Basic kin-selection ideas predict that your close relatives will be more disposed to help you, other things being equal, than unrelated individuals will, and you should be prepared to help them too. One cue to relatedness which humans might use is degree of facial resemblance (this is known as phenotype matching). DeBruine (2005) used a clever technique wherein an image of a stranger’s face is morphed by computer to make it more similar to the participant’s own. The participant is not aware of the nature of the morphing, and people do not consciously recognise themselves. DeBruine showed that people find self-morphed faces more trustworthy than non-self-morphed ones, but they are less physically attracted to them when they are of the opposite sex (see page 173 for a similar study of men and babies).

DeBruine, L.M. (2005). Trustworthy but not lust-worthy: context-specific effects of facial resemblance. Proceedings of the Royal Society, B 272: 919-22. [DOI: 10.1098/rspb.2004.3003]