Saying it's "in the genes" might be a well-worn excuse among those looking to shed weight, but scientists have just put some serious evidence behind the notion. They found that a rare gene mutation can cause obesity in mice, reports the New York Times. The intriguing part is that these mice gained more weight than their siblings even though they ate the same amount of food growing up. The gene, called MRAP2, affects how quickly the body burns calories. When it was disabled in mice, they started packing on the pounds in youth, and the weight gain accelerated in adulthood as their appetites expanded.
“The history of obesity for many many years has been one of blaming people for lack of self control,” says the lead author. “If some of it is due to a slow metabolism, that would completely change the perspectives of parents and patients. It really would change the way we think of the disease.” A complete mutation of the MRAP2 gene is rare in humans—it's been found only in one obese child so far—but the researchers think lesser versions of it are more common, causing "subtle differences in energy regulation and metabolism," reports the Boston Globe. That would make some people more vulnerable to weight gain. (Read more obesity stories.)