The ultimate level of causation refers to the behavior’s evolutionary significance; how the behavior enhances reproductive fitness. The proximate cause refers to the immediate cause of a behavior, whether that be hormonal, neurological, cognitive, interpersonal, or cultural. For example, the proximate cause of humans eating more cookies, cake, and ice cream involves the psychological tendency to desire and enjoy foods with high sugar and fat content. The ultimate cause involves the high caloric content of both sweet and high fat foods, which promotes physical survival in resource-scarce environments.

Such environments were typical until only just recently. However, distinguishing between proximate and ultimate causes in human beings is extremely difficult, far more difficult than it is in simple animals, like insects, whose behavior is much more closely tied to their genetics. This is because one of the most important evolutionary strategies of human beings involves our remarkably developed intelligence. No other animal on earth can learn information of such complexity and modify its behavior in such diverse ways. Therefore, due to our remarkable behavioral flexibility, it is very difficult to distinguish what behavior is learned and what is genetically based.