San Francisco ? Immigrants who come to the United States live an average of three years longer than people born here, new research shows in a surprising finding that challenges some common beliefs.

A growing body of evidence indicates the life span difference reflects both immigrants’ innate vitality and their reluctance to embrace Americans’ drive-through, drive-everywhere mentality. They also smoke less.

The life-expectancy deficit is true for all races but is most dramatic among blacks. Immigrant black men live nine years longer than black men born in the United States, according to an analysis by a National Institutes of Health researcher.

The study reviewed millions of death and health records from 1986-94. Though the numbers are old, more limited studies of recent data suggest the same patterns hold true, although life expectancy is generally rising.

The records showed the average American-born black man could expect to reach 64, while a black man born overseas would likely live beyond 73 if he immigrated. In the case of an African-born man remaining in his homeland, he might have died before his 50th birthday.

Perhaps most astonishing is that immigrants outlive the U.S.-born population even though they’re more likely to be poor and less likely to see a doctor, often a prescription for a shorter life.

It’s all about lifestyle

Such results may seem counterintuitive, but their explanation makes sense.

Lifestyle is a powerful factor. Black immigrants are three times less likely to smoke than American-born blacks, according to NIH research, and far less likely to be obese. Black immigrants drink less and exercise more, according to other federal research.

It is not surprising, then, that national health statistics show black immigrants are far less likely than U.S.-born blacks to die of everything from lung cancer to liver cirrhosis.

Obesity, too, is far more prevalent among American-born residents. Data from the mid-1990s showed that 22 percent of adult immigrants were obese, compared to 28 percent of U.S.-born adults.

The smoking numbers were even more dramatic: 18 percent of immigrants smoked, compared to 26 percent of U.S.-born adults.

Habits from home

There are other factors, too, experts say: Immigrants are likely the most physically active, vigorous citizens in their homelands. They must be resilient to journey here and spread roots. They tend to benefit from stress-reducing social support networks and an outlook that, even when poor, they’re better off than before.

Some doctors have long suspected that immigrants live longer. But the findings surprise some immigrant advocates who focus more on federal policies other than health.

“People have a misconception that immigrants have poorer health, but when you look at the empirical data … you almost always find they do better than their U.S.-born counterparts,” says Gopal K. Singh, an NIH statistician. His research, published this month in the Canadian Journal of Public Health, reported that immigrant life expectancy surpassed 78, while U.S.-born life expectancy hovered at 75. (Current U.S. life expectancy is over 77 years.)

Singh found that immigrants tend to live longer, regardless of race. The difference is greatest among blacks and Hispanics, who have nearly a four-year gap between immigrants and native-born. Implicated to a lesser extent are whites and Asians/Pacific Islanders, the group with the longest life expectancy.

The American way

As they assimilate, however, many immigrants adopt bad health habits. Research by Singh and others suggest that, over time, immigrants behave like the American-born population — more smoke, drink and gain weight.

“Assimilation often means assimilation into eating too much Cheez Whiz,” says Mark Krikorian, executive director Center for Immigration Studies.

In the end, however, immigrants appear to pass on to their children some of the health advantages they enjoy. Not that it’s a piece of cake.

“There is tension over giving their child what they want — chips, fries or soda — when they know that’s not the best thing to be eating,” says Dr. Elena Fuentes-Afflick, an associate professor at the University of California, San Francisco.