The study of individual differences, which is central to personality psychology, has been the most challenging and dif ficult level of analysis for evolutionary psychologists Unlike sex dif ferences, for which scientists have accumulated a lar ge empirical foundation, there is far less of a foundation for adaptive individual dif ferences. Thus, this section must necessarily be more speculative and preliminary than the previous sections.
There are a variety of ways in which individual dif ferences can be explained from the vantage point of evolutionary psychology . The most common is explaining individual dif ferences as a result of environmental dif ferences acting on species-typical (human nature) psychological mechanisms (these are sometimes called facultative traits). An analogy is the phenomenon of calluses that people sometimes develop on their hands and feet. Individual dif ferences in calluses can be explained by suggesting that different individuals are exposed to dif ferent amounts of repeated friction to their skin. All humans are presumed to have essentially the same callus-producing mechanisms, so individual differences are the result of the environmental dif ferences that activate the mechanisms to dif fering degrees. Evolutionary psychologists invoke a similar form of explanation to account for psychological individual dif ferences.
Second, individual dif ferences can emer ge from contingencies among traits (Bouchard & Loehlin, 2001). For example, "a hair-trigger temper may be advantageous if one is big and strong but not if one is small and weak" (Bouchard & Loehlin, 2001, p. 250). These individual dif ferences are a kind of facultative trait. Rather than the trait's expression being contingent on the environment, however , its expression is contingent on other traits the person has—in this case, the size and strength of one' s body.
A third source of individual dif ference stems from frequency-dependent selection: the process whereby the reproductive success (fitness) of a trait depends on it frequency relative to other traits in the population. For example, in a lar ge population of people with a cooperative disposition, selection may favor those with a cheating disposition as long as they do not get too common. As the frequency of cheaters gets more common, cooperators evolve defenses to punish cheaters, and so the success of cheating goes down. Thus, heritable individual dif ferences can be created through frequency-dependent selection.
A fourth source of individual dif ferences comes from the fact that the optimum level of a personality trait can vary over time and space. Consider as an example differences over evolutionary time (or space) in the abundance of food, perhaps due to droughts or ice-ages. In times of food scarcity , selection favors a risk-taking personality trait—one that prompts a person to risk encountering predators in order to venture widely to get food and prevent starvation. In times of food abundance, selection favors a more cautious personality disposition to reduce the risk of venturing widely in the environment. Variations over time and space in the optimum level of a trait can create heritable individual differences in personality that are maintained in the population.
In sum, the evolutionary framework identifies several sources of individual dif ferences: (1) those that arise from individuals possessing universal adaptations whose expression is contingent on the environment; (2) those that arise from contingencies with other traits; (3) those due to variation over time and space in the optimum value of a trait; and (4) those due to frequency-dependent selection. Below we explore some examples of these individual dif ferences.
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