Willpower apparently can be bought. The chance to win or lose $20 a month enticed dieters to drop an average of nine pounds in a year — four times more than participants not paid.

In the study of 100 obese employees at the Mayo Clinic in Rochester, Minn., half got weight-loss counseling, monthly weigh-ins and three-month gym memberships. The others got those things plus financial incentives.

After a year, 27 of the 50 financial-incentive participants came out ahead moneywise — and lost a little more than nine pounds, compared with 2.3 pounds for the others, said results released yesterday.

The study is set to be discussed this weekend at an American College of Cardiology conference in San Francisco.

One participant, Audrey Traun, 29, a lab training specialist who dropped from 215 pounds to 175, said the cash was a big factor — but only at the beginning.

As the study went on, she said, “it was actually more motivating to see my progress — pounds lost and how my clothes were fitting,”

One study leader, Steve Driver of the Mayo Clinic, said incentives are “not like training wheels where people learn healthy habits and then will continue them on their own.”

You have to keep the incentives up for them to work, Driver said.