Natural selection is the force of evolution by which individuals with heritable traits that result in greater survival and reproduction have more offspring than individuals without those traits. By having more offspring (specifically, offspring who themselves survive and reproduce), these heritable traits become more common in future generations. As an example, hominin brain size has increased dramatically over the past two million years. Our ancestors with larger brains were better able to survive and reproduce than those with smaller brains, possibly because they were better able to acquire food or navigate the social complexities of living in a large group (Dunbar 1998; Parker and Gibson 1979).
Human behavioral ecology uses the theory of evolution by natural selection to understand how modern behaviors were advantageous in our evolutionary history. For most of human history, humans lived as hunter-gatherers, meaning they collected or hunted food; they typically resided in small communities with individuals related through blood or marriage; and they had no access to modern medicines or other modern conveniences. It is useful to think about this environment—which is much different than how humans live today—to help us understand how current behaviors may have evolved. For example, humans today enjoy consuming food high in fats and sugars (Figure C.2; see Chapter 16). In the past, eating fatty and sugary food was a good survival strategy since food was limited in this environment and these foods contained a lot of calories. Over time, those individuals who sought out these foods were probably better able to survive and reproduce, resulting in a population of people today who have preferences for these foods. In modern environments, where food is abundant, this preference has likely contributed to the obesity epidemic, which increases people’s risk of cardiovascular diseases and no longer improves people’s ability to survive and reproduce.
In addition to evolutionary history, the field of human behavioral ecology also focuses on the influence of ecology. Ecology is defined as one’s physical environment, including types of resources, predators, terrain, and weather, as well as one’s social environment, including the behaviors of other individuals and cultural rules. For example, if one lives in an environment where there are abundant fruit trees, then the diet likely includes fruit. Since fruits are easy to acquire, children can engage in food gathering at young ages. In contrast, in environments like the Arctic, where there are fewer plant resources, the diet focuses more on hunting and fishing. Since these skills take longer to acquire, children may only be able to contribute to their own subsistence at older ages. One’s environment influences the behaviors in which individuals engage, such as children’s foraging.
Another component of ecology is one’s social environment, including cultural rules. Throughout the world, different cultures have quite different norms of behavior. For instance, in some societies marriages are required to be monogamous, meaning that a marriage is between just two individuals. This is a cultural norm in the United States, and it is illegal to violate this rule. In other societies, marriages can occur between one man and several women or one woman and several men, referred to as polygyny and polyandry respectively. Across the world, polyandry tends to be quite rare, and in cultures with polyandrous marriage, polygynous and monogamous marriages also occur. The age difference of married people frequently depends on the type of marriages allowed in their culture. In cultures with polygynous marriage, the age difference between husbands and wives tends to be larger than it is in monogamous cultures, as the men who are able to attract additional wives tend to have high status or wealth and are typically older than the women who are available for marriage. In cultures with fraternal polyandry, defined as the marriage of one woman to a set of brothers, marriage typically occurs when the eldest brother is ready to marry and he typically marries a woman close in age. This results in the wife being older than some of her husbands, with the exception of the eldest one. The environment (both physical and social) influences one’s behavioral options, and human behavioral ecologists examine how one’s ecology influences people’s behavior. In Figure C.3, we see a visual depiction of the field of human behavioral ecology, using evolutionary history and ecology (physical environment plus culture) to explain modern human behavior.
While physical characteristics (like height) are clearly heritable, we also know that they depend on the environment. When children grow up with poor nutrition and do not ingest enough calories, their growth is stunted. At the same time, if parents are both tall, then their child is more likely to be tall as well. Physical traits are the result of both genes and environment. Behavior is the same—dependent on both genes and environment. While there are no genes for specific behaviors, behavioral tendencies do show some level of heritability. Personality disorders, for instance, may be partially heritable, but it also depends on the environment in which a child is raised, where child neglect or sexual abuse may increase the risk of personality disorders (Johnson et al. 1999).
Human behavioral ecologists assume that even though there are not genes for specific behaviors, genes may influence behavioral tendencies. Additionally, behaviors are flexible and people use information from the environment to determine under which conditions they should behave in particular ways. For example, the ability to cooperate has evolved over evolutionary time, but whether or not an individual cooperates in a particular instance likely depends on the situation. Research shows that people are more likely to cooperate if: (1) their behavior is known to others (that is to say their identity is not anonymous); (2) it will improve their reputation; or (3) they will be punished for not cooperating (Andreoni and Petrie 2004; Fehr and Fischbacher 2003; Milinski, Semmann, and Krambeck 2002).