less advantaged parents to favor their daughters, is one such evolutionary tenet that has received recent attention in mainstream sociological journals. Although many have weighed in on the usefulness of this particular theory, its core assumptions are transported from another, less-studied evolutionary theory—Hamilton’s (1964) theory of kin selection, also known as his theory of inclusive fitness.
Among sociobiologists and other evolutionary theorists, Hamilton’s theory of kin selection is “widely regarded as so well established that it is simply assumed to be correct in its general outlines”. This theory explains parental investment as a form of reproductive survival in which parents display “discriminative parenting”. Hamilton notes that altruistic behavior in humans is adaptive when it increases the genetic fitness of individuals. Because parents incur economic, physical, and mental costs in raising a child, they purportedly invest the most in those who have the greatest amount of shared genetic material—their biological children. Evolutionary theories suggest that because unrelated children offer few reproductive benefits to their parents, they are less likely to garner valuable resources and may even suffer mistreatment at the hands of their parents.
The prevalence of stepfamilies—especially those with stepfathers—in part may explain why they are frequently used by evolutionary scholars to investigate the importance of genetic relatedness in parental investments. These studies cover multiple types of parental investments and consistently support kin selection theory: Stepfathers provide less direct care, monetary support, financial aid for continue dedication, playtime, and homework help to their stepchildren than do biological fathers. In addition, studies connect living with a stepparent to abuse, neglect, and suboptimal growth.
Unlike stepfamilies, adoptive families are not confounded by the presence of one biological parent whose inclusive fitness can be maximized through investment in his or her biological child. Salmon (2005), in The Handbook of Evolutionary Psychology edited by Buss, contends that “We would expect very little to no investment in an adopted child because they are not genetically related at all. With step-parent situations, at least one parent is the biological parent; in adopted situations there is no biological parent present”. Therefore, if evolutionary predispositions are the overriding principle guiding parental investment, we would expect two-adoptive-parent families to invest less in their children than both two-biological-parent families and other alternatively structuredfamilies.
