Jodi Forschmiedt in Metapsychology:

In Us & Them, author David Berreby takes the reader on an exhaustive and thoroughly documented journey through the differences between groups of people, both real and imagined.

Turns out, they are all imagined.

Drawing on history, biology, psychology, and recent events, Berreby demonstrates again and again that we invent categories with which to classify and group one another.  When the circumstances change, we just as easily regroup.  A classic example: the Cagots were a despised caste in fifteenth century France.  The lived apart from others, had separate entrances to churches, married only amongst themselves, had no social or political rights, and were confined to certain occupations.  This discriminatory treatment continued until the French Revolution changed the rules.  Now, though their descendants may still live in France, no trace of the Cagots remains.  The change in French law and culture simply eliminated the category.

In a fascinating chapter titled "Inventing Tradition in Oklahoma, or What I Did on My Summer