Israeli researchers asked half the residents of a large apartment complex to sign a petition favoring the establishment of a recreation center for the handicapped. The cause was good and the request was small, so almost everyone asked agreed to sign.
Two weeks later (on National Collection Day for the Handicapped, to be precise) all the residents of the complex were approached and asked to give to the cause. A little more than half of those who were not asked to sign the earlier petition made a contribution. But an astounding 92 percent of those who did sign donated money.
People feel more obligated to live up to their commitments. Especially when those commitments are active, public and voluntary.