This is the story of a prisoner who committed a horrible crime and says he’s no longer the same person who did it. It’s also the story of why it’s so hard for us to believe him.

In the early 1960s, a young psychologist at Harvard University was assigned to teach a class on personality. Though Walter Mischel was excited to prove himself as a teacher, there was one small problem: He didn’t happen to know very much about personality.

“So, realizing I had to teach this stuff, I decided to look at the literature,” says Mischel, who now works at Columbia University. “And I found myself enormously puzzled.”

Mischel, like pretty much every other psychologist at the time, had some basic assumptions about personality. The first was that people had different personalities, and that those personalities could be defined by certain traits, such as extroversion, conscientiousness, sociability. To read more from Alix Spiegel, click here.