Why do humans and other animals have sex? Is it because they are thinking, robot-like: “Must maximize reproductive success. Must maximize reproductive success”? Of course not. Yet they go about their lives behaving as if they were.

In actuality, natural selection produces organisms that exhibit relatively simple behaviors in response to certain situations or environmental conditions—behaviors that to us may seem complex and sophisticated. Let’s look at an example. An animal that experiences pleasure when it has sex has the incentive to seek out additional opportunities to experience that pleasure. As long as reproductive success is enhanced in the process, it is not necessary (from the reproductive-success standpoint) for the animal to deliberately seek that outcome. In other words, natural selection doesn’t have to produce animals consciously trying to maximize reproductive success. It only needs to produce animals that behave in a way that actually results in reproductive success, and this outcome will occur if organisms experience a pleasurable sensation by having sex. It’s like an evolutionary shortcut. Behaviors that lead to a specific outcome that increases the animal’s relative reproductive success will be favored by natural selection.

We can investigate experimentally whether natural selection sometimes results in such evolutionary short cuts—which we do by trickery. Consider the example of egg retrieval in geese. When an egg falls out of the nest, a goose retrieves it in exactly the same way every time. Is a goose able to do this because it vigilantly keeps tabs on all of its eggs? This seemed to be the case, until researchers decided to trick some geese. The researchers learned that putting any object that remotely resembles an egg near a goose’s nest triggers the retrieval process. Geese will retrieve beer cans, door knobs, and a variety of other objects that will not increase the animal’s reproductive success. Moreover, if multiple egg-like items are just outside the nest, the goose will retrieve the largest item first.

To test the limits of the retrieval behavior, researchers began putting larger and larger models of eggs outside the nests. When given a choice between an actual goose egg and an artificial egg the size of a basketball, the goose always tried to retrieve the giant egg. Rather than keeping tabs on their eggs, geese seem to be following a rule of thumb to retrieve any nearby egg-like objects, preferentially retrieving the larger objects first (FIGURE 9-7). Thus, natural selection can produce organisms that exhibit silly and clearly maladaptive (i.e., fitness-decreasing) behaviors in experiments—the goose trying to retrieve a basketball-sized egg—if, in nature, individuals’ performance of these simple actions nearly always leads to fitness-increasing behaviors—keeping eggs in the nest.

Figure 9-7Programmed to retrieve. The goose will retrieve any egg-like object outside its nest.

Taking an evolutionary approach to the study of the behavior of animals, including humans, is not new. Charles Darwin wrote in On the Origin of Species: “In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation.” And in the past several decades, Darwin’s prediction has increasingly proved true. In a synthesis of evolutionary biology and psychology, researchers taking an approach called “evolutionary psychology” have begun to view the human brain and human behaviors, including emotions, as traits produced by natural selection, selected as a result of their positive effects on survival and reproduction. So, too, has the evolutionary approach to studying behavior begun to influence the field of economics. The 2002 Nobel prize for economics was awarded to researchers who used insights from evolutionary biology and psychology in their analyses of human decision making.

In the rest of this chapter, working from the understanding that organisms’ traits, including their behaviors, have evolved by natural selection, we examine the question of why organisms behave as they do. We begin with an exploration of selfishness and cooperation in animals—in particular, how natural selection has produced organisms that engage in apparent acts of altruism.