In almost every animal species on Earth, equal numbers of males and females are conceived. Why is that? Especially in populations like lions or elephant seals, where most males don’t get to mate? That’s survival of the laziest, not survival of the fittest. It turns out that in most cases, an equal balance of the sexes is the evolutionary stable strategy. From Charles Darwin to Ronald Fisher to W.D. Hamilton to Richard Dawkins, the question of why there’s as many males as females has fascinated biologists. Today, we’ll explain why that is the way it is.