In nations with high per capita energy consumption, women have fewer children. This phenomenon is an unexpected consequence of the biological scaling relationship between metabolism and reproductive rate: larger
species of mammals have higher metabolism but lower birth rates. In the April 2003 issue of Ecology Letters, Moses and Brown show that these same biological scaling rules describe the demographic transition to lower birth rates in human populations. Birth rates decline predictably with increased energy consumption, even though most of that energy comes from fossil fuels, not food.
In pre-agricultural societies, reproductive rates and metabolism are predictable from body mass, about 60 kilograms. In the wealthiest contemporary nations, each person consumes not only 100 watts in food energy, but also up to 10,000 watts of extra-metabolic energy, the metabolism of a 30,000 kilogram primate. Women in these nations have correspondingly low birth rates, resulting in fewer than 2 children per lifetime.