(CNN) Historically, men have been more likely to develop lung cancer than women in the United States, but new research indicates that this sex-based trend has flipped, with the greatest shift occurring among whites and some Hispanics born after the mid-1960s. Overall, younger women are now more likely to get lung cancer than men of the same age, the study authors say.

The new study, a collaboration between the American Cancer Society and the National Cancer Institute, offers a mix of both positive and negative results. The research appeared Wednesday in the New England Journal of Medicine.

"Over the past two decades, the age-specific incidence of lung cancer has generally decreased among both men and women 30 to 54 years of age in all races and ethnic groups," the authors note. That decline, though, has been steeper for men than women, they say.

Smoking behavior trends

Among both men and women, lung cancer is the leading cause of cancer death in the US, according to the American Cancer Society. About 80% of the total 154,000 deaths from lung cancer each year is due to smoking cigarettes.

Generally, it's been thought that fewer women than men are smokers and that female smokers tend to start at older ages and smoke fewer cigarettes than men. Yet recent studies suggest that, increasingly, women are smoking in similar ways to men.

For the new study, the researchers examined the most recent lung cancer data for 1995 through 2014 as they relate to sex, race or ethnic group, age, year of diagnosis and year of birth.

Since the mid-1960s, male-dominated incidence rates for lung cancer no longer hold true, the study shows. Lung cancer incidence rates for all age groups of women and men converged among blacks and Asians/Pacific Islanders and crossed to female predominance among Hispanics ages 40 to 49, according to the study authors.

Among whites, incidence rates for women surpassed those of men in nearly every age group: ages 30 to 34, 35 to 39, 40 to 44, and 45 to 49, the researchers say. In particular, for whites between the ages of 40 and 44, female-to-male incidence went from 12% lower in the 1995-99 period to 17% higher in the 2010-14 period.

Smoking patterns do not fully explain this change, so more research is needed, the authors say. "The prevalence of smoking among white women born after the 1970s and among Hispanics born after the 1960s approached, but did not exceed, that among their male counterparts," they wrote. "Moreover, the average number of cigarettes smoked per day continues to be considerably lower among women than among men."

It is possible that differences in the types of lung cancer affecting men and women contributed to the trend, the researchers speculate. For example, adenocarcinoma is a type of lung cancer that is more common among women -- yet the risk of this lung cancer subtype decreases more slowly than other subtypes when people stop smoking.

Another possible explanation is that women may be more susceptible to the negative effects of smoking than men although this theory has not been proved conclusively, the study authors write. Ultimately, though, they conclude, "future studies are needed to identify reasons for the higher incidence of lung cancer among young women."

'Generally getting better'

Gary Giovino, a professor and chairman of the Department of Community Health and Health Behavior at the University at Buffalo, said the new study is "very thorough."

"It is a little scary," said Giovino, who was not involved in the study but has researched lung cancer rates. "My mother died of lung cancer, so this stuff resonates with me. Things are generally getting better, which is, of course, a positive thing."

Though the researchers considered most of the possible explanations for the new results, they "obviously ran out of gas," he said.

"They didn't look at age of initiation, probably because the data aren't available every year. That would be one thing I might add to their analysis," he said. Has the age of smoking initiation come down more rapidly for women than for men? If so, this might explain the new results, he suggested.

"The second thing is, they dismissed menthol as a possible explanation, when women and young people are more likely to smoke menthol," Giovino said. There's some evidence with African-Americans who smoke fewer cigarettes per day yet have higher lung cancer rates than other groups, he said. "I'm not ready to give up on the menthol hypothesis quite yet."

Follow CNN Health on Facebook and Twitter See the latest news and share your comments with CNN Health on Facebook and Twitter.

Giovino also noted education and socioeconomic differences, since some research has shown an increase in smoking rates among less-educated women since the Virginia Slims cigarette ads of the mid-1970s.

Finally, he said, the study authors say women have lower rates of quitting smoking than men, but they fail to factor in the possibility that men who quit cigarettes are more likely to switch to or continue using cigars or pipes.

Despite these small criticisms, the study is "clearly well done," Giovino said. "If this brings more attention to tobacco use prevention and control, that's a good thing."