Evolution is the change of individual species over time based on the reproductive selection of traits more adaptive to the environment. Evolution does not only work on physical traits, but it also selects for more adaptive behaviors. The study of evolution’s effect on behavior, including human behavior, is sociobiology.

Individual and Group Selection for Behavior

It was once believed behavioral traits evolved to create behaviors for the good of the group (i.e., group selection). However, research shows behavioral traits are selected for in the same way as physical traits: We have evolved to behave in ways that put us at a greater chance of passing on our genes. In other words, our behavior is not governed by group selection but by individual selection. This can lead to quite brutal behavior. For example, male langur monkeys live in male-dominated groups, and whenever a new male takes over, it kills the infants of the group to incite the females into ovulation and then impregnate them to pass on his own genes as opposed to the genes of his failed competitors. This competitive infanticide, first discovered in 1977, has been documented in 119 species. Now accepted, findings on this behavior were once widely criticized.