When millions of Americans abandoned smoking in the 1980s, many health experts and social scientists thought they had tobacco on the run. But in the 90s progress began to slow: From 1990 to 2003, according to federal figures, only 3 percent of Americans gave up their cigarettes.

The slowdown prompted many experts to conclude that most of the smokers who could easily quit had already done so. What remained was a hard-core group of Americans who continued to puff away despite significant health risks and severe social stigma.

So social scientists turned to a new quarry: Understanding the mind of the smoker. By profiling dedicated cigarette addicts, scientists hoped they would find common traits and use that knowledge to design anti-smoking campaigns.

What they have found, however, has proved more controversial than most researchers expected: Smokers are more depressed and suffer a higher rate of anxiety disorders and other psychological maladies. At the same time, nicotine may provide a mental boost that helps them cope. These findings help explain why some people won't quit, experts say.

"We thought understanding the smoker's mind would help us end tobacco use,'' said Gerald Markle, professor of sociology at Western Michigan University and author of Cigarettes: The Battle Over Smoking. "But in some respects we've raised as many new questions as we've answered.''

Recent studies show smokers are 4.7 times more likely than the population at large to suffer from major depression. Dozens of other surveys reveal cigarette users are more liable to struggle with anxiety disorders, schizophrenia, attention-deficit hyperactivity disorder, alcoholism and drug use. Smokers consistently demonstrate higher-than-average levels of neurotic and high-risk behaviors, and show poorer impulse control than nonsmokers.

"People who smoke today are, for the most part, people who are addicted in unique ways,'' said David Gilbert, a professor at Southern Illinois University who examines how nicotine affects the brain.

A recent study also shows daily smokers are more likely to consider or attempt suicide. Published last month in the Archives of General Psychiatry, the study examined the smoking habits of nearly 900 Detroit-area residents during a 10-year period; they were also interviewed about other health habits. Researchers at Michigan State University and the Henry Ford Health System in Detroit found that daily smokers were 174 percent more likely than nonsmokers and occasional cigarette consumers to seriously consider or attempt suicide, even after the results were adjusted for previous depression.

"It is unclear if daily smoking contributes to suicidal impulses or if both smoking and suicidal thoughts stem from the same root issues,'' said Naomi Breslau, one of the study's authors. "But there is a possibility that smoking increases the risk of suicide.''

These and other smoker profiles have influenced public health campaigns, contributing to the development of therapies that treat depression alongside cigarette addiction. But the studies are beginning to have an effect that is worrying to smokers: Some private and public employers are using the findings to justify employment decisions.

In January, two researchers hired by the Naval Postgraduate School in Monterey, California, to profile the smoking habits of recruits suggested possibly excluding longtime smokers because they are at higher risk of expulsion. The researchers found that navy recruits who smoke before enlistment are nearly twice as likely to be expelled for behavioral issues. The study of 6,950 recruits entering active duty in 2001 also found that smokers were five times more likely than nonsmokers to have skipped classes in high school and five times more likely to have been arrested or detained by police for non-traffic offenses.

"Smoking suggests other types of problematic behavior,'' said Eli Flyer, one of the study's authors and a former senior analyst for the Department of Defense.

The navy has not announced plans to revise enlistment policies based on the findings.

Smokers' rights advocates say studies like Flyer's will only encourage discrimination against smokers. "Everyone feels it's okay to beat up smokers and then point to these studies as justification,'' said Wanda Hamilton, a member of Forces International, a nonprofit group that opposes government smoking bans and other restrictions on tobacco use.

Concerned about the possible influence of this research, smokers' rights advocates are pushing for legislation to prevent workplace discrimination. But some employers say the research into smokers' behavior and the negative image of tobacco use provide sufficient justification for policies that are unfavorable to smokers.

Dieter Benz, vice-president at Investors Property Management, said he stopped hiring smokers in 2002 because he did not want the Seattle company associated with cigarette users.

"The image of smokers is they aren't well educated, they don't care about themselves or others, they are less mentally stable,'' said Benz. "We don't want that image associated with our company, so we won't hire them.''

But scientists involved in the studies say such interpretations of their research miss the point. Rather than suggesting smokers should be denied opportunities such as employment, they argue the research suggests that smoking addiction is treatable.

"Smokers are socially isolated and so less likely to search out help,'' said Gilbert of Southern Illinois University, who has studied smokers for more than two decades. "But these studies suggest that better treatments are out there.''

One of the treatments under discussion is using nicotine to combat depression.

Experiments conducted by Gilbert demonstrate that smokers and ex-smokers focus better on positive thoughts and images after receiving nicotine from a patch, suggesting the drug may be useful in treating the lethargy and pessimism associated with depression.

Gilbert said nicotine changes the brain's chemistry and helps some depressed people feel happier.

"Many smokers are essentially self-medicating with nicotine when they smoke, but they are using a delivery device that also contains hundreds of cancer-causing chemicals,'' said Gilbert.

The neurological effects of nicotine are still largely unknown, but results similar to Gilbert's were observed in tests on mice and chimpanzees. There is some evidence that nicotine affects neurotransmitters in a manner similar to antidepressants such as Prozac. Gilbert said scientists are trying to reconcile his results with profiles that indicate smokers are more at risk for depression and suicide.

The emerging evidence that nicotine may offer some benefits has caused a few researchers to suggest that eradicating nicotine consumption is misguided.

"We're learning that some people may need nicotine the same way other people need heart medications or anti-depressants,'' said Matt Barry, a senior policy analyst with the Campaign for Tobacco-Free Kids. "Replacing cigarettes with a less harmful source of nicotine is better.''

At a recent conference of the Society for Research on Nicotine and Tobacco, Swedish tobacco researcher Karl Fagerstrom said studies suggested that some people who are unable to quit smoking may benefit neurologically and have an easier time maintaining focus and avoiding depression through the use of long-term nicotine replacement therapies such as patches, gums and smokeless cigarettes that are used for years.

Currently, nicotine replacement therapies carry labels warning consumers to discontinue use after 12 to 24 weeks. However, those recommendations have not allayed concerns of anti-tobacco employers nor pacified smoker advocates.

Meanwhile, researchers continue to profile the mind of the average smoker, hoping to unshackle the cigarette debate from embittered politics they say hinder progress.

"I didn't expect when I started this research that I would have such varied critics,'' said Gilbert. "But anything that helps us end cigarette use is a good thing.''