In
The selfish gene,
Richard Dawkins explains human social behaviour in terms of reproductive success. For example, women are more faithful than men, because unfaithful men can reproduce more easily than unfaithful women, under the assumption that women put more effort in raising the kids.
Dawkins assumes that genes are crucial in this process: faithfulness is supposed to be a genetic trait. This may be justified in some cases, but blind application smacks of Lamarckianism.