Kindness is not necessarily a uniquely human trait, but humans do seem particularly apt to perform selfless acts at their own expense. Natural selection predicts the evolution of cooperative (benefit to all) and selfish (benefit to individual, cost to someone else) traits because both increase the fitness of the individual who carries them. Altruistic (cost to individual, benefit to someone else) and spiteful (cost to everyone) traits decrease the fitness of their carriers and one predicts that natural selection would not favor those behaviors. Thus, altruism is an evolutionary puzzle; it is a behavior that seems incompatible with natural selection.
How can selection favor costly behaviors, especially those that hinder or actually prevent the individual from successfully reproducing?
The phenomenon of altruism is documented in many animals, especially insects like worker bees. Cape hunting dogs and Florida scrub jays are just two species in which a significant number of adults do not breed and instead they help others rear more offspring than they could possibly handle on their own.
Group selection theory predicts that altruistic traits should spread. Genes for altruism would be perpetuated in future generations because altruism is good for the group as a whole. (Genes for altruism do not necessarily directly cause an individual to act selflessly, they could, for example, code for a rewarding, feel-good natural dopamine high, or stress reducer, that kicks in when an individual behaves altruistically.) The problem with this logic is that a trait will only spread if the genes that shape it are good at getting into the next generation. Altruism genes are bad at getting themselves into future generations. On top of that, altruistic individuals add to the fitness of competing individuals as they are decreasing their own. Under group selection theory, altruists would be selected against. In general, advantages for the group are generally not sufficient to spread a trait.
A combination of natural selection and kin selection solves the altruism conundrum, at least when kin are benefiting. Kin share genes so selection should favor altruism that is selectively aimed at relatives. In this scenario, shown by W.D. Hamilton in 1964, selfishness would be disfavored and cooperation would be favored. An altruist may have low direct fitness, but their overall or "inclusive" fitness would be high if they aided in the successful survival and reproduction of their gene-sharing kin.
Reciprocity, however, is the key to nonkin altruism. More specifically, reciprocal altruism, a concept introduced by Robert Trivers in 1971, can explain the behavior when two conditions are met: 1) costs must be small compared to benefits, and 2) the altruist and the recipient must interact frequently and regularly exchange roles. Such a system of tit for tat turns short-term altruism into long-term cooperation and continues as long as the benefit is always greater than the cost for each actor.
A stable reciprocal altruistic relationship is equivalent to a friendship. The actors do not need to be related for this system to work as long as it is balanced. Many primates like baboons will form male-male coalitions against higher ranking males. Often grooming another individual will be considered like a down payment for future help if attacked by a threatening individual. Chimpanzees that groom one another are more likely to share meat from colobus monkey kills with one another.
Reciprocal altruism works as long as it is withheld from cheaters who do not participate fairly. So cheater detection is crucial and the ability to discriminate between reciprocators and nonreciprocators is essential. Cheaters must be either avoided or punished. Plus, the actors must also be able to track the benefits and costs of altruist acts. This is probably where increased human cognitive functioning comes into play since humans are far better at long-term and complex networks of reciprocal altruism than chimpanzees, and far more dependent on it as well.
Large-scale societies based on agricultural resources that popped up after 10 Kya are relatively new. For most of human evolution, groups were small and subsisted on hunting, gathering, and foraging but the need for the neural wiring to manage multiple networks ofreciprocal altruism across space and time was clearly important even in small groups. Once sedentary lifestyles, based on crops were adopted by some human populations, people were able to thrive in large-scale societies thanks, in part, to the altruistic behaviors that were selected for in their ancestors.
Humans group together with extended families and kin with larger groups on the order of 100-1,000,000 and this is a typically peaceful arrangement because the basis of human sociality is reciprocity (cooperation, coalitions, and exchange), which is rare in nonhuman primates even though most primates live in groups.
There are different reasons primates evolve to live in groups. Group living behavior can be driven by the need to defend resources (strength in numbers) or the need for protection from predators (safety in numbers). But groups can only evolve if the benefits to the individual outweigh the costs. (Remember, selection acts on the individual, not the group or the population.) Benefits to the individual include access to food (e.g., group hunting), access to mates, decreased predation, and communal offspring care. The costs of group living include competition for resources, competition for mates, predator risk, disease risk, and parasite risk. As a consequence, dominance hierarchies form within groups with some individuals having higher or lower rank or status than other individuals.
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