  This argument has drawbacks.  Natural selection does not always provide back-up mechanisms (I have but one liver).  Natural selection sometimes has my desires caused by affect that is produced by a belief rather than directly by the belief (my desire to run away from danger is often caused by my fear, rather than by the mere belief that there is danger).  And in these cases, as in the case of the imperfectly correlated pain and bodily injury, there seems usually to be enough affect.  The altruistic hypothesis also has some of the same problems: for example, just as there might not be enough pain, the non-instrumental desire that the child do well might not be strong enough to defeat other desires.  Indeed, without an estimate of how strong this desire is, there is no reason to think the egoistic hypothesis is less reliable.  It may have more points at which it can go wrong, but produce more care than a direct but weak altruistic mechanism.  (For many of these worries, and others, see Stich, Doris and Roedder 2010.)