if you tell people that they have a genetic predisposition to certain health characteristics, such as a low capacity for exercise or a tendency to overeat, their bodies start to respond accordingly. Even if their DNA does not actually contain the gene variants in question.

Over-the-counter and prescription testing services promise to tell us about our health inheritance, including whether we are prone to weight gain, will respond well to exercise, can metabolize various foods efficiently, and face heightened — or reduced — risks for a broad range of medical conditions.