![Figure][1] Human capital and economic development. Kolkata on World Population Day, 11 July 2011. India is projected to surpass China as the most populous country in the world by 2020. Social scientists can be a contentious lot. Since Thomas Malthus issued his dire warning in 1798—and probably before then—scholars have been arguing over how many people the planet can support. There are “doomsters” who continue to predict the worst, and there are “boomsters” who argue that population growth, while worrisome in many ways, can be an engine of economic growth. In the 1960s, when there were half as many people as there are today, the doomsters had their day as fears of a “population bomb” gripped the world. And today's rapid growth rate (7 billion people will occupy the planet this year, and that number is projected to reach 9.3 billion by 2050 and keep climbing) continues to spark alarm. But so far, the neo-Malthusian worries of global mass starvation and global calamitous environmental degradation and ensuing conflict have not materialized. The startling rate of population growth of the 1960s has actually slowed as families have chosen to have fewer children, freely in some countries and under pressure in others. Since 1950, the global fertility rate has dropped by half. But at regional and local scales, the news is not good. In some parts of the world, most notably across sub-Saharan Africa, fertility and desired family size remain high, and conflict and famine are all too common. Almost all of the population growth between now and 2050 will be concentrated in the world's poorest countries, which lack the resources to support their burgeoning citizenry. In contrast, in many developed countries and even in some still-developing ones, populations are projected to shrink, and worries center on how to support the higher costs of the aging population on the backs of fewer people of working age. Find videos, dynamic graphics, a podcast, and more at [ www.scim.ag/pop2011 ][2] Today these demographic patterns spark concerns, not of a single explosion, but of “cluster bombs” in rapidly growing countries such as Nigeria and Pakistan, which are hobbled by poor governance and limited schooling capacity and already have huge numbers of poorly educated young adults without job prospects. Debate continues over how best to address these and other problems and over whether rapid population growth is best dealt with by expanding family planning programs or implementing policies that will improve livelihoods and increase the education of girls and young women—or both. Still, many experts remain optimistic that with the right mix of policies, countries can harness the opportunities for economic growth and development offered by a young and educated workforce, congregating in dense, networked urban environments. The following special section, which contains News stories by Science 's staff and research assessments by leading experts, enhanced online with videos and dynamic graphics, explores these issues, many of which continue to split demographers. [1]: pending:yes [2]: http://www.scim.ag/pop2011