Reciprocal altruism in humans refers to an individual behavior that gives benefit conditionally upon receiving a returned benefit, which draws on the economic concept – ″gains in trade″.Cosmides, L. & Tooby, J. (1992). Cognitive adaptations for social exchange. In Barkow, J., Cosmides, L. & Tooby, J. (ed.), The adapted mind: Evolutionary psychology and the generation of culture (pp. 163-228). New York: Oxford University Press. Human reciprocal altruism would include the following behaviors (but is not limited to): helping patients, the wounded, and the others when they are in crisis; sharing food, implement, knowledge.Trivers, L. R. (1971). The evolution of Reciprocal Altruism. The Quarterly Review of Biology, 46(1), pp. 35-57.

Overview The term ″altruism″ was firstly coined by the French philosopher Auguste Comte in the 19th century, which was derived from the French word ″altruisme″. Comte believed that altruism is a moral doctrine, which is the opposite of egoism, emphasizing the noble morality of sacrificing themselves and benefiting others. Human beings have both selfish and altruistic motivations, and altruism is used to restrain the egoistical instinct.Comte, A. (1858). The catechism of positive religion. London: John Chapman Press. Comte's altruism describes the nature of human and ethical significance, but it's completely different from the altruism in biological sciences. In evolutionary biology, altruism is an individual behavior that benefits another individual's fitness but reduces their own fitness in population.Graham, B. (2008). Selection: the mechanism of evolution. Oxford: Oxford University Press, pp. 367-368. The concept of ″altruism″ in biology arose from the debate of ″the Problem of Altruism″ in Natural Selection. Charles Darwin suggested that animals behave in the ways that can increase their survival and reproductive chances while competing with others. However, altruistic behavior – the act of helping others even if it accompanies with a personal cost – is common in the animal kingdom, like the vampire batWilkinson, G. (1988). Reciprocal Altruism in bats and other mammals. Ethology and Sociobiology. 9 (2–4): pp. 85–100. and various primates.Seyfarth, R. M. & Cheney, D. L. (1984). Grooming, alliances and reciprocal altruism in vervet monkeys. Nature. 308 (5959): pp. 541–543. Therefore, Charles Darwin regarded ″the Problem of Altruism″ as a potential fatal challenge to his concept of natural selection. In ″The Descent of Man″, Darwin (1859) wrote:Darwin, C. (1859). On the origin of species by means of Natural Selection. London: John Murray.

In 1964, William Hamilton developed mathematical model and put forward to his theory – ″Kin selection″ theory or ″inclusive fitness″ theory reveals that an altruistic gene evolved by natural selection.Hamilton, D. W. (1964). The genetic evolution of social behavior, I and II. Journal of Theoretical Biology, 7, pp. 1-52. The gene can be only shared with relatives, and reduces the individuals own fitness but boosts the fitness of their relatives and off-springs. In this way, this behavior increases the proportion of altruistic gene in population. Hamilton's rule provides mathematical inequality to state that an altruistic gene spread by natural selection only if the following condition can be satisfied: r B > C, where C is the cost to the individual performing the altruistic act, B is the benefit gained by the recipients of the altruistic act, r is the genetic relatedness between individual and recipients.

Hamilton's ″Kin selection″ theory expands the Darwinian definition of ″fitness″ and continues the same Darwinian framework that allows the spread of not only selfish genes but also altruistic genes. Nevertheless, Hamilton's theory did not support an appropriate explanation with unrelated members of other species. In order to solve this problem, Robert Trivers developed the original theory of reciprocal altruism into an attempt to explain the altruism behaviors among unrelated organisms. The idea of reciprocal altruism is straightforward: an altruistic behavior is probably selected only if a return would be obtained in the future. It is similar to the Tit-for-Tat strategy from game theory.

The theory of reciprocal altruism in humanity, based on the biological characteristics of human beings and the realistic society, explicates the interdependence and cooperation between people, as well as its rationality. It also demonstrates the original motivations and the internal mechanisms of the human cooperation, revealing the inevitability and social significance ranging from kin altruism to un-relative altruism in the human population. As a result, the subjective guess and emotion of human cooperation can be refined to a theory, and is gradually become one of the most popular explanations to a variety of social behaviors. In addition, cooperation is the most deep-seated foundation for the formation and existence of human society. Therefore, the proposition of reciprocal altruism is undoubtedly a great theoretical advance in human cognition history.

Human reciprocal altruism seems to be a huge magnetic field to interweave different disciplines closely. New exploration has been made by these disciplines at different levels from different points. Generally, the core of Human reciprocal altruism is located in the puzzle: How to overcome short-term self-interest and achieve cooperation. Ultimately, it reveals that altruistic individuals are more competitive than selfish individuals, on either improving individual fitness or resolving conflicts. The compatibility and complementarity of different theoretical perspectives lay the basis of human reciprocal altruism, and help with exploring human different viewpoints of human reciprocal altruism. The debate of human reciprocal altruism has been the focus of biology, sociology, economics and culture.

