Until not too long ago, most people believed human morality was based on scripture, culture or reason. Some stressed only one of those sources, others mixed all three. None would have thought to include biology. With the progress of neuroscientific research in recent years, though, a growing number of psychologists, biologists and philosophers have begun to see the brain as the base of our moral views. Noble ideas such as compassion, altruism, empathy and trust, they say, are really evolutionary adaptations that are now fixed in our brains. Our moral rules are actually instinctive responses that we express in rational terms when we have to justify them.