[mirror_neuron]

My (unsubstantiated, off-the-cuff) theory is that empathy/altruism and selfishness are held in equilibrium. Altruistic traits lead to cooperation, which leads to civilization and surplus, which makes selfishness less damaging to society (and thus more adaptive for the individual). Too many selfish individuals damage the trust that leads to that surplus, which keeps the number of the selfish in check.