There are some important characters in mating relationships that have not yet been discussed in detail. These of course, are the offspring. In 1974, Trivers applied Hamilton's mathematical formula for kin selection to within-family conflict. In sexual reproducers, the resource allocations and other states of affairs that would maximize parental fitness are not identical to those that would maximize offspring fitness. Individual children will want to receive more than their "fair share," at the expense of other children, in order to maximize their own fitness. In fact, children may even try to gain more resources than the parents can afford to give. Their nieces and nephews would only have an r of .25, while their direct offspring would have and r of .5. Parents are related equally to each of their children and grandchildren, so they would have a tendency maintain resource equality. Hence, sibling rivalries will occur as a result of competition for the attention and resources of parents. Children may try to differentiate themselves from their siblings as a strategy to gain a greater portion of resources.
Haig (1993) extended Trivers' thinking by considering three sets of genes that may have different interests: a) genes in the mother, b) maternally derived genes in the current child, and c) paternally derived genes in the current child. Because the maternal genes have an equal stake in each child, they will be selected to transfer resources to children as a function of the children's likelihood of reproducing. Genes in the current child have a greater interest in the current child than in future children and will be selected to maximize the transfer of resources to the current child.
When their offspring is young, a parent can maximize fitness by investing in the current offspring at the expense of additional offspring. When the offspring is older, the parent can maximize fitness by deferring investment to this individual in favor of younger offspring. Parent-offspring conflict is most intense at intermediate ages of the focal offspring. Older offspring have fewer chances to die before becoming reproductive than younger offspring, and thus have higher reproductive value.
Parental withdrawal of investment seems at first obviously counterselective. In other species, it is typically reproductive competitors who commit infanticide. In primates these are immigrant or out-group males. In humans, stepparents are more likely to abuse or neglect children than genetic parents (Daly & Wilson, 1984). Cross-culturally, deformed or seriously ill newborns are at great risk for infanticide. Having a child at a young age in a time when resources are scarce can actually hurt overall reproductive fitness. As women age and their reproductive value decreases, termination of investment is less likely. Attitudes towards abortion are related to the proportion of women in any group who are "at risk" of unwanted pregnancy (Crawford, 1998).