A new study by Stanford University scholars Jon Krosnick and Neil Malhotra shows that despite most Americans' recognizing that smoking can lead to life-threatening diseases, they don't understand how much that risk increases.

The cause, the two researchers assumed, lies in the misperception of the risk.

Published in PLOS ONE, the study analyzed data from survey interviews of more than 13,000 adults in the United States, including smokers and non-smokers, about the prospects of developing lung cancer.

In some previous studies, researchers had asked survey respondents to report how likely they thought it is that smokers and non-smokers will develop lung cancer. If a respondent thought smokers had a 30 percent chance of developing lung cancer and non-smokers a 10 percent chance, that person thinks that smokers are 20 percentage points more likely to develop cancer.

In the new study, the researchers realized that some people might not be thinking about risk that way. Instead, they proposed, people might perceive risk as the ratio of the two numbers, like answering the question, "How many times more likely to get lung cancer is a smoker than a non-smoker?" This ratio is called the "relative risk." In the 30 percent vs. 10 percent example, a person thinks of the smoker as three times more likely to develop lung cancer.

This seemingly small change in analytic approach has big consequences: most Americans overestimate the difference between the two rates of lung cancer, namely the risk of smoking, but the vast majority underestimate the relative risk. In essence, they are underestimating how much more likely it is for a smoker to develop lung cancer than a non-smoker.

The researchers did find that people who perceived more relative risk were less likely to start and more likely to quit smoking and those who perceived more of a difference between the two cancer rates were no more or less likely to start or stop smoking.

Therefore, Krosnick, a professor of communication and political science at Stanford, and Malhotra, a professor of political economy in the Graduate School of Business, suggested that if people naturally think about such dangers in terms of a ratio, then perhaps rates of smoking would be reduced if Americans are properly informed about such ratios.

For the time being, cigarette package labels in the United States include warnings from the Surgeon General that smoking can cause specific health problems without providing numeric figures to quantify the impact; while in Australia, cigarette labels include quantitative data such as, "Tobacco smoking causes more than four times the number of deaths caused by car accidents."

"Telling people how many times that a person's chances of getting lung cancer increase due to smoking may help the public make more informed choices," Krosnick, the lead author, was quoted as saying in a news release.

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