Whether we love them or hate them, think they're sexy, think they're strange, consider them too big, too small, or anywhere in between, humans have a complicated relationship with butts. It is a body part unique to humans, critical to our evolution and survival, and yet it has come to signify so much more: sex, desire, comedy, shame. A woman's butt, in particular, is forever being assessed, criticized, and objectified, from anxious self- examinations trying on jeans in department store dressing rooms to enduring crass remarks while walking down a street or high school hallways. But why? Here, reporter, essayist, and RadioLab contributing editor Heather Radke is determined to find out.