Pinker took on the task of challenging two ideas that have shaped (or misshaped) contemporary thought about social issues: the blank slate, the noble savage.And he spends a chapter or two debunking another idea: the ghost in the machine.
The idea that humans are a blank slate, infinitely malleable, with capabilities that are limited only by the environment in which they are raised, is one that is oddly appealing across a broad political spectrum, but especially to a certain variety of neo-Marxists. A blank slate can be molded in socially desirable ways, thus enabling profound social change and improvement. A blank slate could be taught to abjure violence and racism, greed and sloth. Moreover, all blank slates are equal: men and women, and people of all ethnicities would have identical capability.
But humans are evolved creatures, with abilities shaped by that evolutionary history. And being evolved, there are variations in ability between individuals and, possibly, between groups. So basing one’s values or political agenda on the idea that such variations are solely induced by culture and environment is risky: what do you do when the science is in and it conflicts with the foundation of your values? Values are values: compassion, empathy, a desire for justice and equality under the law, etc.; none of these depend in any essential way on the idea that every individual is exactly identical in ability.
Pinker gives many examples of evolutionary biologists and ethnologists who have been vilified for publishing their findings, when those findings contradict the blank slate or noble savage presumption. He explains in detail the unreasoning and unreasonable basis of those attacks. He hammers home the idea that (unreasoned) attacks on scientific findings can lead to no good. If public policy is based on false ideas of human nature, bad policy will result.