Full Idea
Two explanations came forward in the neo-Darwinian synthesis. Altruism is either 1) person-based reciprocal altruism, or 2) gene-based kin altruism.
Gist of Idea
For Darwinians, altruism is either contracts or genetics
Source
Owen Flanagan (The Really Hard Problem [2007], 2 'Darwin')
Book Reference
Flanagan,Owen: 'The Really Hard Problem' [MIT 2007], p.47
A Reaction
Flanagan obviously thinks there is also 'genuine psychological atruism'. Presumably we don't explain mathematics or music or the desire to travel as either contracts or genetics, so we have other explanations available.