They say they believe in freedom and share our values. They say a few bad apples shouldn’t bring down judgment on their entire kind. Don’t be fooled. Though they walk among us with impunity, they are, in the words of Henry Farrell, a political scientist at George Washington University, “a group that is notoriously associated with terrorist violence and fundamentalist political beliefs.”

They are engineers.

Farrell, of course, was kidding. He posted that comment on a blog shortly after Umar Farouk Abdulmutallab (confessed Al Qaeda operative and engineering student) tried to blow up an airliner over Detroit last winter. But the satire was rooted in a statistical fact: in the ranks of captured and confessed terrorists, engineers and engineering students are significantly overrepresented. Maybe that’s a numerological accident. The sociologist Diego Gambetta and the political scientist Steffen Hertog don’t think so.

Each month, Gambetta and Hertog’s database grows. Last December, Abdulmutallab’s attempt over Detroit. In February, Joseph Andrew Stack, a software engineer, crashed his plane into I.R.S. offices in Austin, Tex. In March, John Patrick Bedell, an engineering grad student, opened fire at an entrance to the Pentagon. In early May, Faisal Shahzad (bachelor of science in computer science and engineering) was arrested at Kennedy Airport for a failed attempt to set off a bomb in Times Square. Also in May, Faiz Mohammad, a civil engineer, was caught at Karachi’s airport with batteries and an electrical circuit hidden in his shoes. And going back, of the 9/11 conspirators who had been educated beyond high school, eight studied engineering. As this list suggests, the phenomenon isn’t confined to Muslims or Middle Easterners.

In fact, thinking in terms of religion, nation and class has failed to explain why, for example, out of thousands of normal-acting sons of middle-class, moderate Muslim families from Nigeria, apparently only Abdulmutallab took the dive into terrorist plots. On the familiar social-science instruments, he looks the same as countless nonterrorists. What are we missing?