Obesity can run in families, but family lifestyle has just as much to do with teenagers’ weight as their genes do, new research shows.

“What we do as a family - our family lifestyles - matters for weight. Lifestyles aren’t just about individual behaviors,” study author Dr. Molly A. Martin, Pennsylvania State University in University Park told Reuters Health. The study is the first to demonstrate that the connection between parents and children’s weight is social as well as genetic.

“We had a gut sense that this was known or true, but in the research literature it actually had not been proven,” added Martin, a sociologist who studies families, social inequalities, and adolescent health. Instead, she said, scientists studying behavior and genetics have focused solely on the roles of genes and environment, without trying to separate out the effects of a family’s behavior.

Martin developed an equation that allowed her to account for the degree of genetic relatedness in a group 1,704 pairs of twins that ranged from identical twins to half-siblings, to tease out how much a family’s lifestyle contributed to the children’s risk of being obese.

The investigator used data from the National Longitudinal Study of Adolescent Health, a survey of seventh- to twelfth-graders launched in 1994 that included 2 years of follow-up. In 1996, 12 percent of the adolescents were overweight, while 6 percent had two obese parents, 14 percent had obese mothers only, and 5 percent had obese fathers.

A parent’s weight did contribute to the likelihood of a child’s being obese. But Martin found that lifestyle factors also played a role, in particular how many days in the past week that adolescents reported eating three square meals, and inactivity, as measured by how much time the teens spent watching TV or videos or playing computer games in an average week.

In fact, the influence of inactivity and meal frequency on the likelihood that a child would be overweight was as powerful as the effect of having a parent who was obese.

“Families’ adoption, organization and maintenance of sustainable, healthy lifestyles is quite important, especially given the challenges involved in doing so in our postindustrial, time-squeezed economy,” Martin writes in her report in the American Journal of Sociology.

In an interview, she acknowledged that eating regular meals as a family and engaging in regular physical activities can take money and time - both of which are in increasingly short supply among many American families.

“Within that challenge,” Martin added, “I think that if we find little things on a daily basis, like going for walks, playing with the dog, maybe going sledding this winter, those little things actually do matter.”

SOURCE: American Journal of Sociology, December 10, 2008.