Is human altruism a sexually selected trait?

What is it about?

Evolutionary theory predicts competition in nature and that consequently individuals will act selfishly to benefit their genes. So how do we explain apparent examples of selfless or altruistic behaviour? This anomalous behaviour is prominently displayed in humans. We try to answer this question by reasoning that being a good parent would have been especially critical to benefiting your genes in human evolution. This is because the evolution of the human brain would have created enormous additional demands on our ancestors in raising children. But when choosing a mate how could you tell whether s/he would make a good parent? It would have been very easy to deceive. This would have been a particularly serious problem for females as they would have initially born the heaviest burden in child-raising, as they do in other primates. We propose that displays of altruistic behaviour (e.g. caring for an elderly person, bravely defending one’s group from outside attack) would have provided the most reliable clues as to how good a parent an individual would make. Thus genes linked to these altruistic displays would have been favoured and spread in ancestral populations as a result of sexual selection. To test this hypothesis we devised a psychometric scale (i.e. questionnaire) that sought to measure how strong a preference people had towards mates who displayed altruistic traits. If there had been selection in human evolution as we envisaged then genes linked to this preference or sexual attraction are likely to have continued to be expressed in modern populations. Our psychometric scale should therefore be able to pick up on signs of actual mate choice based on altruistic traits and also on signs of stronger female preference towards mates with these traits, as predicted.

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Why is it important?

We found evidence in one study of mate choice by couples based on altruistic behaviour displayed in their spouse/partner. We also found stronger female preference for mates displaying altruistic traits in all three studies. This evidence is therefore consistent with the hypothesis we proposed that human altruism evolved as a result of sexual selection.

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