twin correlations within and
between MPAT and SRA Scale responses
Twin correlations
MPAT SRA
Cross-twin, cross-trait
correlations MPAT–SRA
MZ DZ MZ DZ MZ DZ
.60 (.42, .73) .10 (2.11, .31) .48 (.28, .64) .21 (.00, .40) .25 (.09, .38) 2.08 (2.24, .08)
Note. Significant correlations are given in bold. Results are based on the female-only subsample.
Table 3. Standardized variance components (with 95% CIs) of the full ADE and best-fitting AE model
MPAT SRA
Model a
2
d
2
e
2
a
2
d
2
e
2
ADE .09 (.00, .53) .52 (.07, .71) .39 (.27, .55) .24 (.00, .55) .26 (.00, .58) .50 (.36, .68)
AE .57 (.38, .70) – .43 (.30, .62) .48 (.29, .62) – .52 (.38, .71)
Note. A, additive genetic effects; D, dominance genetic effects; E, non-shared environmental effects;
a
2
,d
2
,e
2
are the standardized variance components for A, D, and E, respectively. Significant values in
bold. Confidence intervals including zero indicate non-significance.
6Tim Phillips et al.
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in the Note to Figure 1 (i.e., pa2
MPAT £ra£pa2
SRA) we found, by using the heritabilities
for the MPAT and SRA Scale responses (see Table 3), that additive genetic effects
explained 67% of the significant phenotypic correlation (i.e., (p.57 £.38
£p.48) ¼.20 which is 67% of rph ¼:30).
Discussion
As predicted, genetic effects exerted a significant influence on variation in responses to
psychometric scales measuring MPAT and ’altruistic personality‘, with significant
additive genetic effects accounting for 57% and 48% of variation respectively.
Importantly, genetic effects were found to exert a significant influence on the
phenotypic correlation between MPAT and SRA Scale responses, with additive genetic
effects accounting for 67% of this covariation. This result is consistent with preferential
mating having occurred between mate preference and preferred trait in ancestral
human populations. Both sets of results, therefore, support the hypothesized link
between sexual selection and altruism towards non-kin.
Several reservations nevertheless need to be made in interpreting these results. The
sample obtained was relatively small. A larger study, and one containing a more
representative proportion of participants of reproductive age, might provide further
insights. Another reservation relates to the use of self-report data. These data are open to
the criticism that how participants report that they would behave may be different to
how they might actually behave in practice (e.g., Feingold, 1992). Continuing checks on
the reliability and validity of the psychometric scales concerned, as took place here, can
only partly answer this point as the possibility exists of some systematic error
confounding the results obtained. An answer lies in exploring the hypothesized link
between altruism and sexual selection by employing alternative methodologies. For
example, direct observation of altruistic behaviour in a context where it could be seen
as a mating signal (Iredale et al., 2008) has given additional support to the hypothesized
link between sexual selection and human altruism towards non-kin.
A further reservation is that the sample contained few male participants and so did
not permit examination of genetic influence on male mate preference and preferred
trait. In many species, it is the females who exercise the mate preference and males who
display the preferred trait (Andersson, 1994). This pattern, according to one prominent
theory, is determined by the relative parental investment each sex makes in its own
offspring (Trivers, 1972). The only way in which the less heavily investing sex (usually
males) can achieve reproductive success is through displays of the preferred trait while
the more heavily investing sex (usually females) can thus afford to be choosy when
Table 4. The genetic and environmental correlations (with 95% CIs) between MPAT and SRA, the
phenotypic correlation (r
ph
) with its components (r
ph-a
and r
ph-e
) predicted by the best-fitting AE model
r
ph-a
r
ph-e
r
ph
r
a
r
e
.20 (.04, .35) .10 (2.004, .24) .30 (.18, .41) .38 (.08, .62) .21 (2.01, .42)
Note. r
ph
, total phenotypic correlation; r
ph-a
and r
ph-e
, phenotypic correlation due to additive genetic and
specific environmental influence. The r
a
and r
e
, genetic, and non-shared environmental correlations
between MPAT and SRA. Significant values are in bold.
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selecting a mate – hence the typical pattern. However, where parental investment
is symmetrical between the sexes it is predicted that each sex would express both mate
preference and preferred trait similarly (Trivers, 1972). There is now growing evidence
for sexually selected traits being widespread in females (Clutton-Brock, 2007, 2009) and
that where mutual sexual selection is found both sexes contribute to parental
investment ( Jones & Hunter, 1993).
