Page "Altruism" Paragraph 37

Every single instance of altruistic behavior need not always increase inclusive fitness ; altruistic behaviors would have been selected for if such behaviors on average increased inclusive fitness in the ancestral environment.

This need not imply that on average 50 % or more of altruistic acts were beneficial for the altruist in the ancestral environment ; if the benefits from helping the right person were very high it would be beneficial to err on the side of caution and usually be altruistic even if in most cases there were no benefits.