http://www.time.com/time/health/article/0,8599,1700821,00.html?cnn=yes

That link didn't work for me.

Decades ago, when behavior modification was just getting rolling good, they ran experiments with "token" economies. That is, animal subjects could perform for tokens and then trade the tokens for food pellets.

The first time they ran that sort of study with chimpanzees that had more autonomy (all the chimps lived on a small island and carried their tokens with them), a strange thing happened... within 24 hours, the female chimps had all of the tokens... true story...