Although parental investment is difficult to quantify (e.g., Knapton, 1984), there is
evidence that, in humans, males make a far heavier parental investment than in other
mammals (Kenrick, Sadalla, Groth, & Trost, 1990). Evidence from societies that are likely
to reflect conditions present during human evolution (i.e., modern hunter/gatherer and
foraging societies) show that males provide an average of approximately 66% of calories
consumed (Gangestad, 2007; Kaplan, Hill, Lancaster, & Hurtado, 2000) while the
absence of a father has been found to triple the rate of childhood mortality (Geary,
2000). There are thus grounds for inferring that a more symmetrical pattern of parental
investment between the sexes would have developed during human evolution. On the
basis of Trivers’ theory, we should thus expect some form of mutual sexual selection to
be present in humans (Miller, 2000). Exploration of whether genetic influence acts on
variation in both male MPAT and male altruistic behaviour would therefore provide an
important subject for future research.
In establ ishing that genes influence both female MPAT and female altruistic traits, the
study made no attempt to explore how this link might operate. As discussed in the
introduction, altruistic traits might have evolved as a means of demonstrating superior
genetic and phenotypic quality through successfully overcoming the handicap of the
altruistic act (Zahavi & Zahavi, 1997). It might also have evolved as a result of another
sexual selection mechanism, such as the ‘runaway’ process (Fisher, 1958), or even some
combination of these mechanisms over evolutionary time (Phillips et al., 2008).
The findings of this study highlight the importance of interactions between genes
and environment in understanding human altruistic behaviour (e.g., Hauser, 2006) and
focusing on this interaction offers a promising way forward for future investigation.
For example, it may be that more altruistic individuals seek out environments in which
they have more opportunity to express these traits (cf. Dickens & Flynn, 2001; Meaney,
2001). ‘Gene by environment’ interactions were not measured in this study and would
have required careful measurement of the environmental indices of interest. These
would relate to social norms associated with altruistic behaviour present in the shared
family environment or the twins’ unique environment and could provide a useful
subject for future research.
We believe that the sexual selection hypothesis for the evolution of human altruistic
traits should now be considered alongside other more established theory (Bshary &
Bergmu
¨ller, 2008; Lehmann & Keller, 2006), particularly as there is the possibility that
multiple mechanisms might underlie a complex behaviour such as altruism. Empirical
testing of contrasting theories might even be possible. For example, reciprocal altruism
(Trivers, 1971) does not strictly predict the genetic correlation between MPAT and
‘altruistic personality’ found here as ongoing reciprocation towards others would not
necessarily result in such a selective process. Indirect reciprocity (Leimar &
Hammerstein, 2001) concerns reputation directed towards all other group members
while the sexual selection hypothesis focuses solely on altruistic displays that can be
evaluated by potential mates (Phillips et al., 2008). A study that examined ‘costly
signalling’ of altruistic behaviour through personal donation to a children’s charity
found a significant effect on male behaviour when witnessed by a female observer while
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no such effect was found when male participants were observed by same sex others
(Iredale et al., 2008), a finding that could be seen to be at odds with indirect reciprocity.
Additional studies could further elucidate the effects of altruistic reputation when
directed towards same sex others as opposed to potential mates, thus testing the relative
claims of indirect reciprocity against the sexual selection hypothesis.
This study extends previous work on the sexual selection hypothesis for the
evolution of altruism towards non-kin, which showed both predicted sex differences in
MPAT and mate choice on the basis of altruistic traits (Phillips et al., 2008). Here, we
have demonstrated that (1) variation in both MPAT and altruistic traits themselves were
subject to genetic influence and (2) the correlation found between the mate preference
and preferred trait was subject to genetic influence. The novel methodology employed
to obtain these results could enable further progress to be made in exploring the basis of
human altruistic behaviour. This study also suggests ways in which fresh hypotheses
might be formulated to test the relative claims of reciprocal altruism, indirect reciprocity
theory, and the sexual selection hypothesis.
Acknowledgements
We thank Tom Reader, Mike Kearsey and various anonymous reviewers for their comments during
the preparation of the manuscript. We thank the Wellcome Trust for their support of the twin
database and Tim Spector for allowing access to it. We extend thanks to Lynn Cherkas and Janice
Hunkin for their help during and after the ‘twin days’ and to all those twins who participated. This
paper is dedicated to the memory of Professor Chris Barnard who was a member of the team and
who sadly died during this project.
References
Andersson, M. B. (1994). Sexual selection. Princeton, NJ: Princeton University Press.
Andersson, M. B., & Simmons, L. W. (2006). Sexual selection and mate choice. Trends in Ecology
and Evolution,21, 296–302. doi:10.1016/j.tree.2006.03.015
Bouchard, T. J., & Loehlin, J. C. (2001). Gene, evolution and personality. Behavior Genetics,31,
243–273. doi:10.1023/A:1012294324713
Bressan, P., Colarerlli, S. M., & Cavalieri, M. B. (2009). Biologically costly altruism depends on
emotional closeness among step but not half or full siblings. Evolutionary Psychology,7,
118–132.