Scientific viewpoints

Biology In 1902, Peter Kropotkin published his monograph – Mutual Aid: A Factor of Evolution, and demonstrates the survival mechanisms of cooperation, based on various examples of animal and human societies.Kropotkin, P. (1902). Mutual aid: a factor of evolution. New York: Alfred A. Knope. He attempted to reveal that the law of biological evolution is mutual aid rather than survival competition. Mutual aid and cooperation are the principles of all species’ biological evolution including human beings’, and the concepts resulting in a profound influence upon biological evolution. E.O. Wilson applied the term of ″sociobiology″ as an attempt to explain social behavior of insect and thus explored the evolutionary mechanism of other animals including human such as the social behavior, altruism.Wilson, E. O. (1975). Sociobiology: the new synthesis. New York: Harvard University press. He argued that human altruistic behavior, as one of the human nature characteristics, is the result of the genetic inheritance.

In 1971, Trivers published one of the most important biological articles of the 20th century - The Evolution of Reciprocal Altruism and introduced the term of ″reciprocal altruism″ to explain the evolution of cooperation. The idea of reciprocal altruism is almost the same as kin selection, except to put emphasis on reciprocation instead of the need for genetic relatedness. It described that an altruistic trait or behavior may be selected, because the recipient will very likely to return benefits to the altruistic sender. If the reproductive benefit that the altruistic sender receives in return is larger than the cost initially incurred by the altruistic action, individuals who engage in this kind of reciprocal altruism will outbreed those who do not. Therefore, a seemingly altruistic trait can spread in a population. The reciprocation can be delayed as long as the individuals stay in the trade, and, so to speak, have sufficient long-term memory. This explains many features of human social life, for example, we do a favor for someone with the expectation that the favor would be remembered, and lead to a return in the future.

As Trivers supported the foundation for reciprocal altruism, Axelrod and Hamilton Axelrod, R. & Hamilton, W. D. (1981). The evolution of cooperation. Science, 211, pp. 1390-1396. applied the Game Theory to study the mechanism of reciprocal altruism, and attempted to answer the key question: How altruism spreads when cheating is an all-win strategy used by members of the population. In this paper, Axelrod and Hamilton revealed that reciprocating the assistance from another individual is stable in evolution as long as there are enough altruists in the population. They also demonstrate that a population of altruists can initially emerge through inclusive fitness, helping and receiving help among genetically related individuals. Subsequent work indicates that only a few altruists are initially needed for the emergence of reciprocation in a heterogeneous population.Nowak, M. & Sigmund, K. (1992). Tit for Tat in heterogeneous population. Nature, 355, pp.250–253.

Some evolutionary biologists, like Richard Dawkins, wholly endorse Axelrod and Hamilton's work in individual selection. In describing genes as being selfish, DawkinsDawkins, R. (1976). The selfish gene. Oxford: Oxford University Press. states that the organisms act altruistically against their individual interests in order to help copies of themselves in other bodies to replicate. Essentially, reciprocal altruism is the individual behavior behind selfish motivations. The bird is a prime example in the narrative of Dawkins: those altruistic birds who sacrifice their own interests by reproducing late or less during hard times would not have been able to pass their altruistic genes to the future generations, which will be dominated by the selfish genes from birds who take advantage of the situation by using up others’ food supply to reproduce their own offspring. Nevertheless, some scholars such as K. Lorenz and W. Edwards have been strongly opposed to the individual selection. In contrast, they spearhead the campaign of group selection.

The debate whether individuals/gene or group/species are the basic level/unit of selection has occurred since the 1960s. The major idea of group selection is that individuals may sacrifice their own reproductive interests for the benefit of the survival of the group to which they belong. W. EdwardsEdwards, W. V. C. (1962). Animal dispersion in relation to social behavior. Edinburgh: Oliver and Boyd. builds this argument mainly on birth related behaviors of birds. He points out that many bird species with small clutches have prolonged periods before reaching reproductive maturity, and have long breeding seasons sometimes in excess of one year. Other group selection supporters also argue that these behaviors must be social and altruistic in that, for example, when food supply is abundant, clutches are bigger than when food supply is more scarce. Birds thus can regulate their population densities below starvation levels through social conventions. All of these characteristics run contrary to the idea that fitness is defined by individual organisms attempting to selfishly reproduce. However, it soon became clear that group selection was losing the battle. In 1966, George WilliamsWilson, G. C. (1966). Adaptation and natural selection: a critique of some current evolutionary thought. Princeton: Princeton University Press. published the influential Adaptation and Natural Selection: a critique of some current evolutionary thought. By the end of the 1960s, a Neo-Darwinian interpretation of the modern synthesis had taken hold and it has become almost a gold standard that the unit of evolutionary analysis is at the individual's and the gene's level. Dawkins, Hamilton, and Trivers represent the mainstream view of individual selection.

Sociology