Nicholas Kristof, a New York Times columnist and co-author of the book Half the Sky, said the inhuman reality many girls face in the world became crystal clear when he purchased two girls from a brothel in Cambodia for about $200 each, and was given receipts.

"It's no exaggeration to talk about this as truly slavery," he said, speaking to the World Affairs Council tonight at Town Hall.

At the peak of the transatlantic slave trade, about 80,000 people were sold. Today there are 800,000 women and girls being trafficked around the world, he said.

Anywhere from 60 million to 100 million girls have disappeared from the world's population because of female infanticide and inadequate care for girls' health, Kristof said, showing photos of a skeletal child being treated in a feeding center, whose brothers were well fed and healthy.

"Every kid in the feeding center was a girl," Kristof said.

But he argued that even small interventions can transform the situation, and education is the best place to focus resources.

The U.S. has spent $11 billion in aid to Pakistan since 9/11, money which has accomplished "next to nothing," he said. If some of it had gone to education, the impact would be felt by now.

Bangladesh, by contrast, invested in girls education after it split off from Pakistan. Now there are more girls in school than boys, the country is doing relatively well and tackling its remaining problems with home grown solutions such as microcredit.

Supporting local grassroots movements for female education and economic opportunity is one way Americans can encourage change without forcing their cultural values on others, he said.

He finds the rise of social entrepreneurs a revolution that will change the world.

People want to engage in causes larger than themselves because it makes them happy, he said. Asked how he remains hopeful in the face of so much suffering, Kristof said it's because he witnesses so many selfless acts by people working in terrible conditions to save lives.

But when he comes back and sees "people who express their humanity by buying the latest car or having the latest iPod -- that is truly depressing," he said.

He advised young people to travel abroad, go outside their comfort zone, be embedded in the home of a local family.

Some people ask him why we should care about the fate of people in other countries many miles away.

"When you actually see a girl in a Cambodian brothel with her eye gouged out you don't ask that question."

What happened to the girls he bought out of slavery five years ago? Kristoff said he stayed in touch and still visits them. One is married to a good husband who doesn't know her past. The other went back to the brothel temporarily to feed her meth addiction, and later married a police officer. But now the brothel no longer exists. U.S. government pressure on Cambodia to crack down on trafficking made it risky and expensive, so the proprietor turned it into a grocery store.