This type of study takes advantage of the fact that although identical and non-identical twins grow up in very similar environments, only identical twins share 100% of their genes. This means that if a trait is much more common in identical twins than non-identical twins, then that trait must be the result of genetic influences.

Statistical analysis of their responses revealed that genes influenced variation in both the subjects’ preference towards a mate and their own altruistic behaviour - an indication that sexual selection might be at work. 

Interestingly, there was also a genetic correlation between the two. This suggested that, in our evolutionary past, those with a stronger mate preference towards altruistic behaviour mated more frequently with more altruistic people, thus further supporting a link with sexual selection. 

Tim explained: "These results are consistent with a link between human altruism towards non-relatives and sexual selection and throws an exciting new light on the puzzle of altruistic behaviour - which appears, at first sight, to be at odds with evolutionary theory." 

"The expansion of the human brain would have greatly increased the cost of raising children so it would have been important for our ancestors to choose mates both willing and able to be good, long-term parents. Displays of altruism could well have provided accurate clues to this and so led to a link between human altruism and sexual selection." 

You can download the full research for free from BPS Journals.

Source: BPS (Press Release)