Joel E. Cohen is professor of populations and head of the Laboratory of Populations at the Rockefeller University and Columbia University. He is an applied mathematician and author of “How Many People Can the Earth Support?”

The demographic future is not carved in stone.

If women have, on average, half a child more or half a child less than assumed in the U.N.’s medium projection from now to 2100, the population projected for 2100 rises to 15.8 billion or falls to 6.2 billion, which was the world’s population around 2001.

For relatively little investment, we have the ability to turn back the clock on population growth.

If people in wealthy countries with already low fertility prefer a world in 2100 of 6.2 billion prosperous people to a world of 15.8 billion mostly poor people, they should put their pennies where their preferences are -- now.

They should invest in making contraceptives available cheaply or free to 215 million women who want and need contraception. They should invest in research to improve contraception for men and women. They should immunize, de-worm, and educate all children well at least through the secondary level. They should assure adequate food and perinatal care to pregnant women, lactating women, and all children through the age of 4. These are no brainers, and they are relatively cheap.

Nor is demography destiny. In the 2009-2010 crop year, the world produced 2.26 billion metric tons of cereals. Approximately 0.2 metric tons (440 pounds) of cereal grains provide the food energy an average human needs for a year. Dividing the 2.2 billion metric tons produced by 0.2 metric tons required per person shows that current grain production could feed 11 billion people.

Of today’s (almost) 7 billion people, nearly one billion are chronically hungry. Why? Roughly one third of grain is consumed by domestic animals. More than one sixth of grain goes into industrial products like biofuels and starch, seeds and other uses. Less than half of world cereal production feeds humans. The world chooses to feed its machines and its domestic animals before it feeds its people.

Hunger is economically invisible because poor people exert little or no demand in world grain markets. This is a problem, not solely of demography, but also of economics and of human values. The wasted capacity of children whose brains are malnourished before and after birth is an enormous cost ignored in present economic accounting.