When I was twenty I went off a cliff. I was returning from Afghanistan and I’d picked up a local bus in eastern Turkey. During the night, the driver fell asleep. The bus crossed the midline, ricocheted off a truck coming in the opposite direction, and became airborne as it left the road. When it finally came to a stop, nine people were dead. I staggered from the wrecked vehicle, in darkness, hurting all over from the impact, covered in blood from a wound in the upper part of my body that I had no way of locating. In moments like these you discover what matters most. The first thing I checked was whether I had vision in both eyes. The brain agrees with this priority. And more: As far as the brain is concerned, the hands’ first order of business is to protect the face. Whatever compelling job your fingers are presently busy with, the slightest flicker of a shadow, of a looming object coming in fast toward your eyes, will reflexively interrupt that task and bring them up in a protective stance, like ten secret service agents shielding a president. This threat reflex is hard-wired. Because the fingers and the face have this special relationship, the brain always has to know where they are in relation to one another. How far are the ten soldiers from the expected impact zone? How long will it take them to get up there? Computing how far the hands are from the face, locating them in three-dimensional space, calculating possible trajectories, takes up processing capacity. When we listen to someone, we may cup our chin in a hand; we may even have both elbows on the table and support our head with the palms of both hands on our cheeks. There’s no mechanical reason for this—the neck muscles are more than able to support the head. But if hands and face are in the same place, the brain can switch off the tracking module and divert those resources to more important—and more demanding—things, like reading facial expressions.  The person with something to hide is vulnerable. In lying about his vital interests, the liar puts himself at risk. He will likely be confronted; if he’s found out, he’ll face angry retribution. The liar is both under threat and—because lying eats up a lot of brain capacity—in a computing crunch. Although the odds are against anyone actually hitting him, to the brain, a threat is a threat. So the liar is likely to let his fingers drift upward to squeeze a lip, scratch the nose, wipe the brow, or tug on an earlobe. Or the fingers may simply curl about the chin and stay there with the elbow supported by the other hand, close to where they’ll be needed in case of trouble.
When I suspect someone’s just lied to me, I like to let several seconds go by. I maintain a neutral, interested expression and I remain silent. This interval puts stress on the liar, gives no clue to whether I’ve bought the lie, and doesn’t waste my processing capacity thinking up a response. Instead, I’m intently watching his hands. I find people tend to touch their faces not before they lie or while they’re lying, but during the five seconds after they’ve lied. The action may be quite minor—just an index finger coming up to touch the nose, for example—but it’s telling in someone who hasn’t brought his hands up to his face for several minutes previously. In experienced liars, I watch for hand movements that they censor—the hand start to move up, then the movement’s cancelled.  Since the brain takes in the face as a whole, anything that prevents you from seeing the entire pattern makes it more difficult to read the other person’s emotional expression. I won’t interview a person wearing dark glasses for this reason.
The fingers that come up to protect the face also hide it. A man in a “deep thought” posture who’s using his entire hand to squeeze the lower part of his face below the nose is effectively wearing a mask. So is the person who’s thinking so hard she shields her eyes with a hand that clasps her forehead. In a false gesture of fatigue or exasperation, the hand may be drawn down the face in a smoothing motion, so that the ten soldiers literally erase the telltale expression, covering the tracks of their master. | 
