Helping More Likely with Close Kin than More Distant Kin
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Evolutionary theory predicts that when all else is equal, individuals will help their closely related relatives more than their distantly related relatives.
Humans are prosocial species and typically provide help to one another. The term help can be defined as active and voluntary acts, including the transfer of any resources (e.g., time, money, material) from one individual to another. In evolutionary studies, contact and emotional closeness are often used as proxies for help because they tend to strongly correlate with several forms of support. According to kin selection theory, with all other things being equal, the likelihood of providing help should correspond to the degree with which individuals are related to each other and individuals are predicted to provide more help to their closely related kin than their distantly related kin (Hamilton 1964).
On average, individuals share 50% of their genes with their parents, children, and full-siblings; 25% with half-siblings, grandparents, grandchildren, nieces, aunts, and uncles; and 12.5% with their great grandparents, great grandchildren, and first cousins. An abundant amount of empirical evidence shows that individuals tend to prefer kin over non-kin and close kin over more distant relatives (Salmon and Shackelford 2011). For instance, individuals provide more help and are more willing to make sacrifices if the beneficiary is a close relative and the readiness to help decreases with the degree of relatedness (Burnstein 2005; Madsen et al. 2007).
The exclusion of all other things besides the degree of relatedness is difficult because all else is never equal in human populations, meaning that several factors can affect the conditions under which helping takes place. However, different variations of sibling and twin relationships provide a research frame where all other things are closer to equal, but the genetic relatedness varies. Although individuals share an average of 50% of their genes with their full-siblings, the relatedness is only 25% between half-siblings. Moreover, dizygotic twins’ relatedness is equivalent to full-siblings, but monozygotic twins are genetically identical.
Research on family relationships shows that the sibling ties are stronger between full-siblings than half-siblings and that the degree of relatedness predicts interactions and support between siblings (Pollet and Hoben 2011; Tanskanen and Danielsbacka 2014). A twin study, where different twin-pairs played the prisoners’ dilemma game, found that identical twins were noticeably more cooperative with each other than other twin-pairs (Segal and Hershberger 1999). Furthermore, identical twins are also “genetic parents” of their co-twin’s children, while dizygotic twins have customary relatedness to their nephews and nieces. A prior study found that monozygotic twin aunts and uncles expressed greater social closeness and gave more gifts to their nieces and nephews than dizygotic twin aunts and uncles (Segal and Marelich 2011).
Prior empirical studies have shown that individuals tend to favor their close kin over more distant kin and that the likelihood of providing help is associated with the relatedness of the individuals. These findings are consistent with the kin selection theory.
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