  Broadly speaking, sociobiology, which arises out of work in population genetics, population ecology and ethology, is the evolutionary study of human and non-human social behavior. Sociobiologists postulate that some behaviors are traits, just like height or hair color, that are subject to evolution by natural selection. Ideally, to show that a behavior is an evolutionary adaptation, researchers must demonstrate that (1) the behavior is heritable, (2) there is or was behavioral variability among individuals in a population, and (3) that differential reproduction, caused by the presence of the behavior in question, led to an increase in the frequency of individuals tending to exhibit that behavior in a population. Since researchers cannot go back in time to directly observe the evolution of current behaviors, they most often rely on indirect evidence. Sociobiology is most often associated with E.O. Wilson, either his more general work exemplified in his book Sociobiology a New Synthesis (1975) or On Human Nature (1978), which focuses on human sociobiology. There is feminist work in sociobiology such as Sarah Hrdy's work on mother-infant relations (1981/1999, 1986).