The second lie-detection technology uses f.M.R.I. machines to compare the brain activity of liars and truth tellers. It is based on a test called Guilty Knowledge, developed by Daniel Langleben at the University of Pennsylvania in 2001. Langleben gave subjects a playing card before they entered the magnet and told them to answer no to a series of questions, including whether they had the card in question. Langleben and his colleagues found that certain areas of the brain lighted up when people lied.

Two companies, No Lie MRI and Cephos, are now competing to refine f.M.R.I. lie-detection technology so that it can be admitted in court and commercially marketed. I talked to Steven Laken, the president of Cephos, which plans to begin selling its products this year. “We have two to three people who call every single week,” he told me. “They’re in legal proceedings throughout the world, and they’re looking to bolster their credibility.” Laken said the technology could have “tremendous applications” in civil and criminal cases. On the government side, he said, the technology could replace highly inaccurate polygraphs in screening for security clearances, as well as in trying to identify suspected terrorists’ native languages and close associates. “In lab studies, we’ve been in the 80- to 90-percent-accuracy range,” Laken says. This is similar to the accuracy rate for polygraphs, which are not considered sufficiently reliable to be allowed in most legal cases. Laken says he hopes to reach the 90-percent- to 95-percent-accuracy range — which should be high enough to satisfy the Supreme Court’s standards for the admission of scientific evidence. Judy Illes, director of Neuroethics at the Stanford Center for Biomedical Ethics, says, “I would predict that within five years, we will have technology that is sufficiently reliable at getting at the binary question of whether someone is lying that it may be utilized in certain legal settings.”

If and when lie-detection f.M.R.I.’s are admitted in court, they will raise vexing questions of self-incrimination and privacy. Hank Greely, a law professor and head of the Stanford Center for Law and the Biosciences, notes that prosecution and defense witnesses might have their credibility questioned if they refused to take a lie-detection f.M.R.I., as might parties and witnesses in civil cases. Unless courts found the tests to be shocking invasions of privacy, like stomach pumps, witnesses could even be compelled to have their brains scanned. And equally vexing legal questions might arise as neuroimaging technologies move beyond telling whether or not someone is lying and begin to identify the actual content of memories. Michael Gazzaniga, a professor of psychology at the University of California, Santa Barbara, and author of “The Ethical Brain,” notes that within 10 years, neuroscientists may be able to show that there are neurological differences when people testify about their own previous acts and when they testify to something they saw. “If you kill someone, you have a procedural memory of that, whereas if I’m standing and watch you kill somebody, that’s an episodic memory that uses a different part of the brain,” he told me. Even if witnesses don’t have their brains scanned, neuroscience may lead judges and jurors to conclude that certain kinds of memories are more reliable than others because of the area of the brain in which they are processed. Further into the future, and closer to science fiction, lies the possibility of memory downloading. “One could even, just barely, imagine a technology that might be able to ‘read out’ the witness’s memories, intercepted as neuronal firings, and translate it directly into voice, text or the equivalent of a movie,” Hank Greely writes.

Greely acknowledges that lie-detection and memory-retrieval technologies like this could pose a serious challenge to our freedom of thought, which is now defended largely by the First Amendment protections for freedom of expression. “Freedom of thought has always been buttressed by the reality that you could only tell what someone thought based on their behavior,” he told me. “This technology holds out the possibility of looking through the skull and seeing what’s really happening, seeing the thoughts themselves.” According to Greely, this may challenge the principle that we should be held accountable for what we do, not what we think. “It opens up for the first time the possibility of punishing people for their thoughts rather than their actions,” he says. “One reason thought has been free in the harshest dictatorships is that dictators haven’t been able to detect it.” He adds, “Now they may be able to, putting greater pressure on legal constraints against government interference with freedom of thought.”

In the future, neuroscience could also revolutionize the way jurors are selected. Steven Laken, the president of Cephos, says that jury consultants might seek to put prospective jurors in f.M.R.I.’s. “You could give videotapes of the lawyers and witnesses to people when they’re in the magnet and see what parts of their brains light up,” he says. A situation like this would raise vexing questions about jurors’ prejudices — and what makes for a fair trial. Recent experiments have suggested that people who believe themselves to be free of bias may harbor plenty of it all the same.

The experiments, conducted by Elizabeth Phelps, who teaches psychology at New York University, combine brain scans with a behavioral test known as the Implicit Association Test, or I.A.T., as well as physiological tests of the startle reflex. The I.A.T. flashes pictures of black and white faces at you and asks you to associate various adjectives with the faces. Repeated tests have shown that white subjects take longer to respond when they’re asked to associate black faces with positive adjectives and white faces with negative adjectives than vice versa, and this is said to be an implicit measure of unconscious racism. Phelps and her colleagues added neurological evidence to this insight by scanning the brains and testing the startle reflexes of white undergraduates at Yale before they took the I.A.T. She found that the subjects who showed the most unconscious bias on the I.A.T. also had the highest activation in their amygdalas — a center of threat perception — when unfamiliar black faces were flashed at them in the scanner. By contrast, when subjects were shown pictures of familiar black and white figures — like Denzel Washington, Martin Luther King Jr. and Conan O’Brien — there was no jump in amygdala activity.

