You don't pay back your parents. You can't. The debt you owe them gets collected by your children, who hand it down in turn. It's a sort of entailment. Or if you don't have children of the body, it's left as a debt to your common humanity. Or to your God, if you possess or are possessed by one. The family economy evades calculation in the gross planetary product. It's the only deal I know where, when you give more than you get, you aren't bankrupted - but rather, vastly enriched.