9.5–9.9: Cooperation, selfishness, and altruism can be better understood with an evolutionary approach.

An arctic ground squirrel in Alaska.

If we look closely, we see many behaviors in the animal world that appear to be altruistic behaviors—that is, they seem to come at a cost to the individual performing them, while benefiting a recipient. When discussing altruism, we define costs and benefits in terms of their contribution to an individual’s fitness.

Take the case of the Australian social spider (Diaea ergandros). After giving birth to about 50 hungry spiderlings, the mother’s body slowly liquefies into a nutritious fluid that the newborn spiders consume. Over the course of about 40 days, her offspring literally eat her alive; they ultimately kill their mother, but start their lives well-nourished. The cost to the mother is huge—she is unable to produce any more offspring—but there is a clear benefit to her many offspring (FIGURE 9-8).

Figure 9.8: Parental care to the extreme. The female Diaea ergandros spider feeds her offspring with her own body.

Such altruistic-appearing behavior is so common in the natural world that it puzzled Darwin. Natural selection, he believed, generally works to produce selfish behavior. The alleles that cause the individual carrying them to have the greatest reproductive success should become more common in any given generation, relative to other alleles for that same gene. At the same time, alleles that cause the individual carrying them to help increase other individuals’ reproductive success at the expense of their own success should decrease their relative frequency in a population. Put another way, natural selection should never produce altruistic behavior. Darwin worried that if the apparent instances of altruism were indeed truly altruistic, they would prove fatal to his theory.

As it turns out, Darwin’s theory is safe. Virtually all of the apparent acts of altruism in the animal kingdom prove, on closer inspection, to be not truly altruistic; instead, they have evolved as a consequence of either kin selection or reciprocal altruism.

Kindness Toward Close Relatives: Kin Selection Kin selection is a strategy by which one individual assisting another can compensate for its own decrease in fitness if it is helping a close relative in a way that increases the relative’s fitness. Kin selection can lead to the evolution of apparently-altruistic behavior toward close relatives. Suppose that, for one gene, you carry allele K, which causes you to behave in a way that increases the fitness of a close relative, while decreasing your own fitness. Because you and your relatives tend to share many of the same alleles, including allele K, your altruism may increase the frequency of K in the population. Thus the increased fitness of a relative you help might compensate for your own reduced fitness, because allele K will, overall, increase its frequency in the population when you help your relatives increase their reproductive output.

Kindness Toward Unrelated Individuals: Reciprocal Altruism Reciprocal altruism can lead to the evolution of apparently-altruistic behavior toward unrelated individuals. Suppose that, for another gene, you carry allele R, which causes you to help individuals to whom you are not related. This helps to increase their fitness and decreases yours in the process. Allele R might still increase its market share in the population, if the individuals whom you help become more likely to return the favor and help you in the future, increasing your fitness.

Seen in this light, both kin selection and reciprocal altruism can lead to the evolution of behaviors that are apparently altruistic but, in actuality, are beneficial—from an evolutionary perspective—to the individuals engaging in the behaviors.

In the next two sections, we explore kin selection and reciprocal altruism in more detail. We also investigate some of the many testable predictions about when acts of apparent kindness should occur, whom they should occur between, and how we could increase or decrease the frequency of their occurrence through a variety of modifications to the environment.

Occasionally, we see individuals engaging in behavior that is genuinely altruistic. We’ll examine how and why certain environmental situations cause individuals to behave in a way that decreases their fitness and, as such, is evolutionarily maladaptive. It is important to note here that even as genes play a central role in cooperation and conflict, people’s ability to override impulses toward selfishness is responsible for some of the rich diversity in human behavior that we see around us.