According to classical fitness, members of a species that sacrifice their resources for the benefit of others will have fewer successful offspring, on average, and hence an inherent tendency towards this behavior would be selected against. Hamilton (1964a) expanded the basis of reproductive success to include relatives who become ancestors of offspring with similar genetic material. Hamilton's inclusive fitness theory leads to the prediction that natural selection will favor those characteristics that cause a person's genes to be passed on, regardless of whether or not the person is the actual direct ancestor. By assisting in a time of need, one would help his/her relative become an ancestor of offspring with similar genes. A genetic influence on the tendency to help relatives would increase of proportion of genes that promote giving assistance to relatives in the population. This would be adaptive from an evolutionary perspective, since it is not the survival of the organism that is critical for natural selection, it is the survival of the genes. Despite the catchphrase "survival of the fittest," the ultimate criterion which determines whether an "altruistic gene" is spread is whether it benefits the gene itself, not whether it is of benefit to the bearer of the gene (Hamilton, 1964b).
The term "r" is used to indicate the proportion of genes (not common to the species) that individuals share. Identical twins have an r of 1.0, an individual's siblings, parents, and offspring have an r of 0.5, first cousins have an r of 0.125, etc. Hamilton (1964a) proposed that actions benefiting the aggregate distribution of the relevant gene would be selected for. The cost of the behavior is considered for the donor(s). The benefit is the sum of the benefits to others, multiplied by their r with respect to the donor(s). For such a gene to successfully replicate, the benefits multiplied by the r values must exceed all costs multiplied by the r value of the donor(s). Thus, one can calculate whether a genetically influenced action is likely to spread across a population (if: benefit x r > cost).
The term Nepotism originally referred to the bestowal of patronage on the bastard sons, euphemistically called nephews, of popes and other high Vatican officials. Today, it is defined as the (usually illicit) use of one's social position to bestow the benefits on genetic and marital relatives, (Daly, Salmon & Wilson, 1997). Since humans are particularly adept at amassing material wealth, it is likely that no other species exhibits the degree of resource transfer that can take place through inheritance within human families. Resources are typically passed to older sons, and to sons at the expense of daughters. Within polygynous marital systems, inheritance is strikingly male-biased (Crawford, 1998). The reason for this gender bias will be discussed in the following section on sexual selection.
For altruistic actions towards relatives, usually called kin selection, to be evolutionarily successful, potential altruists must be able to distinguish (consciously or subconsciously) between kin and non-kin (Schroeder, Dovidio & Piliavin, 1995). For example, if a costly behavior was indiscriminately performed for the benefit of neighbors, an individual may just be breaking even or worse in terms of inclusive fitness since the benefits to close relatives could be outweighed by the costs to the donor.
A gene allowing discriminatory behavior itself benefits inclusive fitness and would be selected for (Hamilton, 1964c). This would allow one to maximize his/her reproductive fitness through selectivity in the provision of assistance. Evidence for kin recognition mechanisms include: physical resemblance (Porter, Cernoch, & Balogh, 1984); olfactory cues (Porter, Cernoch, & Balogh, 1984); social cues such as proximity and familiarity (Sherman, 1985); and neurochemical mechanisms such as endogenous opioids stimulated by reunions with kin after long periods of separation (D'Amato & Pavone, 1993).
Kin recognition mechanisms have an impact on sexual attraction. Incest avoidance mechanisms to prevent damaging genetic combinations and promote out-breeding have been expressed in both traditional biologically related families and artificially constructed families of non-relatives. Studies of Israeli kibbutzim have revealed the remarkable fact that the children raised together do not marry within their own group. The critical factor is that the children are raised together from an early age, since the only exceptions involved pairs who were in fact separated from one another for a large part of their childhood. Many comments by the Israeli youngsters reveal their awareness that being reared together kills sexual attraction (Shepher, 1971).
In the Shim-pua marriage of Taiwan, the couple is wedded as children and reared together. Such couples have more difficulty consummating their marriages, more extramarital affairs and divorces, and fewer children than do couples whose marriages are arranged post-pubertally. This indicates that socially based kin recognition may be generalized to genetically unrelated individuals (Wolf, 1970).