Haynes understands the objection to these scans—he calls it "mental privacy"—but he buys only half of it. He doesn't like the idea of companies scanning job applicants for loyalty or scanning customers for reactions to products (an emerging practice known as neuromarketing). But where criminal justice is at stake, as in the case of lie detection, he's for using the technology. Ruling it out, he argues, would "deny the innocent people the ability to prove their innocence" and would "only protect the people who are guilty."
I hear what he's saying. I'd love to have put Khalid Sheikh Mohammed through an fMRI before Sept. 11, 2001, instead of waiting six years for his confession. And I wish we'd scanned Mohamed Atta's brain before he boarded that flight out of Boston. But what Haynes is saying—and exposing—is almost more terrifying than terrorism. The brain is becoming just another accessible body part, searchable for threats and evidence. We can sift through your belongings, pat you down, study your nude form through your clothes, inspect your body cavities, and, if necessary, peer into your mind.
FMRI is just the first stage. Electrodes, infrared spectroscopy, and subtler magnetic imaging are next. Scanners will shrink. Image resolution and pattern-recognition software will improve.
But don't count out free will. To make human choice predictable, you first have to constrain it so that it's not really free. That's why Haynes confined his participants to arithmetic, gave them only two options, and forbade them to change their minds. They could have wrecked his experiment by defying any of those conditions. So could you, if somebody came at you with a scanner or an electrode helmet. To look into your soul and get the right answer, science, too, has to cheat. Somewhere, Woody Allen is laughing. I can feel it.
A version of this article also appears in the Outlook section of the Sunday Washington Post.