It might seem a total wonder that a smoker won't quit after hearing that puffing away is a leading cause of death, or that an obese person can't shed a few pounds after learning that lethal ailments loom for the overweight.

But scientists have come up with a host of reasons why humans stick to bad habits, and they are zeroing in on what to do about it.

Killing Ourselves A study by scientists at the U.S. Centers for Disease Control and Prevention (CDC) found that avoidable behaviors like cigarette use, poor diet and lack of exercise were the underlying cause of half of the deaths in the United States in the year 2000. Deaths cause by: Killing Ourselves A study by scientists at the U.S. Centers for Disease Control and Prevention (CDC) found that avoidable behaviors like cigarette use, poor diet and lack of exercise were the underlying cause of half of the deaths in the United States in the year 2000. Deaths cause by:

Tobacco: 435,000

435,000 Inactivity and bad eating: 400,000

400,000 Alcohol consumption: 85,000

SOURCE: March 10, 2004 issue of the Journal of the American Medical Association

Among the reasons:

Innate human defiance.

Need for social acceptance.

Inability to truly understand the nature of risk.

Individualistic view of the world and the ability to rationalize unhealthy habits.

Genetic predisposition to addiction.

You'd think people were on a one-track mission to self-destruct rather than desiring immortality.

"We have found that people aren't changing their behaviors," said Cindy Jardine of the University of Alberta. "But it's not because they haven't gotten the information that these are big risks." She added, "We tend to sort of live for now and into the limited future—not the long term."

Killer knowledge

In a recent study, a group led by Jardine surveyed 1,200 people in Alberta, Canada in 1994 and again in 2005 about what they perceived to be risky behaviors. Many of the participants ranked lifestyle behaviors, such as smoking, drinking and sun tanning, as more dangerous than ozone depletion and chemical pollution.

In a related study that wrapped up this year, the scientists asked groups of indigenous Canadians why they ranked behaviors dangerous or not.

For instance, when asked about drinking and driving, most participants mentioned that you could hurt yourself or somebody else. If people know cigarettes can kill them or drinking and driving could be lethal, logic suggests they might quit it. Yet even with this knowledge, Jardine said, people continue to undertake these lifestyle risks.

Everybody's doing it

Jardine suggests several reasons for the contrary findings. For one, when a behavior is socially accepted or even considered desirable people tend to reconcile the fact that it's bad for them with the idea that "everybody's doing it," she said.

"I know this is bad for me but in social circles this makes me more accepted," Jardine said of the common reasoning. "It ends up being something people rationalize one way or another. And it's often easier to rationalize it in favor of trying to fit into your social group."

Addiction in Your Genes Addiction in Your Genes

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One way of making it okay to smoke like a chimney or eat like a pig is with individual experiences that support your action. For instance, you could say, "It hasn't hurt me yet," or, "My grandmother smoked all her life and lived to be 90."

In 2004, Jardine found that stress moved past cigarette smoking as the most dangerous habit.

"Most of us wear our stress as a badge of honor these days," Jardine said. So rather than thinking about stress as causing physical damage to your body and perhaps hurting family relationships, "people often boast of their stress as a success."

Risky interpretations

Typically the likelihood of contracting a disease or dying from a substance or activity is reported numerically as a percentage or ratio [see The Odds of Dying].

Ellen Peters of the University of Oregon has found that people who are better at processing numbers look at the same information differently than people not as number-minded, who tend to rely more on fear than actual hard evidence. Being afraid of cancer could drive their decisions on whether or not to smoke or the importance of treatment for particular cancers. It comes down to emotions, which Peters suggests act as guiding lights in choices.

That's one reason she thinks the "truth" campaign by the American Legacy Foundation and other anti-cigarette campaigns have been so effective. The truth ads show gruesome images such as a bleeding brain or inflamed heart with text stating cigarettes as the cause. One video ad shows a human-size rat walking up from a subway station and then collapsing on the sidewalk with a sign about how cigarettes contain rat poison.

The Truth Hurts ... and Helps The Truth Hurts ... and Helps

A study by the American Legacy Foundation showed that 22 percent of the overall decline in youth smoking from 2000 to 2002 was attributable to their "truth" campaign.

No bad behavior vaccine

Social and physical environments also play large roles in fueling poor habits.

For example, if you perceive that all of your friends are staying up all night, baking in the sun every day at the beach or taking multiple smoke breaks during work, this will affect whether you also take part in the activities.

Couch potatoes might be glued to the TV by external factors more than a lack of desire to be healthy.

"We tell people they need to become physically active, but in certain neighborhoods if you get out and go for a walk you could be putting yourself in harms way from either traffic that's not well controlled or other kinds of things like violence in your neighborhood," said Andrea Gielen of Johns Hopkins University's Bloomberg School of Public Health.

Coming up with successful pro-health campaigns requires more research and multiple strategies, experts say.

"There's no single strategy or single bullet. We're not going to be able to find a vaccine for healthy behavior," Gielen said. "We have to be more creative. We have to have different kinds of partners and work with many different folks."