For some, the joy of exercise is enough to inspire early morning runs in the park.

Others can easily decline a second helping of chocolate cake, knowing moderation today may one day lead to better health.

But many of us need extra motivation to reach for our running shoes or forgo that second treat. And there is growing consensus that when it comes to losing weight, a big-time motivator can be cold hard cash.

But there is a HUGE caveat.  

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