A recent survey showed that 30 % of German women are childless intentionally. David P. Barash, a professor of psychology at the University of Washington, asks, "... isn't it curious — indeed, counterintuitive — that people choose, and in such large numbers, to refrain from participating in life's most pressing event?"
... [I]ntentional childlessness is indeed curious — but in no way surprising. It is also illuminating, because it sheds light on what is perhaps the most notable hallmark of the human species: the ability to say no — not just to a bad idea, an illegal order or a wayward pet but to our own genes.
When it comes to human behavior, there are actually very few genetic dictates. Our hearts insist on beating, our lungs breathing, our kidneys filtering and so forth, but these internal-organ functions are hardly "behavior" in a meaningful sense. As for more complex activities, evolution whispers within us. It does not shout orders.
People are inclined to eat when hungry, sleep when tired and have sex when aroused. But in most cases, we remain capable of declining, endowed as we are with that old bugaboo, free will. Moreover, when people indulge their biologically based inclinations, nearly always it is to satisfy an immediate itch, whose existence is itself an evolved strategy leading to some naturally selected payoff. A person doesn't typically eat, for example, with the goal of meeting her metabolic needs but to satisfy her hunger, which is a benevolent evolutionary trick that induces the food-deprived to help out their metabolism.