But for Berrebi and others, the shift came quickly. Many of Berrebi's friends on campus knew he was a Parisian who had lived in Israel and lost friends to terror bombings. They pressed him to explain why the terrorists had acted as they did. But he could not explain it, he says. And that void in understanding troubled him.
"After 9/11, I felt terrorism was no longer a problem of distant regions in a state of clear conflict, but a global threat," Berrebi says. "I realized that I might be able to contribute in a field mostly unexplored ... to be one of those who might help understand the factors that motivate terrorism."
One of the questions that most intrigued him: Did a fetid mix of poverty and lack of education drive some people to commit acts of terror, as pundits often claimed on TV? Berrebi hoped to find out.
Within days, he was conferring with his adviser, economist Alan Krueger, about changing his research topic. Dr. Krueger might have discouraged him: Getting data to support a new thesis would be tough.
But Berrebi had an idea for how to get the information. So Krueger gave the go-ahead, and Berrebi began by focusing on Palestinian suicide bombers. Krueger, meanwhile, would do the same, focusing on Hizbullah terrorists.
For almost two years, Berrebi prowled the Internet archives of websites belonging to the Palestinian Islamic Jihad (PIJ) and Hamas. From each, he painstakingly translated from Arabic details about individual suicide bombers, the "martyrs," as the websites called them. Krueger, for his part, culled other data sources for information.
What the two men discovered surprised them both.
Among Hamas and PIJ members, Berrebi found, only 20 percent were poor - fewer than the 32 percent who qualified as poor among a similar slice of the general Palestinian population between ages 18 and 41. But among suicide bombers, the contrast was even more pronounced: Just 13 percent were from poor families.