By Pleistocene hangovers, Buchanan means: Biological traits that we have today that are the result of selective pressures that existed thousands of years ago but no longer today. Biological traits that we have today but would not have had in the environment that existed thousands of years ago. Biological traits that are better in terms of quality of life to have today, even though the selective pressures that allowed for those traits do not exist today. Selective pressures toward perfection that feel like a hangover because perfection is impossible. Selective pressures that exist today that tend to push human beings away from the pursuit of perfection.