"Everything can be inherited except sterility. None of your direct ancestors died childless. Consequently, if we are to understand how human nature evolved, the very core of our inquiry must be reproduction, for reproductive success is the examination that all human genes must pass if they are not to be squeezed out by natural selection. Hence I am going to argue that there are very few features of the human psyche and nature that can be understand without reference to reproduction. I begin with sexuality itself."