Here is an excerpt from the authors of Freakonomics in the column in the NY Times:

Deborah Kattler Kupetz is a Los Angeles businesswoman and mother of three who tries to watch her weight. That’s why she recently bought two lifelike plastic models of human body fat from a medical-supply company, a one-pound blob and a five-pound blob, and put them on display in her kitchen.

By doing so, Kattler Kupetz wouldn’t seem to have much in common with Han Xin, a legendary Chinese general who lived more than 2,000 years ago. But she does.

Upon entering one battle, Han assembled his soldiers with their backs to a river so that retreat was not an option. With no choice but to attack the enemy head-on, Han’s men did just that.

This is what economists call a commitment device — a means with which to lock yourself into a course of action that you might not otherwise choose but that produces a desired result. While not as severe as Han’s strategy, Kattler Kupetz’s purchase of those fat blobs was a commitment device, too: every mealtime, they force her to envision what a few extra pounds of fat looks like.