Altruism was long thought to be unexplained by biology and therefore evidence for 'nurture.' However, in the 1970s Richard Dawkins popularized gene-centered evolution with his book The Selfish Gene , which proposed that genes could explain altruism. Geneticists worked out mathematically that altruism could be explained by its degree of relatedness to the beneficiary. In other words, you're more likely to rescue your brother than a second cousin, and more likely to rescue a cousin than a stranger. The model had a good deal of explanatory power and is still in use today.