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  • Created by: imanilara
  • Created on: 06-06-16 18:55


  • Extent to which a mother or father contributes time/resources eg food into the survival of their offspring 
  • however means that less investment can be made in the survival and success of other poss offspring- cannot ensure survival
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Parental investment theory- Trivers AO1

  • Trivers 1973- says that there are biological differences in the basic investment made by males and females in the production of offspring - give rise to differences in PI.
  • Female gametes larger+ fewer of them- can only produce limited in a lifetime (anisogamy)
  • men can produce up to 500million sperm in one day 
  • due to preganancy- human female can only have a small number of children - can only carry one at a time (record is 69 according to Daly+wilson), whereas men could potentially have thousands 
  • Pregnancy- physiological cost- extra resources, risks to health + following this breast feeding and childcare
  • Man can potentially opt out of investing after conception in most species (D+W)
  • For these reasons, Trivers says that women are choosier about quality of mates compared to males - wants someone who will invest in the offspring and to her to increase survival 
  • By contrast, man increases rep success by putting all resources into courtship to trick female into believing he is a high quality mate+ then making no investment afterwards- moving on to another mate ensuring a high quantity of offspring  
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Emperical evidence:

  • men gain more from polygyny- more sexual partners- able to spread out genes and invest less 
  • Women should favour monogamy- one partner who gives all resources to her and offspring 
  • When resources limited - women have more to gain from polygyny- several females can share resources of one powerful male rather than having one male w few resources. Maximises benefit to child 
  • Evidence shows polygyny is the most common mate strategy in human history prior to western influence - 80% societies polygynous (Murdoch 1967) 
  • Indicates PI does influence human behaviour 
  • Sex-role-reversed species- men invest more - females compete for the choosy males - Trivers 1985- this supports PIT bc relative parental investment explains behaviour 
  • In support of female choosiness/male promiscuity- Clark+Hatfield - 75% of male students agreed to have sex w random girl who approached, whereas no females did - supportive
  • But more penalties for women who are sexually promiscuous- could explain this - social norms influences. SSR! Male students - ethics+pop val
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Human males do invest considerably in their offspring, where according to PIT this shouldnt happen. Cannot explain why men invest in step children who do not have their gene or why women have one-night stands! Issue is that it is deterministic and we have the free will to do as we please. 

Theory justifies sexual inequality- men allowed to invest less and be more promiscuous, women are not allowed to do this - issues in power, childcare and choice - perhaps women are only dependant for resources because they want more equality- if they had more financial independence and power would female choice be similar to male choice?? 

We cannot presume what was adaptive in the EEA as we have no emperical evidence to back it up = speculative - we dont know what the childcare arrangements were or what type of PI existed - no way of retrospectively discovering this

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Reductionism- evol exp's reductionist- over simplify human behaviour 

Rowe 2002- exp of paternal investement into offspring looking at evol factors alone is limited- input of a male in the rearing of offspring is depenadant on a large number of other factors, eg relationships w mother, personal+social conditions + personality characteristics of the father. Belsky improves reliability of theory - correlation between parental divorce and degree to which male invests into infant 

Animal research- in evol exps we look at beh of primates and assume this emulates beh in the EEA. Males dont usually show any parental investment, and so male HUMAN parenting is a drastic evolutionary change - could be explained by our cultural learning

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