Demography is a powerful thing. Only two years ago, the UN projected a “medium variant” global population in 2050 of 9.3 billion; just recently the UN upped this projection by 300 million people to 9.6 billion. What’s striking about this upward drift in the UN’s projections is that the growth comes from a surprisingly small set of countries, mainly in sub-Saharan Africa. These include Nigeria and the Democratic Republic of Congo, as well as Niger, which currently has the world’s highest fertility rates.

Elsewhere the world has recorded general declines in fertility. Family size is falling and incomes are rising. And the relationship between those two trends is the subject of ongoing debate over whether one causes the other, and if so, which way the causality flows.

The experience of some emerging economies demonstrates that what’s most important to general societal health and well-being is not necessarily how rapidly the population is growing, but rather what policies are in place to leverage these additional brains into productive uses for society. The demographic dividend that India has reaped, for example, means that even though huge social challenges remain, the growing Indian population has been able to power middle class economic growth and social progress. Population matters, but so do policies. And policies may matter the most when a country’s population is growing fastest due to the lag between reduced infant mortality on the one hand and the built up momentum of the desire for high family size on the other. Up to two additional percentage points of India’s GDP growth in coming years may be attributable to having the right education, workforce training and economic policy frameworks in place to make the most of the demographic dividend, according to the IMF.

And what about those few parts of the world where fertility seems to keep rising? These countries will make the difference between the UN’s “high” and “medium” projections: whether the world population stabilizes around 9 billion by the end of the century, or whether we head towards 15 billion and keep going north from there. Such is the power of demography. The aim of the 2012 London Summit on Family Planning was to mobilize the resources to provide 120 million women in the world’s poorest countries with access to contraception by 2020. This isn’t about dictating anyone’s choice of family size; this is simply about respecting the choices women would like to make about spacing and limiting their pregnancies.

World Population Day is a good reminder of the promise, the power and the potential of demography. Put the right policies in place, and demographic growth will drive society forward. Fail to meet women’s need for modern contraception, and demographic power will drive us all into distress. We must be agents of change, or we will be its victims.

Author: Karl Hofmann is President and Chief Executive Officer of Population Services International (PSI) and is a member of the World Economic Forum’s Global Agenda Council on Population Growth.

Image: People walk into the entrance of a university in Wuhan REUTERS/Stringer