Another factor driving jihadists to drop out is a general lack of respect for the group’s leadership. Ridi testified during the embassy bombings trial that he resented having to take battlefield orders during the Afghan jihad from bin Laden and others who lacked military experience. For Ridi, the final straw was a battle in which many jihadists died thanks to inept leadership, but that al-Qaida nonetheless declared a victory. Jarrah, the 9/11 pilot, felt cut out by ringleader Mohamed Atta’s leadership style.

Another reason bad guys bail out is money. Like the rest of us, some terrorists see inadequate compensation as a sign of unfair treatment. Fadl, the Sudanese radical, fumed over his salary while al-Qaida was based in Sudan and began embezzling funds — stealing approximately $100,000 from bin Laden. When bin Laden got wind of Fadl’s theft, he ordered Fadl to repay the money; after forking over about $30,000, Fadl fled, fearing retribution.

The final factor seems to be good old family ties. Terrorists who maintain contact with friends and family outside their cell or organisation seem more likely to drop out.

There’s no obvious silver bullet here, of course. But the tales of the terrorists who weren’t are of more than academic interest. We’d do well to spend some time trying to understand how Mr. Hyde turns back into Dr. Jekyll.

Jacobson, a former staff member of the 9/11 commission, is a senior fellow at the Washington Institute for Near East Policy.