Endowed progress effect · Nunes & Drèze, 2006
A head start makes people finish.
At a car wash, customers got one of two cards. Both needed 8 paid washes for a free one. One card had 8 empty spaces; the other had 10 spaces with 2 already stamped.
8-stamp card, none stamped
10-stamp card, 2 pre-stamped
Empty 8-stamp card
19%
10-stamp card, 2 given
34%
Share of customers who completed the card. Same effort, almost twice the completions.