Bshary, R., & Bergmu
¨ller, R. (2008). Distinguishing four fundamental approaches to the evolution
of helping. Journal of Evolutionary Biology,21, 405–420. doi:10.1111/j.1420-9101.
2007.01482.x
Cesarini, D., Dawes, C. T., Fowler, J. H., Johannesson, M., & Lichtenstein, P. (2008). Heritability of
cooperative behaviour in the trust game. Proceedings of the National Academy of Sciences
of the USA,105, 3721–3726. doi:10.1073/pnas.0710069105
Clutton-Brock, T. (2007). Sexual selection in males and females. Science,318, 1882–1885.
doi:10.1126/science.1133311
Clutton-Brock, T. (2009). Sexual selection in females. Animal Behaviour,77, 3–11.
doi:10.1016/j.anbehav.2008.08.026
Darwin, C. (1859). The origin of species by means of natural selection. London: John Murray.
Darwin, C. (1871). The descent of man and selection in relation to sex. London: John Murray.
Dickens, W. T., & Flynn, J. R. (2001). Heritability estimates versus large environmental effects:
The IQ paradox resolved. Psychological Bulletin,108, 346–369.
Altruism and sexual selection 9
CORRECTED PROOF
Copyright © The British Psychological Society
Reproduction in any form (including the internet) is prohibited without prior permission from the Society
Feingold, A. (1992). Gender differences in mate selection preferences: A test of the parental
investment model. Psychological Bulletin,112, 125–139. doi:10.1037/0033-2909.112.1.125
Ferguson, E., Farrell, K., & Lawrence, C. (2008). Blood donation is an act of benevolence rather
than altruism. Health Psychology,27, 327–336. doi:10.1037/0278-6133.27.3.327
Fisher, R. A. (1958). The genetical theory of natural selection. New York: Dover Publications.
Gangestad, S. W. (2007). Reproductive strategies and tactics. In R. I. M. Dunbar & L. Barrett (Eds.),
Oxford handbook of evolutionary psychology (pp. 321–323). Oxford: Oxford University
Press.
Geary, D. C. (2000). Evolution and proximate expression of human paternal investment.
Psychological Bulletin,126, 55–77. doi:10.1037/0033-2909.126.1.55
Gregory, A. M., Light-Haeusermann, J., Rijsdijk, F. V., & Eley, T. C. (2008). Behavioural genetic
analyses of prosocial behaviour in adolescents. Developmental Science,12, 165–174.
doi:10.1111/j.1467-7687.2008.00739.x
Hamilton, W. D. (1963). The evolution of altruistic behavior. American Naturalist,97, 354–356.
doi:10.1086/497114
Hauser, M. D. (2006). Moral minds. How nature designed our universal sense of right and
wrong. New York: Harper Collins Publishers.
Hur, Y.-M., & Rushton, J. P. (2007). Genetic and environmental contributions to prosocial
behaviour in 2- to 9-year-old South Korean twins. Biology Letters,3, 664–666.
doi:10.1098/rsbl.2007.0365
Iredale, W., Van Vugt, M., & Dunbar, R. I. M. (2008). Showing off in humans: Male generosity as a
mating signal. Evolutionary Psychology,6, 386–392.
Jones, I. L., & Hunter, F. M. (1993). Mutual sexual selection in a monogamous seabird. Nature,
362, 238–239. doi:10.1038/362238a0
Kaplan, H., Hill, K., Lancaster, J., & Hurtado, A. (2000). A theory of human life history evolution:
Diet, intelligence, and longevity. Evolutionary Anthropology,9, 156–185. doi:10.1002/
1520-6505(2000)9:4,156::AID-EVAN5.3.0.CO;2-7
Kelly, S., & Dunbar, R. I. M. (2001). Who dares, wins. Human Nature,12, 89–105.
doi:10.1007/s12110-001-1018-6
Kenrick, D. T., Sadalla, E. K., Groth, G., & Trost, M. R. (1990). Evolution, traits, and the stages of
human courtship: Qualifying the parental investment model. Journal of Personality,58,
97–116. doi:10.1111/j.1467-6494.1990.tb00909.x
Knapton, R. W. (1984). Parental investment: The problem of currency. Canadian Journal of
Zoology,62, 2673–2674. doi:10.1139/z84-388
Krueger, R. F., Hicks, B. M., & McGue, M. (2001). Altruism and anti-social behaviour: Independent
tendencies, unique personality correlates, distinct etiologies. Psychological Science,12,
397–402. doi:10.1111/1467-9280.00373
Lande, R. (1981). Models of speciation by sexual selection on polygenic traits. Proceedings of the
National Academy of Sciences of the USA,78, 3721–3725. doi:10.1073/pnas.78.6.3721
Lehmann, L., & Keller, L. (2006). The evolution of cooperation and altruism – a general framework
and a classification of models. Journal of Evolutionary Biology,19, 1365–1376.
doi:10.1111/j.1420-9101.2006.01119.x
Leimar, O., & Hammerstein, P. (2001). Evolution of cooperation through indirect reciprocity.
