Population growth is both an asset and a liability for development. It is an asset in that a growing population is a source of youthful labor, social security contributions, and expanding markets. From a simple arithmetical standpoint, population growth subtracts from gross domestic product growth in determining growth in per capita income. Age–gender population pyramids are an effective way of visualizing population growth as affected by birth and death rates and other events. Countries with high population growth rates have high dependency ratios. The dependency ratio is the ratio of non-working-age to working-age population. The demographic transition is a one-time population explosion that occurs as death rates fall ahead of birth rates. It comes to an end when birth rates also decline, eventually converging with death rates. For Reverend Thomas Malthus, who in 1798 wrote An Essay on the Principle of Population, the “passion between the sexes” always pushes population growth ahead of growth in food supply.