 As humans societies have evolved from hunter-gatherers into civilized societies, there have been substantial changes in relations between people, with perhaps one of a few remaining biological constants being that both adult women and men must have sexual intercourse for human procreation to happen.Humans have been compared to other species in terms of sexual behavior.Neurobiologist Robert Sapolsky constructed a reproductive spectrum with opposite poles being tournament species, in which males compete fiercely for reproductive privileges with females, and pair bond arrangements, in which a male and female will bond for life.