The legal implications of the new experiments involving bias and neuroscience are hotly disputed. Mahzarin R. Banaji, a psychology professor at Harvard who helped to pioneer the I.A.T., has argued that there may be a big gap between the concept of intentional bias embedded in law and the reality of unconscious racism revealed by science. When the gap is “substantial,” she and the U.C.L.A. law professor Jerry Kang have argued, “the law should be changed to comport with science” — relaxing, for example, the current focus on intentional discrimination and trying to root out unconscious bias in the workplace with “structural interventions,” which critics say may be tantamount to racial quotas. One legal scholar has cited Phelps’s work to argue for the elimination of peremptory challenges to prospective jurors — if most whites are unconsciously racist, the argument goes, then any decision to strike a black juror must be infected with racism. Much to her displeasure, Phelps’s work has been cited by a journalist to suggest that a white cop who accidentally shot a black teenager on a Brooklyn rooftop in 2004 must have been responding to a hard-wired fear of unfamiliar black faces — a version of the amygdala made me do it.

Phelps herself says it’s “crazy” to link her work to cops who shoot on the job and insists that it is too early to use her research in the courtroom. “Part of my discomfort is that we haven’t linked what we see in the amygdala or any other region of the brain with an activity outside the magnet that we would call racism,” she told me. “We have no evidence whatsoever that activity in the brain is more predictive of things we care about in the courtroom than the behaviors themselves that we correlate with brain function.” In other words, just because you have a biased reaction to a photograph doesn’t mean you’ll act on those biases in the workplace. Phelps is also concerned that jurors might be unduly influenced by attention-grabbing pictures of brain scans. “Frank Keil, a psychologist at Yale, has done research suggesting that when you have a picture of a mechanism, you have a tendency to overestimate how much you understand the mechanism,” she told me. Defense lawyers confirm this phenomenon. “Here was this nice color image we could enlarge, that the medical expert could point to,” Christopher Plourd, a San Diego criminal defense lawyer, told The Los Angeles Times in the early 1990s. “It documented that this guy had a rotten spot in his brain. The jury glommed onto that.”

Other scholars are even sharper critics of efforts to use scientific experiments about unconscious bias to transform the law. “I regard that as an extraordinary claim that you could screen potential jurors or judges for bias; it’s mind-boggling,” I was told by Philip Tetlock, professor at the Haas School of Business at the University of California at Berkley. Tetlock has argued that split-second associations between images of African-Americans and negative adjectives may reflect “simple awareness of the social reality” that “some groups are more disadvantaged than others.” He has also written that, according to psychologists, “there is virtually no published research showing a systematic link between racist attitudes, overt or subconscious, and real-world discrimination.” (A few studies show, Tetlock acknowledges, that openly biased white people sometimes sit closer to whites than blacks in experiments that simulate job hiring and promotion.) “A light bulb going off in your brain means nothing unless it’s correlated with a particular output, and the brain-scan stuff, heaven help us, we have barely linked that with anything,” agrees Tetlock’s co-author, Amy Wax of the University of Pennsylvania Law School. “The claim that homeless people light up your amygdala more and your frontal cortex less and we can infer that you will systematically dehumanize homeless people — that’s piffle.”

V. Are You Responsible for What You Might Do? The attempt to link unconscious bias to actual acts of discrimination may be dubious. But are there other ways to look inside the brain and make predictions about an individual’s future behavior? And if so, should those discoveries be employed to make us safer? Efforts to use science to predict criminal behavior have a disreputable history. In the 19th century, the Italian criminologist Cesare Lombroso championed a theory of “biological criminality,” which held that criminals could be identified by physical characteristics, like large jaws or bushy eyebrows. Nevertheless, neuroscientists are trying to find the factors in the brain associated with violence. PET scans of convicted murderers were first studied in the late 1980s by Adrian Raine, a professor of psychology at the University of Southern California; he found that their prefrontal cortexes, areas associated with inhibition, had reduced glucose metabolism and suggested that this might be responsible for their violent behavior. In a later study, Raine found that subjects who received a diagnosis of antisocial personality disorder, which correlates with violent behavior, had 11 percent less gray matter in their prefrontal cortexes than control groups of healthy subjects and substance abusers. His current research uses f.M.R.I.’s to study moral decision-making in psychopaths.

Neuroscience, it seems, points two ways: it can absolve individuals of responsibility for acts they’ve committed, but it can also place individuals in jeopardy for acts they haven’t committed — but might someday. “This opens up a Pandora’s box in civilized society that I’m willing to fight against,” says Helen S. Mayberg, a professor of psychiatry, behavioral sciences and neurology at Emory University School of Medicine, who has testified against the admission of neuroscience evidence in criminal trials. “If you believe at the time of trial that the picture informs us about what they were like at the time of the crime, then the picture moves forward. You need to be prepared for: ‘This spot is a sign of future dangerousness,’ when someone is up for parole. They have a scan, the spot is there, so they don’t get out. It’s carved in your brain.”

