Families and Development

Families are collections of individuals who

share many genes by common descent

• Genetic overlap —> overlapping interests

• Family members are not identical —> they are bound to disagree to some extent

Relatedness

• The basis of conflict within families

• Evolutionary theory predicts a certain amount of conflict within families

• Each individual’s fitness inevitably conflicts to some degree with others in the family

• Each individual is related more closely to himself or herself than anyone else

Parent-offspring Conflict

• Parents are sources of investment and resources

• This can be distributed equally or unequally

• Parent-offspring conflict revolves around the distribution of this investment

Although parents and offspring are closely related, they are not genetically identical

• There will be one distribution that maximizes the mom’s genes

• There will be a different distribution that maximizes each offspring’s genes

For any maternal investment, the mother has a choice

• Invest in the current offspring

• Prepare her body for another pregnancy

All of the mother’s offspring with carry 50% of her genes

• She will favor an equal distribution among offspring

• Anytime the benefits of giving investment to a present child do not outweigh the costs of withholding for future offspring, a mother should invest in future offspring

The offspring’s viewpoint is different from the mother’s

• When the mother compares a child to a future child, she sees two equally valuable genetic entities

• When the child makes this comparison, he/she reaches a different conclusion

• In terms of spreading his genes, he is twice as good as any sibling he could ever have

Offspring should have been shaped by natural selection to

devalue the costs his/her own rearing inflicts on his siblings

Half siblings will devalue each other ______ _______ than full siblings

More steeply

Parent-offspring conflict in mammals

The conflict is universal among mammals

• Babies of all species will demand nursing long after mother becomes reluctant

• There is no “mutually beneficial” time for weaning to take place

The offspring is ___1___ in eliciting the investment from the mother

This helps explain the ___2___ to less mature behavior when a new baby appears

1. active

2. regression

The mother is designed to weigh the value of investing in this particular fetus against the value of potential future fetuses

Negative maternal evaluation

• Believed to explain why nearly half of all fetuses are spontaneously aborted

The fetus is more interested in its own survival than the mother

The fetus has evolved hormonal means of manipulating the mother

• This provides the fetus with more resources than are in the mother’s best interests

• Can cause maternal diabetes, maternal high blood pressure

Parents should want their children to share ____ ____ than their children would want to

more often

This goes beyond the immediate family

• A mother is related to her niece by 25%

• A child is related to his cousin by 12.5%

• A parent should have evolved to want altruism towards cousins more than offspring would

The occurrence of identical twins may have been too rare in human history for this unselfishness to have evolved

There does exist some evidence that identical twins are less selfish towards each other

The mother has a motive to evaluate each child as a vehicle for her fitness

• A younger child needs more care than an older child

• A sick child needs more care than a well child

Discriminative parental solicitude

investment in offspring that is conditional on parental resources and offspring’s relatedness, need, health, etc.

Daly and Wilson (1988): Post-Partum Depression

Propose that post-partum blues and post-partum depression are mechanisms to give the mother an objective frame of mind

• If the mother were euphoric at birth, she would not be able to ask herself difficult, but adaptive questions

Women who kill their babies are mostly young, poor, and unmarried

These are the exact conditions in the EEA that would predict low success in raising a child

The degree of conflict between the father and mother depends on the father’s confidence that his mate’s children are his

If humans were completely monogamous, the interests of father and mother in their children would be identical

• We have seen that this is not the case

If a father is sure that his mate’s children are not his, he has an interest in minimizing the investment their mother gives them

• This investment could be going to his children

In some species it is common for the male that has taken a new female to kill all her existing dependent offspring

• And sometimes those born before the required gestation time

The incentive is to get the female to stop investing in another male’s offspring and to become pregnant with his offspring as soon as possible

Lightcap et al. (1982): Child Abuse in Step Children

• Studied families with 2 parents who each had a biological child and a stepchild

• If bad parenting was the explanation, both children should be equally abused

• Found that adults abused 12 of 21 stepchildren (57%), but 0 or 20 (0%) biological children

Daly and Wilson (1988): Homicides

Studied homicides of children

• Extreme cases of abuse

A child living with one natural and one stepparent is as many as 100 times more likely to be killed than a child living with two natural parents

• This is an extremely strong finding

Daly and Wilson (1982): Perception of Paternity

• Babies are most often said to look like the father

• These claims are mostly made by the mothers and the mother’s relatives

Handicapped children are abused more often than non-handicapped children

• Children with physical handicaps would have had a difficult time competing in the EEA

• Handicapped children were less likely to convert maternal investment into grandchildren

Bertzig (1989): Causes of Divorce

Looked at cross cultural data on divorce from 186 societies

Found that in every region, infidelity and infertility were the more common bases for divorce

• These led by a high margin over personality differences, economic problems, problems with in-laws, etc.

These two major causes of divorce involve threats to the reproductive fitness of one or both partners

Why do humans take such a long time to develop?

It has been suggested that childhood allows for time to learn the many skills necessary to function in society

• Linguistic skills, social skills, practical skills, etc.

Children’s play changes with age

Infants’ play consists largely of manipulating single objects

• Shaking rattles, banging blocks, mouthing dolls, etc.

During the second year, children begin pretend play

• Feeding dolls, resting dolls with naps, driving model cars around, etc.

Groos (1901): Child Play

Suggested that play serves the purpose of giving the child practice at skills that would be necessary as an adult

• For the infant, this means learning the nature of physical objects

• Older children are learning about social relations

Bjorklund (1997): Immaturity

• Suggests that immaturity may play an adaptive role in a child’s life and development

• Some aspects of childhood are designed by evolution to adapt the child to its current environment, not its future one

• Chicks have temporary structures on their beaks called an “egg tooth” that helps it break out of its shell. It disappears soon afterwards.

Children are oblivious to how poor they are at tasks

• A lack of awareness of limitations may prevent them from being discouraged

• Allows children to keep attempting to learn necessary life skills

• This contributes to a child’s optimism and willingness to compete

• This also may be related to a children’s failure to detect sarcasm

Reproductive value

the number of future children a person can be expected to produce

Reproductive value is not the same as fertility

A child has zero fertility, but high prospects for producing children in the future

Reproductive value increases until puberty, when it then begins to decrease with age

• The older you are the fewer children you can expect in the future

In hunter gatherer societies, reproductive value increases markedly up to about age 15

Maximum grief was reported for children who died at an age corresponding to the peak of reproductive value for hunter-gatherers (15)

This pattern is consistent with conditions in the EEA, not today

The Grandmother Hypothesis

proposes that a time comes when women can further their fitness more by investing in existing kin (i.e. grandchildren) than in trying to produce more of their own children

• Menopause may have evolved to switch women from investing in producing children of their own to investing in kin