A gene that’s been shown to strongly influence obesity can help make people fatter—but only if they were born after 1942, a new study shows.

The findings bolster evidence that environmental or lifestyle changes could be impacting our health. They also show that while people have genes shaping all sorts of traits, these genes may only be activated under certain conditions. In this case, that environment might be a modern lifestyle, with cars, TV and fast food.

The gene is called FTO, and about 20 percent of white people have a variant of the gene that raises their risk of obesity. The links are clear and widely accepted by scientists. In 2007, British scientists found that people who carry two copies of this variation of the FTO gene weighed, on average, seven pounds more than people who lack it.

In the new study, researchers examined whether time affected the gene’s influence.

They used the Framingham Heart Study, a giant, ongoing study of more than 10,000 people who fill out questionnaires and get medical exams every few years. About three-quarters of them also have had their DNA sequenced and, consequently, it’s known which version of the FTO gene they have.

As might be expected, just about everyone gained weight as he or she got older.

“People born in the early 1940s or before had no increased risk for higher body mass index or obesity."

“What we wanted to see was whether there was a difference for people born in the earlier part of the cohort, during the 1920s, (when compared to) to people born in the later part if the cohort, in the 1940s and 1950s,” said Dr. James Niels Rosenquist of the Massachusetts General Hospital, who led the study.

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There was.

“People born in the early 1940s or before had no increased risk for higher body mass index or obesity” if they had the “bad” version of FTO, Rosenquist told NBC News.

"These results— to our knowledge the first of their kind — suggest that this and perhaps other correlations between gene variants and physical traits may vary significantly depending on when individuals were born, even for those born into the same families."

The study, published in the Proceedings of the National Academy of Sciences, does not attempt to explain what changed after 1942 to activate the gene. But there are some good guesses. After World War II ended, people moved to more sedentary desk jobs, they began to rely more on cars, fast food became more popular and cheaper, and television entered almost every home.

“This gene, which up until this point in time had no effect … may be some kind of McDonald’s gene or some kind of office chair gene,” Rosenquist said.

Just about all Americans are exercising less and eating more unhealthy foods, but it could be that people are more susceptible to the effects when they are younger, the researchers said.

The study also shows that time is one of the environmental factors that scientists should consider when studying how genes affect health, said Yale University medical sociologist Dr. Nicholas Christakis, who also worked on the study.

“It suggests caution or suggests humility when scientists work trying to understand the effect of genes on outcome,” Christakis told NBC News.

“The same FTO gene has been there for hundreds of years but its impact is changing."

Many scientific studies are conducted by using old data because it’s easier to place large numbers of people into a study if they’ve given blood samples over time. It’s difficult and expensive to collect fresh, new data for every study — but environment does change over time and it might be risky to make assumptions based on what happened to people living in the 1960s, 1970s or even the 1980s.

“The same FTO gene has been there for hundreds of years but its impact is changing. And our ability to see that impact changes,” said Christakis, perhaps best known for his studies showing that obesity, happiness and quitting smoking can be contagious.

The researchers hope other scientists in other parts of the world can start similar studies to see if they find the same effect, and launch fresh research to pinpoint what aspects of the environment make people with FTO gene variants more or less likely to become obese.

Earlier studies have suggested it might be exercise – Amish people with the ‘bad’ FTO gene, for example, often stay slim.