From Kermit Rose:

Re: Genes affecting generosity may be found (Oct. 31): The author of this article indicated that some biologists were puzzled why *al­tru­ism could evolve since it seemed to be a disadvantage to be altruistic. I feel strongly that altruism is not a disadvantage.

It is a considerable advantage to the individual and the species. People who are altruistic are attractive to potential mates. They are loyal and faithful, and the potential mate knows it.