Human personality is the product of a long evolutionary history. As individuals, our behavior is determined by genetic composition and especially by our personal histories of reinforcement. As a species, however, we are shaped by the contingencies of survival. Natural selection plays an important part hi human personality (Skinner, 1974, 1987a, 1990a).
Individual behavior that is reinforcing tends to be repeated; that which is not tends to drop out. Similarly, those behaviors that, throughout history, were beneficial to the species tended to survive, whereas those that were only idiosyncratically reinforcing tended to drop out. For example, natural selection has favored those individuals whose pupils of then eyes dilated and contracted with changes hi lighting. Their superior ability to see during both daylight and nighttime enabled them to avoid life-threatening dangers and to survive to the age of reproduction. Similarly, infants whose heads turned in the direction of a gentle stroke on the cheek were able to suckle, thereby increasing then chances of survival and the likelihood that this rooting characteristic would be passed on to their offspring. These are but two examples of several reflexes that characterize the human infant today. Some, such as the pupillary reflex, continue to have survival value, whereas others, like the roothig reflex, are of diminishing benefit.
The contingencies of reinforcement and the contingencies of survival hiteract and some behaviors that are individually reinforcing also contribute to the survival of the species. For example, sexual behavior is generally reinforcing to an individual, but it also has natural selection value because those individuals who were most strongly aroused by sexual stimulation were also the ones most likely to produce offspring capable of similar patterns of behavior.
Not every remnant of natural selection continues to have survival value, hi humans' early history, overeathig was adaptive because it allowed people to survive during those times when food was less plentiful. Now, hi societies where food is continuously available, obesity has become a health problem, and overeating has lost its survival value.
Although natural selection helped shape some human behavior, it is probably responsible for only a small number of people's actions. Skhuier (1989a) claimed that the contingencies of reinforcement, especially those that have shaped human culture, account for most of human behavior.
We can trace a small part of human behavior ... to natural selection and the evolution of the species, but the greater part of human behavior must be traced to contingencies of reinforcement, especially to the very complex social contingencies we call cultures. Only when we take those histories into account can we explain why people behave as they do. (p. 18)
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