Breast Cancer, Out of the Dark

While trying to sleep in his apartment in the mid-1980s, Mr. Stevens noticed that the street lamp outside was so bright he could almost read in his bedroom. "You couldn't do that a thousand years ago or even a few hundred years ago," he says. "People just don't experience dark anymore."

That observation sent his brain racing. As an epidemiologist, Mr. Stevens had been puzzling over a curious trend in breast-cancer statistics: The rates shoot up as societies grow more industrialized. Many other cancers — stomach, colon, and liver cancers — become less prevalent as a country modernizes, largely because people benefit from better sanitation and safer food. But epidemiologists could not figure out why breast-cancer rates in modernized societies were five times the levels documented in the developing world.