Evolutionary Psychology


– study of human behaviour as a result of evolved psychological mechanisms

– how evolution and genetic variables influence adaptive behaviour

– understanding behaviour by considering environmental pressures that led to their evolution

Proximate explanation: understanding the details of how a mechanism works

Ultimate explanation: understanding the evolved function of a psychological mechanism or why it exists



– Roots in Darwin’s (1859) theory of natural selection

– variant traits helping survival and reproduction will be transmitted to future generations

– this process leads to adaptations: inherited characteristics that solved problems related to survival and reproduction (e.g. fear of snakes)

– similarly psychological adaptations evolved for same reason (e.g. reading fertility cues)



Ilardi et al (2007) – depression

– ancestors outdoors in sunshine, active, tending kids, tight knit groups, specific diets

– told depressed to get sun, increase omega-3, exercise, socialise daily

– high success rate vs controls



Deterministic: genes determine behaviour with little environmental influence

Response: misunderstanding – environment in (a) selection pressures (b) proper development of mechanisms (c) activation of mechanisms


Learning: behaviour can be explained by culture or learning

Response: there are adaptations underlying different types of learning (e.g. learned food aversion & incest avoidance) or the transmission of culture (e.g. what is transmitted and filtering what is taken in)


Historical knowledge: don’t have detailed knowledge of historical selection pressures

Response: we know enough using multi-disciplinary approach and can also infer from current behaviour (e.g. snake fear or bitter taste aversion)


Individual & cultural differences: can’t explain

e.g. women generally better at spatial location memory than men but lots of variation among women and some men better than some women


Homosexuality & suicide: don’t increase individual’s reproductive success – not explained


Testability: can’t test & disprove because we don’t know how particular behaviours evolved

Response: can test like any other hypothesis e.g. Buss & paternity uncertainty adaptation theory




Trivers (1972) – MAMMALS & POLYGYNY

– female mammals bear low numbers of helpless slow developing young

– it’s adaptive for males to stay and promote successful development

– natural selection promotes bonding with females they’ve had sex with

– also selection pressure on females to behave in ways get males to bond so she can pass on her genes

– Polygyny (one male, many females) is most prevalent type of bonding in mammals

– females contribute far more to rearing of young

– females can only produce a few offspring but males can father many

– so females need to mate with fit males: 1) fit offspring to pass on genes

2) effective parental support

– so evolutionary pressure on females to be selective in bonding, not males

– theory supported because polyandry (one female, many males) is never found in mammals; only occurs where male contributes more


Parental investment theory: human sex differences in parental investment predict sex differences in sexual strategies.


ARTICLE: BUSS(1994) in BJORKLAND & SHACKLEFORD (1999) – Differences in Parental Investment Contribute to Important Differences Between Men and Women

  • men & women face different adaptive problems that threatened survival & reproduction
  • so they evolved different psychological mechanisms that can be seen today
  • males invest much less than females in parenting [across cultures]
  • but human males spend more time caring than other mammals due to extended childhood
  • men can have lots of offspring if they spend more effort on mating
  • women can’t spend more time on mating because costs of pregnancy are high
  • [offspring are more successful with paternal presence & care]
  • male mammals can have great reproductive success by inseminating many females
  • for females the cost of copulation can be high, so selective females favoured
  • women’s reproductive interests served by good mate genes & support
  • it also evolved in men’s reproductive interest to support kids to reproductive age
  • but still conflict because amount of investment was still less for men (than women)



  • for long-term partners men are selective: want attractive, intelligent, kind
  • women value financial resources most, as predicted by parental investment theory
  • women are selective in granting sex, as predicted by parental investment theory
  • females have certainty of maternity, males don’t
  • women’s ovulation is concealed (so men don’t know best time for copulation)
  • women can lose mate’s investment to another woman
  • so women are more upset by emotional infidelity, signalling loss of resources
  • men more upset by partner’s sexual infidelity



human sex differences in parental investment predict differences in sexual strategies:

