Why do Hispanics live longer, on average? They smoke less.

Two graduate students say they think they've solved a public health riddle that has stumped researchers over the years--why Hispanic people in America tend to live longer their white neighbors, even though they are, on average, poorer and less educated.

Income and education are directly correlated to longevity, so this exception has had scientists scratching their heads for years.

In the past, researchers have tried to explain away the so-called Hispanic Paradox by pointing to family structure and diet, but no one has been able to isolate the answer. Now, graduate students Laura Blue and Andrew Fenelon write in Scientific American that the answer is simple. Hispanic people in America tend to smoke significantly less than white people.

Blue writes:

In 2009, for instance, only 9 percent of Hispanic women were current smokers, compared with 21 percent of non-Hispanic white women; 18 percent of Hispanic men smoked, compared with 25 percent of non-Hispanic white men. Among smokers, Hispanics also consumed far fewer cigarettes on average.

Smoking accounts for about 75 percent of the difference in life expectancy between 50-year-old white and Hispanic men, Blue says. The Centers for Disease Control and Prevention says Hispanic Americans live, on average, about two and a half years longer than white Americans. The average Hispanic lives 7.7 years longer than the average African American.

Recent immigrants of all nationalities tend to live longer than the native population, even though the immigrants are poorer.

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