Throughout prehistoric, written, and recent history, human warfare has been commonplace. Nearly all societies engage in regular or periodic war. In many examples, human warfare has characteristics similar to chimpanzee war: an in-group fights with and kills members of the out-group. This information is not to be misinterpreted as either justifying human violence or considering it inevitable. When it comes to births and fecundity, though, humans are very different from the other great apes. Chimpanzees reproduce once every five to eight years; humans can give birth again within 18 months. It is likely that an increase in male contribution to child rearing allowed this greater fecundity.