It is an active point of research, but the problem is the feedback. It's hard to get metrics people can't fake. Inasmuch as body weight is one of those unfake

It is an active point of research, but the problem is the feedback. It's hard to get metrics people can't fake. Inasmuch as body weight is one of those unfakeable metrics, the weekly Weight Watchers weight-in (note that you pay less if you weigh less) is an example in this area.