Parental investment theory proposes that the differences in reproductive strategies between males and females stem from their differing levels of minimal parental investment. For females, pregnancy and lactation require extensive energetic resources, limiting the number of offspring that can be produced. In contrast, males can father many offspring with minimal investment. As a result, females evolve to be more selective in choosing mates who can provide resources, while males benefit from pursuing multiple mating partners. This theory helps explain sex differences in mate preferences, jealousy, and parent-offspring conflicts as organisms strive to maximize their inclusive fitness within the constraints of their biological roles.

1. PARENTAL
INVESTMENT THEORY
Describe and Evaluate the Parental Investment theory

2. Parental Investment Theory
◦One explanation of mate selection and human
reproductive behaviour is ‘Parental Investment Theory’
◦Robert Trivers (1972)

3. Parental Investment (PI)
◦“Any investment by the parent in an individual offspring
that increases the offspring’s chance of survivng at the
cost of the parent’s ability to invest in other offspring”

4. Parental Investment
◦Includes provision of resources:
◦ Food
◦ Energy
◦ Territory
◦Time spent teaching offspring
◦Risks taken to protect young

5. Parental Investment
◦Trivers believes that the differences between male
and female reproductive behaviour is as a result of
differences in parental investment made by males and
females

6. Males…
◦ Limitless sperm
◦ Fertile throughout life
◦ Number of potential offspring determined by number of
potential female partners
◦ Required level of parental investment = Low

7. Oldest natural parents…

8. Females…
◦ Egg is more than 100x larger than sperm
◦ Only release 1 (usually) a month
◦ Fertility lasts for around 30 years
◦ Average 40 weeks pregnancy
◦ Nutrients for foetus comes from mother’s own supplies
◦ Painful birth with potential complications
◦ Breastfeeding (in the past this would have lasted 2 yrs)
◦ Required level of Parental Investment = Very High

9. Best chance of
Reproductive
Success
◦Men = many female
partners to increase
inclusive fitness
◦Women = ensure survival
of few precious offspring

10. Brain Size
◦At birth, the human brain is only 27% of it’s adult size.
◦(Macaque monkeys are born with 70% brain development)
◦The adult human brain is 4 times larger than would be
expected in a primate with our size body

11. Brain Size
◦A human baby’s brain is very large at birth compared to the
size of the body
◦Human females have evolved to have wider hips and pelvis to
cope with this extra pressure
◦Childbirth has become more painful for a female as a result

13. Brain Size
◦As a result of our larger brains, humans must be born at this lower
level of brain development
◦This results in babies having restricted motor capabilities when
they are born
◦It takes humans twice as long as a chimpanzee to develop the
ability to walk and hold on to it’s mother
◦It takes a long time for a child to reach a state where it could
survive without it’s parents

14. ◦ Trivers (1972) defined parental investment as “Any investment by the parent in an individual
offspring that increases the offspring’s chance of surviving”
◦ The female investment is greater as eggs are less numerous and more costly than sperm. A
female is limited in the number of offspring she can produce whereas a male can have a virtually
unlimited number of offspring.
◦ Human mothers make a greater pre-natal contribution through the demands of pregnancy and
carrying and nourishing a child for 40 weeks.
◦ Women also make a greater post-natal contribution through breastfeeding and care.
◦ An increase in brain size has made childbirth more difficult, human babies are born prematurely
and so need extra care and are more dependent on their mothers than other species, resulting in
greater maternal investment.
◦ Alternatively, the required parental investment from males is much lower and the best way for a
male to increase his inclusive fitness is to have many fertile, female partners
◦ These factors explain why women select quality over quantity in potential partners and prioritise
good parenting characteristics as well as good resources (high salary or characteristics that could
translate to resources, i.e. intelligence) to ensure the survival of her few, precious offspring and
men opt for youth as more desirable in a partner

15. AO1
Number of sexual partners
Longevity of relationships
Sexual Jealousy
Sexual Fantasies

16. AO2 support for PI theory
◦ Buss (1989)
◦ Buss (1992)
◦ Daly & Wilson (1988)
◦ Geher et al (2007)

17. Buss (1989)
◦Demonstrated gender differences in modern
relationships
◦37 cultures in 6 continents
◦10,000+ participants
◦Questionnaire on mate preferences

18. Buss (1989)
◦ Across all cultures…
◦ Women showed a preference for males with resources or possessing
characteristics that would translate to resources (intelligence/ambition)
◦ Women preferred a high-status male on average 5 years older than
themselves
◦ Men placed more emphasis on physical attractiveness (indication of
woman’s health/fertility)
◦ Men desired a partner who was younger than themselves

19. Buss (1992)
◦Sexual jealousy may have evolved as a solution to
these problems…
◦Buss (1992)
◦Which is worse?
◦ A) The knowledge that your partner has had sex with
someone else
◦ B) The knowledge that your partner has developed a deep
emotional bond with someone else

