Moral decisions can often feel like abstract intellectual challenges, but a number of experiments such as the one by Grafman have shown that emotions are central to moral thinking. In another experiment published in March, University of Southern California neuroscientist Antonio R. Damasio and his colleagues showed that patients with damage to an area of the brain known as the ventromedial prefrontal cortex lack the ability to feel their way to moral answers.

When confronted with moral dilemmas, the brain-damaged patients coldly came up with "end-justifies-the-means" answers. Damasio said the point was not that they reached immoral conclusions, but that when confronted by a difficult issue -- such as whether to shoot down a passenger plane hijacked by terrorists before it hits a major city -- these patients appear to reach decisions without the anguish that afflicts those with normally functioning brains.

Cheney, who told the commission he was operating on instructions from Bush given in a phone call, issued authority for aircraft threatening Washington to be shot down. But the commission noted that "among the sources that reflect other important events that morning there is no documentary evidence for this call, although the relevant sources are incomplete." Those sources include people nearby taking notes, such as Cheney's chief of staff, I. Lewis "Scooter" Libby, and Cheney's wife, Lynne.

Bush and Cheney told the commission that they remember the phone call; the president said it reminded him of his time as a fighter pilot. National security adviser Condoleezza Rice, who had joined Cheney, told the commission that she heard the vice president discuss the rules of engagement for fighter jets over Washington with Bush.

Within minutes, Cheney would use his authority. Told -- erroneously, as it turned out -- that a presumably hijacked aircraft was 80 miles from Washington, Cheney decided "in about the time it takes a batter to swing" to authorize fighter jets scrambled from Langley Air Force Base in Hampton, Va., to engage it, the commission reported.