By examining 200 years of demographic data from 237 countries and territories, Kenji Itao from the RIKEN Center for Brain Science in Japan has discovered that universal mathematical laws describe how fertility declines as societies develop, providing a new framework for understanding one of the most profound transformations in human history: the demographic transition from high birth and death rates to longer lifespans and lower fertility.
The study, published in Evolutionary Human Sciences, found that despite vast differences in geography, culture, and economic development, the different countries and territories he analyzed follow two remarkably consistent pathways linking fertility and life expectancy.
Itao focused on the relationship between the crude birth rate—the number of births per 1,000 individuals—and life expectancy at birth. He found that there are two distinct phases of demographic transition, each governed by a different universal relationship.
The first phase, which was common in European countries up until around 1950, was characterized by high child mortality and relatively high fertility. During this phase, improvements in life expectancy are accompanied by decreases in birth rates in a manner that maintains a stable balance between reproduction and survival, supporting steady population growth. In this phase, the population growth is conserved.
In the second phase, which began around 1950 in many European countries, fertility and life expectancy follow a different universal law. Many developing countries also follow this same law, in some cases without first passing through the earlier phase. Itao’s model suggests that this shift reflects a fundamental change in how families make investments—from investing into fertility, meaning having many children, to making investments into the education of children. In this phase, GDP per capita consistently increases. Itao’s work is important in that it situates the global diversity within a quantitatively rigorous framework.
To explain the transition between the two phases, Itao developed a theoretical model based on a trade-off between reproduction and investment in education. In societies where survival is uncertain and child mortality is high, producing more offspring is an effective strategy for maintaining population and economic growth. However, when survival improves and economic opportunities expand, greater investment in each child’s education and skills becomes more economically advantageous.
The model identifies key aspects of modernization that have accelerated the transition to the second phase, including reductions in education costs and increased social mobility.
According to Itao, these findings may also provide insights into one of the major demographic challenges facing many developed countries: persistent low fertility. Rather than viewing lower birth rates as an inevitable consequence of modernization, the study suggests that policies reducing the economic burden of raising and educating children may help societies maintain or recover fertility while preserving high educational standards.
According to Itao, “Beyond the practical implications, this research demonstrates how principles from statistical physics can be used to identify simple, universal laws hidden within the complexity of human societies. By combining ideas from physics, evolutionary science, and demography, this study offers a new perspective on the forces that have shaped human populations over the past 200 years—and that will continue to influence their future.”