Other scholars see little wrong with using brain scans to predict violent tendencies and sexual predilections — as long as the scans are used within limits. “It’s not necessarily the case that if predictions work, you would say take that guy off the street and throw away the key,” says Hank Greely, the Stanford law professor. “You could require counseling, surveillance, G.P.S. transmitters or warning the neighbors. None of these are necessarily benign, but they beat the heck out of preventative detention.” Greely has little doubt that predictive technologies will be enlisted in the war on terror — perhaps in radical ways. “Even with today’s knowledge, I think we can tell whether someone has a strong emotional reaction to seeing things, and I can certainly imagine a friend-versus-foe scanner. If you put everyone who reacts badly to an American flag in a concentration camp or Guantánamo, that would be bad, but in an occupation situation, to mark someone down for further surveillance, that might be appropriate.”

Paul Root Wolpe, who teaches social psychiatry and psychiatric ethics at the University of Pennsylvania School of Medicine, says he anticipates that neuroscience predictions will move beyond the courtroom and will be used to make predictions about citizens in all walks of life.

“Will we use brain imaging to track kids in school because we’ve discovered that certain brain function or morphology suggests aptitude?” he asks. “I work for NASA, and imagine how helpful it might be for NASA if it could scan your brain to discover whether you have a good enough spatial sense to be a pilot.” Wolpe says that brain imaging might eventually be used to decide if someone is a worthy foster or adoptive parent — a history of major depression and cocaine abuse can leave telltale signs on the brain, for example, and future studies might find parts of the brain that correspond to nurturing and caring.

The idea of holding people accountable for their predispositions rather than their actions poses a challenge to one of the central principles of Anglo-American jurisprudence: namely, that people are responsible for their behavior, not their proclivities — for what they do, not what they think. “We’re going to have to make a decision about the skull as a privacy domain,” Wolpe says. Indeed, Wolpe serves on the board of an organization called the Center for Cognitive Liberty and Ethics, a group of neuroscientists, legal scholars and privacy advocates “dedicated to protecting and advancing freedom of thought in the modern world of accelerating neurotechnologies.”

There may be similar “cognitive liberty” battles over efforts to repair or enhance broken brains. A remarkable technique called transcranial magnetic stimulation, for example, has been used to stimulate or inhibit specific regions of the brain. It can temporarily alter how we think and feel. Using T.M.S., Ernst Fehr and Daria Knoch of the University of Zurich temporarily disrupted each side of the dorsolateral prefrontal cortex in test subjects. They asked their subjects to participate in an experiment that economists call the ultimatum game. One person is given $20 and told to divide it with a partner. If the partner rejects the proposed amount as too low, neither person gets any money. Subjects whose prefrontal cortexes were functioning properly tended to reject offers of $4 or less: they would rather get no money than accept an offer that struck them as insulting and unfair. But subjects whose right prefrontal cortexes were suppressed by T.M.S. tended to accept the $4 offer. Although the offer still struck them as insulting, they were able to suppress their indignation and to pursue the selfishly rational conclusion that a low offer is better than nothing.

Some neuroscientists believe that T.M.S. may be used in the future to enforce a vision of therapeutic justice, based on the idea that defective brains can be cured. “Maybe somewhere down the line, a badly damaged brain would be viewed as something that can heal, like a broken leg that needs to be repaired,” the neurobiologist Robert Sapolsky says, although he acknowledges that defining what counts as a normal brain is politically and scientifically fraught. Indeed, efforts to identify normal and abnormal brains have been responsible for some of the darkest movements in the history of science and technology, from phrenology to eugenics. “How far are we willing to go to use neurotechnology to change people’s brains we consider disordered?” Wolpe asks. “We might find a part of the brain that seems to be malfunctioning, like a discrete part of the brain operative in violent or sexually predatory behavior, and then turn off or inhibit that behavior using transcranial magnetic stimulation.” Even behaviors in the normal range might be fine-tuned by T.M.S.: jurors, for example, could be made more emotional or more deliberative with magnetic interventions. Mark George, an adviser to the Cephos company and also director of the Medical University of South Carolina Center for Advanced Imaging Research, has submitted a patent application for a T.M.S. procedure that supposedly suppresses the area of the brain involved in lying and makes a person less capable of not telling the truth.

As the new technologies proliferate, even the neurolaw experts themselves have only begun to think about the questions that lie ahead. Can the police get a search warrant for someone’s brain? Should the Fourth Amendment protect our minds in the same way that it protects our houses? Can courts order tests of suspects’ memories to determine whether they are gang members or police informers, or would this violate the Fifth Amendment’s ban on compulsory self-incrimination? Would punishing people for their thoughts rather than for their actions violate the Eighth Amendment’s ban on cruel and unusual punishment? However astonishing our machines may become, they cannot tell us how to answer these perplexing questions. We must instead look to our own powers of reasoning and intuition, relatively primitive as they may be. As Stephen Morse puts it, neuroscience itself can never identify the mysterious point at which people should be excused from responsibility for their actions because they are not able, in some sense, to control themselves. That question, he suggests, is “moral and ultimately legal,” and it must be answered not in laboratories but in courtrooms and legislatures. In other words, we must answer it ourselves.