Thin people have thin parents.

And genes are only partially responsible.

“Perhaps 30 percent of being thin is genetic — the rest is environment,” says James O. Hill, Ph.D., director of the Center for Human Nutrition at the University of Colorado Health Sciences Center, in Denver, and cofounder of the NWCR. If you’re raised playing sports and eating healthy, unprocessed foods, chances are you’ll continue those habits into adulthood, significantly raising your odds of staying slim.

Holly Johnson, age 45, a co-owner of a Sarasota, Florida–based marketing and public-relations firm and the mother of an eight-year-old, describes her father as a “beanpole” and says her mother still weighs “within three pounds of what she did when she married my dad.”

But while genetics were clearly in her favor, Johnson credits healthful home-cooked meals for creating a model of good eating that helps her maintain her weight. “We always had breakfast and dinner together,” she says. “I was brought up with family meals, and now my family sits down every night and lights candles. Dining and healthy eating are important to me.”