  • men more likely to have short-term casual sex
  • women more selective in choosing a mate [especially in short term]
  • male distress with sexual fidelity; female distress with emotional infidelity


Dewsbury (1988) – MAMMALS & MONOGAMY

– evolved because these mammals can raise more fit young with support

– 2 evolved changes for females:

1) drive female competitors away

2) don’t have sex until male has stayed around for long time


Buss (1994) :

– so male strategy changed from polygyny to bonding with single fit female

BUT polygyny appears more prevalent for human species in society today

– many people in West bond with several partners in their life

– infidelity common esp. for men



– good theories of behavioural evolution have predictive power, otherwise no good

– otherwise could just postulate a theory from a hypothetical past


Buss (1992)

– men in most cultures value youth & attractiveness (i.e. fertility signs) more than women

– women value power and earning capacity more

– men with high occupational status will bond with physically attractive females

– women’s mate attraction strategy is increasing their attractiveness; for men it’s displaying their power and resources

– men more likely to commit adultery




Buss (1992)

– jealously is an adaptation to the problem of paternity uncertainty

– found sex differences where men’s jealousy triggered by sexual infidelity cues

– women’s by emotional infidelity cues (i.e. that man is diverting commitment & resources to another woman)





Daly & Wilson (1988)


– male aggression in mammals because females selective (investment) so competition high

– only fittest most successful competitors pass on their genes

– great differences in fitness means more intense competition

– humans have history of mild polygyny meaning that comparisons can be made



– Male violence toward women if suspects infidelity, because risk of cuckoldry & investment in another’s child

– Adaptive jealousy psychology to detect sexual infidelity (as in Buss)

Campbell (2007) – review of studies doesn’t show this aggression link convincingly


ARTICLE: Campbell (2007) – “sex differences in aggression” review

– sex differences in aggression are clear & universal (e.g. crime statistics)

– men compete to dominate & get resources attractive to females (qualifications, money)

– 16-24 is peak of violence, when reputation is made and mates acquired

– females use aggression selectively to compete for resources & protect their child investment

– young women use indirect aggression against competitors: gossip, exclusion, manipulation




– altruism contradictory because it’s less adaptive than competitive or selfish behaviour

– have been theories around empathy, rewards, emotional states, social norms, no.of bystanders

– theses are proximate but not ultimate explanations

[- Dawkins (1986): altruistic behaviour is way that genes maximise inclusive fitness (reproductive success)]

– Hamilton (1964): inclusive fitness (reproductive success) or kin selection – natural selection favours behaviours that benefit others that share our genes >> altruistic behaviour

– Altruism decreases as relatives become more distant

– can’t explain altruism where individuals unrelated

Trivers (1971) – reciprocal altruism: cooperative behaviour between unrelated individuals that benefits everyone involved

– Near et al (1988) – support for reciprocal altruism: we are primed to detect cheaters and partners valuable in social exchange

Buss (1995) – kindness & dependability (altruism type traits) are the most valued characteristics in potential mates


Wilson (1997) – multilevel selection theory (MST)

– builds on inclusive fitness and reciprocal altruism theories (indiv level)

– groups can be adaptive units in their own right when competing against other groups

– groups of altruists are more fit than groups of non-altruists in certain situations

– fortunes of each individual depends on success of group

– this leads to evolutionary increase in altruists in population


1) conditions for group selection to happen don’t happen in the real world – R: need evidence

2) sacrifice for the good of the species discredited – R: MST is different – in competing group altruism is adaptive & works for the benefit of individual


Grafen (1990) – cost signalling theory (CST)

– these theories don’t explain altruism to non-kin that will never be returned (e.g. beggar)

– CST: indivs display costly signals so they will be chosen as a mate, ally or deferred to by rivals

– e.g. extreme philanthropy signals resources

– cross-cultural evidence that generous individuals are rewarded when times get tough