There are gender differences in hormones (Testosterone and Oestrogen) and evolution. In Evolutionary terms, success is measured by the production of offspring that survive to reproduce. In the EEA, successful males were physically bigger and stronger so would be those most capable of providing food and protecting their mates and their offspring. Psychologically, males who were naturally more aggressive when their resources were threatened, or when out hunting would also have an adaptive advantage. This gave them an advantage in terms of mate choice. Successful females chose mates who provided good genes, so big, strong men were favoured. Such men were also more likely to provide better resources in terms of food and protection, suggesting that those who displayed aggressive traits would be more successful than those who did not. Thus competition for mates has driven male aggression.