Turhan Canli has an odd photo collection. It includes several shots of people's faces. He flashes photos of the words "death," "happiness" and others printed in various colors. He also has images of fanged snakes and snarling dogs, babies and white supremacists. Canli, a psychologist at the State University of New York at Stony Brook, recently showed several of the pictures to Robert Sheiman, 22, a theater manager from New Haven, Conn. Before viewing them, Sheiman was put in an fMRI scanner, and as each photo flashed up, he indicated his reaction--positive, negative or neutral--by pressing buttons. The type of image that came up most often was a person's face along with a word. Sometimes the connection between face and word was vaguely comical--such as happy face along with the word "stool"--and at other times disturbing, the person crying and "happiness." After an hour in the scanner, "I came out of it shaking," says Sheiman. "It was overwhelming."

Canli thinks that the way Sheiman and others respond emotionally to these peculiar word-image combinations--and specifically what happens inside their brains--can tell a lot about what kind of people they are. "Certain types of people can't help but notice the emotional meaning of certain words," he says. Neurotic personalities tend to focus on ugly or repellent images, their brains lighting up at pictures of rotting food or mutilated bodies. Extroverts, on the other hand, prefer pleasurable objects, such as sunsets or giggling babies. For years Canli has been putting test subjects like Sheiman under the scanner and cataloging the personality types. "Can you use a brain scan to read personality?" he says. "Of course you can."

Anybody who's ever interviewed a prospective job applicant knows how important the ability to judge personality can be, and how difficult it is to do well. Personality--an ambiguous, amorphous and subjective concept--has always resisted quantitative measurement. But lately scientists like Canli have begun to zero in on the mechanisms of human physiology that bear on the perennial problem: what makes some people better suited to a particular task than others? For the first time, science may be able to offer useful insights for employers by drawing accurate conclusions about deep-rooted character traits. At --the same time, that possibility is also raising questions about ethics and privacy.

In a competitive global economy, the demand for personality tests in job recruitment has skyrocketed. Big corporations started testing for hiring in earnest more than a decade ago, but many have taken to using such tests for internal promotions as well. And small firms, governments, schools and other institutions are getting into the act. Testing is a $400 million industry in the United States. The problem, though, is that current quizzes are often little better at predicting than astrologers.

The most popular tests tend to be questionnaires based on generalized notions of personality types. In the widely used Myers-Briggs Type Indicator, test subjects answer questions about their own tendencies and are classified along four opposing scales--introversion and extroversion, sensing and intuition, thinking and feeling, judging and perceiving. Then each person is bracketed within a four-letter category. "ESTJs" for instance--extroverted, sensing, thinking, judging types--make natural leaders; they direct action and organize projects logically. Today 89 of the companies in the Fortune 100 use Myers-Briggs in hiring and promoting. Although experts concede the test seems to capture valid aspects of personality, they say it's too blunt. "It has vastly oversimplified personality, making it sound like people are walking 'types'," says Dan McAdams, a Northwestern University psychologist. "Most people are blends."

The Minnesota Multiphasic Personality Inventory, or MMPI, is widely used to ferret out substance abuse and other symptoms of social maladjustment. Although psychologists say the test has value, it tends to flag otherwise normal test takers as pathological. Still, 60 percent of U.S. police departments use the test, as do industries ranging from banking to retail. The test has been translated into more than 100 languages, including Russian and Arabic.

A more accurate generation of personality tests has now hit the market. These aren't yet the product of brain scans and other technologies, but they're derived from empirical studies about how types of people behave in certain situations. One is the NEO (Neuroticism, Extroversion and Openness) Personality Inventory, a subtler model than Myers-Briggs that measures five main traits: extroversion, neuroticism, agreeableness, conscientiousness and openness. Unlike the MMPI and notorious Rorschach test, it was designed to measure normal personalities.

Another promising test is the OPQ--Occupational Personality Questionnaire, which measures 32 factors, like sociability and the ability to persuade others. Results can be created on a country-by-country basis, which means answers given by a Lithuanian management trainee to a question, such as "I like to take charge," can be compared with those given by Lithuanians of similar background. SHL, the London-based firm that developed the test, worked with Neiman Marcus to increase sales per associate by 42 percent and reduce staff turnover by 18 percent by clearly identifying the characteristics needed for the job and designing a test to select employees who showed those traits.

As businesses grow to appreciate such tools, scientists like Canli are digging even deeper, honing in on genetic and neural predictors of personality. "If I know what the conditions are under which I see [a certain] activation pattern, I can make a good prediction as to what this person's personality traits are," Canli says. His brain scans have shown that people differ profoundly in their brain wiring, even down to primitive structures--like the amygdala, the seat of emotions--that humans share with such animals as squirrels and lizards.

Scientists are also exploring the role of genes in setting the range of personality traits a person can develop. "There are certain genes that we all share but which differ in subtle ways from one person to another," says Canli. For instance, two different forms of one gene regulate serotonin, a neurotransmitter associated with mood and emotional stability. One is short and one long, and every individual has two copies of the gene--one from each parent. Those with two short copies are more likely to be neurotic, anxious and risk-averse.

The potential of brain scans and genetic testing to reveal personality raises hard questions. Will a company someday pick a CEO by evaluating competing scans? Will divorce courts use a cotton swab to determine child custody? What to do with a job applicant if a personality test also revealed a brain tumor? Science is inexorably moving toward more-accurate knowledge of our elusive inner selves. How businesses choose to harness the new information will be just as important to watch.