Nature versus nurture may be one of the greatest and most broadly applicable arguments of medical history, especially when it comes to obesity risk. At a time when one-third of Americans are either overweight or obese, it’s no surprise the average person wonders who’s at risk and why. Researchers at Harvard Medical School’s largest teaching hospital, Massachusetts General Hospital, found a new risk factor: the year a person is born.

"Looking at participants in the Framingham Heart Study, we found that the correlation between the best known obesity-associated gene variant and body mass index increased significantly as the year of birth of participants increased," the study’s lead author Dr. James Niels Rosenquist, from Massachusetts General Hospital (MGH) Department of Psychiatry, said in a press release. "These results — to our knowledge the first of their kind — suggest that this and perhaps other correlations between gene variants and physical traits may very significantly depending on when individuals were born, even for those born into the same families."

Researchers analyzed the year each person was born in the Heart Study and compared it to their body mass index (BMI), and found their birth year significantly affected their chance at an obese life. Strangely, participants born in the year 1942 or earlier had no correlation, but then again, the country wasn’t undergoing an obesity epidemic then. Their BMI was measured eight times throughout the study, and found risky years were grouped together, such as those born following World War II. It was a time when the country began relying more on technology rather than physical labor, and a boom of new high-calorie processed foods began to take over the market.

The amount of calories people eat and drink has undoubtedly been a contributor to weight gain, but so has the sedentary lifestyle America has become so dangerously accustomed to. The obesity epidemic is on a trajectory to spread thickly throughout our already thick country. Over the last 30 years, obesity rates have tripled in children and adolescents, promising a breed of heavier and unhealthier generations, according to the Centers for Disease Control and Prevention.

"We know that environment plays a huge role in the expression of genes, and the fact that our effect can be seen even among siblings born during different years implies that global environmental factors such as trends in food products and workplace activity, not just those found within families, may impact genetic traits," Rosenquist said. "Our results underscore the importance of interpreting any genetic studies with a grain of salt and leave open the possibility that new genetic risk factors may be seen in the future due to different genetically driven responses to our ever-changing environment."

Source: Rosenquist JN, Christakis N, Smoller J, Lehrer S, O’Malley J, and Zaslavsky A. Proceedings of the National Academy of Sciences. 2014.