Researchers have discovered that the reason women live longer than men is that women have smoked fewer cigarettes in their lifetimes, according to a new study in Public Health Reports, a journal published by the Department of Health and Human Services.

The study concluded that the "overwhelming" reason for the difference in longevity between men and women is cigarette smoking. The conclusion contradicts the speculation of researchers over the years that job stress and style of life might explain the eight-year gap between the life spans of men and women.

The paper, by Dean R. Gerstein of the National Research Council and Gus Miller of Indiana University of Pennsylvania, near Pittsburgh, drew some strong conclusions from the new data:

* The study warns that the widening gap between the longevity of men and of women eventually may be erased. Both women and men will die at earlier ages statistically, because women now smoke almost as much as men.

Young women are now taking up smoking at a rate much higher than young men. If women's smoking continues to increase, women will begin to be afflicted with higher and higher rates of disease at all ages, and thus pull down their average longevity, said Gerstein.

* Insurance companies that now may charge women lower premiums but also give them lower benefits based on sex are "probably doing this on the basis of the wrong factor", Gerstein said. Insurance benefits should be calculated on the basis of a history of cigarette smoking, which is the actual cause of longer female longevity, not sex.

"The new study does not necessarily contradict the idea that stress may be a factor in men dying earlier, because it could be that stress causes men to smoke and so both contribute," said Gerstein. "But I am not inclined to think that. I think that whatever other factors are left over after smoking is subtracted doesn't account for much."

The new study was conducted on a sample population of 8,300 people in Erie County, Pa. Researchers discovered that if cigarette smoking were eliminated as a factor, and the higher rate of violent death among young men were discounted, there would be no difference in the life span of men and women.

The findings are the first large study to try to explain the differences in longevity by subtracting these two factors.

American women today can expect to live to be 77.9 years old, and men can expect at birth to live to age 70.3, according to 1981 figures from the National Center for Health Statistics. The gap, which was very small at the turn of the century, has been widening ever since, and even with increased smoking among women, it is expected to continue to widen for some years.

Eventually the rise in women's smoking, however, is expected to at least match the men's rate. The gap will narrow, the researchers say, when the younger women begin to contract cancer and other diseases from smoking.

Gerstein and Miller compared the life span of men who neither died violently nor smoked more than 20 cigarettes in their lifetimes, with the life span of women and found the two statistically identical.

Gerstein said he eliminated death by violent causes in the study because there is a wide imbalance in the number of deaths by accident, homicide, and suicide between young men and women. So, he said, men who are not killed violently and do not smoke can be expected to live to the same age as women of their same social level.

"The resulting life expectancy figures for non-smoking men and women of parallel age were virtually identical. Thus, differential rates of cigarette smoking are apparently the overwhelming cause for the male-female longevity difference," the study concluded.

"Actuarial tables should be divided by smoking behavior to reflect this finding," the study added.

"When . . . women who have smoked as much as men reach the later decades of life . . . our study suggests that their lives will be shortened as much as men's and that the present differences in longevity between men and women will disappear," the study concludes.