For thousands of years, scholars have been theorizing about why human groups structure themselves the way they do, allowing more power and freedom to certain individuals and less to others. This article takes an evolutionist approach to that question, using a variety of mathematical models to predict changes in social organization throughout the evolutionary history of humanity and its ancestors, accounting for changes in brain size and global human populations. The authors validate their models using data from the ethnographic record. In general, they find that tendency towards hierarchy decreases with bigger brain size but increases with global population growth.