After reading the first few comments two things stand out: 1. Genetic Predisposition is always a factor (basically for everything) and therefore has an impact

After reading the first few comments two things stand out: 1. Genetic Predisposition is always a factor (basically for everything) and therefore has an impact on prices and ultimately "fairness" (whatever your definition). One can have basically the same argument about intelligence and salaries. 2. It is also never the only factor. Once we start to acknowledge the above two facts, we can start discussing percentages - To what extend does Generic Predisposition affect Obesity? - What other factors exist that affect it similarly or more? (Gender, smoking, alcohol, white vs blue collar, leisure activities, marital status, salary, age ...) Only after that we should discuss whether it's fair: It's highly unlikely that anyone is in the worst of all above categories and a simple cap on expenses (or a progressive negative tax) may help to soften the blow to those cases while still incentivizing everyone to live a healthier life.