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There tends to be large differences between maternal and paternal investment, with women
tending to make the biggest prenatal and postnatal contribution when bringing up a child. A female
produces fewer gametes in her lifetime, meaning she can only give birth to one child in a year,
whereas a male can father an unlimited amount of children. The enlargement of the skull through
evolution means childbirth is more difficult and so infants are relatively immature at birth. This and the
need for breastfeeding means women are more burdened by the extended childcare period.
Males, on the other hand, only require a few minutes copulation and a teaspoon of semen in order to
produce a child (Symons). Although internal fertilisation means males lack parental certainty, men are
not committed to invest in a child, and could potentially walk away and father more. Because of this,
indiscriminate mating is more costly for females (Goetz and Shackelford).
This is supported by Geher et al., who used the `Parental Investment Perception Scale'. They found
that males showed an increased heart rate when given a scenario which emphasised parenting costs.
This suggests that males are less biologically prepared to confront parenting issues, supporting the
idea that women are more prepared and willing to invest.
Large maternal investment could justify extra-marital affairs; because females can only give birth
once every year, it is important they raise good quality offspring so their investment isn't wasted. An
affair means they can marry a caring man with good resources but shop for good genes through
infidelity. This shows that the evolutionary idea of parental investment is capable of explaining
behaviour in our society today, supporting its validity.
Non-human species have also been used when researching parental investment. Chimpanzees and
bonobos in particular show little or no investment in their offspring. These studies show us the origins
of parental behaviour, suggesting that women traditionally made a greater investment before
evolution. However this is a non-human study and may not be generalisable to humans as monkeys
do not have an identical physiology to us.
Paternal investment, however, is supported by Reid, who pointed out that men invest resources
through giving their family a stable food supply. This allows their offspring to live in a healthier
environment, decreasing infant mortality. This contribution can be seen as a vital investment,
contradicting the idea that women invest more.
Investment in offspring does have costs for both genders. For males, cuckoldry is a big risk if the
father is going to invest, as men fear using their resources for a rival's child. Buss has suggested that
jealousy has evolved in order to protect males from cuckoldry and females from a diversion of
resources. Sexual and emotional jealousy, therefore, attempts to protect individuals from the
consequences of infidelity.
It has been found, however, that cuckoldry does not necessarily prevent investment. Anderson
found that stepfathers do not tend to discriminate between their biological and step children in
terms of resources. This suggests that even if cuckoldry has occurred, men are still willing to invest.
Cuckoldry also has benefits for females (additional social support, higher quality genes etc.) but Daly
and Wilson have pointed out that females do risk abandonment and the use of mate-retention. The
fact that females may risk this for more protected and genetically superior offspring suggests they
do make a greater investment.
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Overall, the theory of parental investment can be seen as reductionist. Rowe has stated that
personal and social conditions (like the father's personality) affects how much males invest into their
offspring. The theory tries to break down complex behaviour into a basic process of evolution,
ignoring other factors.
Finally, on the other hand, Buss's theory of sex differences in jealousy is supported by his own study.
He found that male students tended to fear sexual infidelity, while female students were more
concerned about emotional infidelity.…read more