Identity Economics provides an important and compelling new way to understand human behavior, revealing how our identities--and not just economic incentives--influence our decisions. In 1995, economist Rachel Kranton wrote future Nobel Prize-winner George Akerlof a letter insisting that his most recent paper was wrong. Identity, she argued, was the missing element that would help to explain why people--facing the same economic circumstances--would make different choices. This was the beginning of a fourteen-year collaboration--and of Identity Economics.

The...