Thinfluence Thin-flu-ence (noun) the powerful and surprising effect friends, family, work, and environment have on weight

Tackling a weight problem is often viewed as a personal responsibility that requires making healthier choices. The latest research, however, shows that external factors—from family and friendships to advertising and the workplace environment—make an equal, if not greater, contribution. Just look at the stats: A person's chance of becoming obese increases by 57 percent if a close friend is obese, 40 percent if a sibling is obese, and 37 percent if a spouse is obese.

That's where Thinfluence comes in. Through a research-based examination of various social, environmental, and policy-based issues, renowned Harvard researchers Dr. Walter Willett and Dr. Malissa Wood examine how relationships, workplace, media, and other factors are affecting readers' weights. Thinfluence doesn't tell readers to ditch their friends and family, change jobs, or move to another state. It offers a clear three-step action plan—analyze, act, influence—for readers to identify hidden factors affecting weight, develop a personal toolbox to combat external effects, and become positive influences on others around them.

WALTER WILLETT, MD, DrPH, is a professor of epidemiology and nutrition and chairman of the department of nutrition at Harvard School of Public Health and a professor of medicine at Harvard Medical School. His bestselling book Eat, Drink, and Be Healthy helped change the USDA Food Pyramid. He lives in Cambridge, MA.

MALISSA WOOD, MD, is a clinical cardiologist and staff physician at Massachusetts General Hospital in Boston, codirector of the Corrigan Women’s Health Program, assistant professor of clinical medicine at Harvard Medical School, and the lead investigator of the HAPPY Heart trial. She lives in Concord, MA.

DAN CHILDS is the managing editor of medical coverage for ABC News. He lives in New York City.