Hamilton’s Rule and Kin Investment
Hamilton’s rule predicts that, all else being equal, individuals will invest more in their closely related relatives compared to their distantly related relatives.
Inclusive fitness theory (Hamilton 1964) argues that individuals can spread their genes in future generations (inclusive fitness) by investing in the reproduction of their relatives (indirect fitness) in addition to reproducing themselves (direct fitness). Hamilton’s rule is an equation of the above-described theory and can be described as follows: Br > C. In the equation, B stands for benefits and C refers to the costs, which are both measured in terms of reproductive success. The coefficient r represents the genetic relatedness between the investor and the recipient, indicating the probability that these individuals have an identical allele by common descent at a random locus (Salmon and Shackelford 2011). According to Hamilton’s rule, the benefits of an investment, weighted by the degree of relatedness, should exceed its costs. The theory has been investigated especially in nonhuman populations and empirical results have provided extensive support for the validity of Hamilton’s rule in many species, such as eusocial insects, birds, primates, and many other mammals (Hepper 2011).
Hamilton’s Rule and Human Populations
The benefits and costs of investments are difficult to measure because they are defined in terms of the overall reproductive success of an individual. Therefore, research on humans typically focuses on the genetic relatedness of individuals, which is the more measurable parameter of Hamilton’s rule. On average, the degree of relatedness is 0.5 with parents, children and full-siblings, 0.25 with half-siblings, grandparents, grandchildren, nieces, nephews, aunts, and uncles, and finally 0.125 with great-grandparents, great-grandchildren, and first cousins. According to Hamilton’s rule, all other things being robust, the investment between individuals should correspond to the degree of genetic relatedness.
The prediction is relatively well supported by ethnographic, experimental and self-report-based studies, which have shown that people typically invest more in their close relatives than their distant ones and, in general, the investments among kin or the willingness to invest reflects the patterns of their genetic relatedness (Burnstein 2005; Madsen et al. 2007). For instance, individuals are more willing to make sacrifices for the benefit of their siblings than their cousins and, respectively, individuals are closer with and provide more support to their full-siblings than their half-siblings (Neyer and Lang 2003; Pollet and Hoben 2011). Moreover, twin studies have shown that genetically identical monozygotic twin-pairs have higher levels of closeness and cooperation than dizygotic twins, whose relatedness is equivalent to full-siblings (Segal et al. 2007).
Hamilton’s rule is widely applied in evolutionary studies. Nonetheless, it has also been criticized. The universal applicability of inclusive fitness theory was questioned in a recently published article by Nowak et al. (2010). In the following debate, almost 150 scholars stated that the critics had ignored an extensive amount of empirical studies that has accumulated over several decades and provide strong evidence for Hamilton’s rule (Abbot et al. 2011). Overall, Hamilton’s rule can provide a prediction and an explanation for kin altruism and it has gained extensive support from empirical studies.
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