In this blog post, we’ll explore how altruistic behavior in humans can be explained through the repeated reciprocity hypothesis and the “tit-for-tat” strategy.
“The Selfish Gene,” a book by world-renowned evolutionary biologist Richard Dawkins, has had such a profound impact on the scientific community that discussions regarding its validity and limitations have continued vigorously ever since its publication. In this book, the author argues that human genes are inherently selfish and hyperrational. Here, “hyperrational” means that genes always choose the option most advantageous to themselves from all possible choices. He also asserts that even altruistic behavior can be explained through the selfishness of genes. Various hypotheses have been proposed to support this claim, which may seem paradoxical at first glance, and in this post, I will attempt to explain one of them—the repeated reciprocity hypothesis—in simple terms.
In evolutionary theory, altruistic behavior is generally explained as an evolutionarily disadvantageous strategy. To understand this more easily, let’s consider the following scenario: Imagine that a highly altruistic person moves to an island inhabited solely by selfish people. The people there do not act for the benefit of others, and even if they receive help, they do not reciprocate. In such a situation, could an altruistic person really survive? Altruistic behavior comes at a cost, but if everyone around you is selfish, you cannot expect any reward. However, in reality, we frequently encounter examples of good deeds through the internet or the news, and various forms of altruistic behavior—such as donations and blood donations—exist in South Korean society. One of the leading theories explaining this phenomenon is the Repeated Reciprocity Hypothesis.
If, as the theory suggests, altruistic behavior were evolutionarily disadvantageous and everyone chose only selfish strategies, how would our daily lives change? For example, if the bill at a class reunion were split evenly among the group, everyone would order only the most expensive dishes, ultimately increasing each person’s share of the cost. But reality is not like that. Many people voluntarily choose to act altruistically. The repeated reciprocity hypothesis explains this phenomenon relatively simply. It posits that if the same situation repeats itself multiple times, people engage in altruistic behavior in the present because they expect to be rewarded in the future. An example of this might be taking over cleaning duty for a day to help a friend who has an urgent matter during school cleanup time. Since it’s a relationship that will continue into the future, there’s an expectation that the other person will willingly help in return when you need assistance someday. In this way, many altruistic behaviors are naturally explained by the repeated reciprocity hypothesis.
This hypothesis also explains behaviors observed in nature. Vampire bats share food with hungry companions when food is plentiful. Research has shown that individuals who previously shared food are more likely to receive help from others when they themselves become hungry later on. Another classic example supporting the repeated reciprocity hypothesis is Milinsky’s experiment on the giant spiny loach. Large spiny loaches travel in schools, and when they spot an object that appears to be a predator, some individuals risk their lives by approaching it to assess the danger. This behavior, which could endanger their lives, has long been considered a mystery.
Milinsky installed a transparent glass wall at one end of a long aquarium and placed a large fish that appeared to be a predator on the other side. On the opposite side, he placed a single large spiny loach and encouraged it to swim toward the “predator.” He then installed mirrors next to the tank—once parallel to the tank wall and another time at an angle. This was designed to make the fish mistake its reflection in the mirror for a fellow loach scouting alongside it. When the mirror was installed parallel to the tank wall, the fish perceived that its companion was also moving forward and continued to advance. Conversely, when the mirror was positioned at an angle, the fish stopped moving forward because it appeared as though its companion was lagging behind. This experiment is widely regarded as a clear demonstration of the principle of reciprocity: “I took a step forward, so now it’s your turn to take a step forward.”
Specifically, there are several strategies within the repetition-reciprocity hypothesis. Representative examples include the TFT strategy, the Trigger strategy, the GTFT strategy, and the TF2T strategy. Among these, the TFT strategy deserves particular attention. TFT stands for “Tit for Tat Strategy,” which translates to “an eye for an eye, a tooth for a tooth” in Korean. It is a very simple strategy: if the opponent cooperates, one cooperates; if the opponent defies, one defies. It is well known that in Prisoner’s Dilemma tournaments where different strategies compete, this simplest TFT strategy emerges victorious. In other words, the familiar expression “an eye for an eye, a tooth for a tooth” is one of the key strategies that explains altruistic behavior in humans.
The various altruistic behaviors commonly observed around us can be explained to a large extent by the repeated reciprocity hypothesis. This hypothesis applies particularly well in small-scale relationships—such as those with friends or coworkers—where there is a high likelihood of continued interaction in the future. Meanwhile, kin selection theory—one of the early theories explaining altruistic behavior—uses the concept of relatedness, as proposed by Hamilton, as a key concept. Relatedness refers to the genetic similarity between the agent of altruistic behavior and the beneficiary. While kin selection theory effectively explains altruistic behavior in cases of high relatedness, it fails to fully account for the phenomenon of altruistic behavior frequently occurring even among people with almost no genetic relationship. In contrast, the repeated reciprocity hypothesis explains this phenomenon by assuming relationships characterized by repeated interactions, cooperation, and the ability to retaliate. However, this theory also has its limitations. Subsequent studies have reported that altruistic behavior occurs more frequently than expected even in situations where relationships are not repeated or where precise retaliation against free-riders is not possible; this phenomenon is being further explained through other evolutionary and behavioral science theories.