20. Buss (1992)
◦85% of women chose option B – deep emotional bond
◦60% of men chose option A
◦ (The majority of the 40% of males who had chosen option A
had never been in a committed sexual relationship)

21. Sexual Jealousy (Buss, 1992)
◦ For a man, an unfaithful mate means risk of investing in a child who is not
their own
◦ For a woman, an unfaithful mate may mean a diversion of resources away
from the family
◦ Sexual Jealousy may have evolved as a solution to these problems
◦ Risk of cuckoldry means men are more jealous of the sex act itself
◦ Women are more concerned with the emotional focus and loss of resources

22. AO2
◦Buss supports the idea of sex differences in parental
investment influencing their mate choice (1989)
◦And their differences in sexual jealousy (1992)
◦Parental uncertainty can only occur in the male, therefore it is
important men are sure the offspring is theirs and they don’t
invest in another man’s offspring

23. Daly & Wilson (1988)
◦ Daly & Wilson (1988)
◦ Children under the age of 2 are 60 times more likely to be killed by a
step-parent (usually step-father) than by a natural parent
◦ Step parents-infants are genetically unrelated, therefore investing
resources, time & energy in them is evolutionary pointless

24. Daly & Wilson (1988)
◦Younger mothers are more likely to kill their infants than older
mothers
◦Older mothers are less likely to abuse or harm their infants
◦(even when financial resources and marital status are held
constant)

25.  Geher et al (2007)
◦Studied 91 non-parent heterosexual undergraduates
◦1. Filled out questionnaire on how prepared they are to have a
child
◦Both males and females scored equally…
◦2. ANS arousal measured when given parenting scenarios
◦Males had higher response

26. Geher et al (2007)
◦Consistent with PI theory…
◦…males are biologically less prepared than females to
confront issues associated with parenting
◦(highest inclusive fitness from multiple partners, not high
paternal investment to one singular infant)

27. Parental Investment Theory…
◦ 1. The costs of maternal investment (reproductive success)
◦ Longevity and quantity/quality of mates (Buss, 1989)
◦ 2. Sexual and emotional jealousy + cuckoldry risk
◦ Buss (1992)
◦ 3. Paternal investment
◦ Daly & Wilson
◦ Geher (2007)
◦ 4. Parent-Offspring Conflict

28. Parent-Offspring Conflict
◦ Offspring are not completely passive in this process
◦ Trivers (1974)
◦ Children desire greater investment than their parents provide
◦ Parents try to allocate resources to ensure that the maximum number of
offspring survive
◦ Conflict occurs when each child wants more resources than parents are
prepared to give

29. Parent-Infant Conflict
◦Pre-natal conflict
◦Post-natal conflict
◦Sibling rivalry

30. Pre-natal conflict
◦ food supply to the fetus.
◦ When a fetus perceives that it needs
more nutrition, it releases a hormone
into the mother’s bloodstream that
causes her arteries to constrict.
◦ This raises the mother’s blood
pressure and therefore delivers more
blood (and hence nutrition) to the
fetus
◦ this can damage the mother’s tissues
and in severe cases can cause
maternal death.
◦ Clearly the adaptation has evolved to
benefit the fetus, even at the expense
of the mother.

31. Post-Natal Conflict
◦ We originally would have
breast-fed for the first 2 years of
life
◦ After this time period, infants
would be ‘weaned off’ and put
onto solid foods
◦ This makes the infant less
dependent on the mother
◦ Child Development
◦ 2 years is often associate with
increased temper tantrums and
aggression

32. Sibling Rivalry
• Offspring will want more than their “fair share” at the expense of
their siblings in order to maximise their own fitness.
• As a result sibling rivalries develop as children compete for the
attention and resources of parents.

33. Andrews (2006)
◦ Analysed responses from a survey of 1600 US adolescents.
◦ Results showed that severe suicide attempts were significantly more common
among middle-born compared to first and last-born children.
◦ This study supports the view that that suicidal behaviour may be an adaptive
response in line with the predictions of the PI theory and that middle-borns
would make risky suicide attempts in an effort to extort increased investment
from parents.

34. AO1
◦ Trivers (1975) – PI theory
◦ Maternal vs. Paternal investment (pre&post natal)
◦ Brain Size = immature infants requiring more investment
◦ Short/Long term strategies reflecting required PI
◦ Parent-Infant conflict (pre&post natal)
◦ Sibling Rivalry

35. AO2
◦Buss (1989) – sexual selection reflect investment
◦Buss (1992) – paternal investment risk from cuckoldry
◦Daly & Wilson (1988) – investment in non-related infants
◦Geher (2007) – males strong reaction to investment
◦Andrews (2006) – sibling rivalry (middle child)

36. AO3
Gender
Reductionist
Ethical Guidelines?
Nature vs. Nurture
Animal Studies
Determinsm vs. Free Will
Ethnocentricism