Oh the evolutionary answer is easy. If your parents have a gene that can make you behave in a way that reduces the chance of you reproducing, but changes your b

Oh the evolutionary answer is easy. If your parents have a gene that can make you behave in a way that reduces the chance of you reproducing, but changes your behavior in such a way that increases the chance of your kin surviving, then that gene is in good position to spread in the population through your siblings. Of course that doesn't work so well when the average fertility rate is less than 2 children per couple. Also, the same gene can sometimes have a positive and negative effect on your chances of reproducing at the same time. For instance, suppose there's a gene that makes you want to work out instead of have sex. That might just increase your chances more than decrease in an environment where you need strength to survive (or that partners are attracted to that).