Proceedings of the Royal Society of London B: Biological Sciences,268, 745–753.
doi:10.1098/rspb.2000.1573
Lumsden, C. J., & Wilson, E. O. (1981). Genes, mind and culture. Cambridge, MA: Harvard
University Press.
Meaney, M. J. (2001). Nature, nurture and the disunity of knowledge. Annals of the New York
Academy of Science,935, 50–61.
Miller, G. F. (2000). The mating mind. London: Heinemann.
Neale, M. C. (1997). Mx: Statistical modelling (4th ed. software). Richmond, VA: Department of
Psychiatry, Medical College of Virginia.
10 Tim Phillips et al.
CORRECTED PROOF
Copyright © The British Psychological Society
Reproduction in any form (including the internet) is prohibited without prior permission from the Society
Neale, M. C., & Cardon, L. R. (1992). Methodology for genetic studies of twins and families.
Dordrecht: Kluwer Academic Publishers.
Phillips, T., Barnard, C., Ferguson, E., & Reader, T. (2008). Do humans prefer altruistic mates?
Testing a link between sexual selection and altruism towards non-relatives. British Journal of
Psychology, 99, 555–572. doi:10.1348/000712608X298467
Ridley, M., & Dawkins, R. (1981). The natural selection of altruism. In J. P. Rushton & R. Sorrentino
(Eds.), Altruism and helping behaviour: Social personality and developmental perspectives
(pp. 19–39). Hillsdale, NJ: Erlbaum.
Roberts, G. (1998). Competitive altruism: From reciprocity to the handicap principle. Proceedings
of the Royal Society of London B: Biological Sciences,265, 427–431. doi:10.1098/
rspb.1998.0312
Rushton, J. P. (2004). Genetic and environmental contributions to pro-social attitudes: A twin
study of social responsibility. Proceedings of the Royal Society of London B: Biological
Sciences,271, 2583–2585. doi:10.1098/rspb.2004.2941
Rushton, J. P., & Bons, T. A. (2005). Mate choice and friendship in twins. Psychological Science,
16, 555–559. doi:10.1111/j.0956-7976.2005.01574.x
Rushton, J. P., Chrisjohn, R. D., & Fekken, G. C. (1981). The altruistic personality and the
self-report altruism scale. Personality and Individual Differences,2, 293–302. doi:10.1016/
0191-8869(81)90047-7
Rushton, J. P., Fulker, D. W., Neale, M. C., Nias, D. K. B., & Eysenck, H. J. (1986). Altruism and
aggression: The heritability of individual differences. Journal of Personality and Social
Psychology,50, 1192–1198. doi:10.1037/0022-3514.50.6.1192
Steele, W. R., Schrieber, G. B., Guiltinan, A., Nass, C., Glynn, S. A., Wright, D. J., …Garratty, G.
(2008). The role of altruistic behaviour, empathic concern, and social responsibility
motivation in blood donation behaviour. Transfusion,48, 43–54.
Trivers, R. L. (1971). The evolution of reciprocal altruism. Quarterly Review of Biology,46,
35–57. doi:10.1086/406755
Trivers, R. L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual
selection and the descent of man, 1871–1971 (pp. 136–179). London: Heinemann.
Wallace, B., Cesarini, D., Lichtenstein, P., & Johannesson, M. (2007). Heritability of ultimatum
game responder behavior. Proceedings of the National Academy of Sciences of the USA,104,
15631–15634. doi:10.1073/pnas.0706642104
Wilson, D. S., & Sober, E. (1994). Reintroducing group selection to the human behavioral sciences.
Behavioral and Brain Sciences,17, 585–654. doi:10.1017/S0140525X00036104
Zahavi, A. (1977). ‘Reliability’ in communication systems and the evolution of altruism.
In B. Stonehouse & C. Perrins (Eds.), Evolutionary ecology (pp. 253–260). London: MacMillan
Press Ltd.
Zahavi, A. (1995). Altruism as a handicap – the limitations of kin selection and reciprocity. Journal
of Avian Biology,26, 1–3. doi:10.2307/3677205
Zahavi, A., & Zahavi, A. (1997). The handicap principle. New York/Oxford: Oxford University
Press.
Received 20 October 2009; revised version received 19 January 2010
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