darwin identifies two intrinsic compulsions facilitating mans reproductive decisions: the sexual instinct and the parental instinct. despite moral and religious prohibitions, the pleasurable sensation of sexual activity all but guarantees its perpetual propagation. historically, the ineluctable outcome of rampant sexual intercourse resulted in continuous population expansion. however, in the developed world, the advent of birth control drastically altered mans reproductive behaviors. therefore, the impetus for sexual reproduction increasingly requires intellectual incentives: for the reproduction of the race, there are two instincts needed, the sexual and the parental, and the way these are organized is to say the least curious. the sexual instinct, though much complicated by all sorts of taboos, is for most of mankind nearly as violent as the fear of death, though it has the advantage of being pleasant instead of unpleasant. among animals it brings about the inevitable consequence of reproduction, and until very recently the same was true for man, so that the malthusian increase of population was assured. this is still true for a large proportion of the human race, but the existence of birth-control has entirely altered the situation among the more highly developed peoples. the consequence has been to make reproduction depend for them not on an intense instinctive impulse, but rather on intellectual reasoning, and this for very many people is an exceedingly tepid motive.
United States