Recipes in one of America's classic cookbooks keep getting bigger and richer per serving, according to a new Cornell study.

By examining the 18 recipes that have been continuously published in "The Joy of Cooking" since it was first published in 1936, Cornell Professor Brian Wansink has found that the average calories per serving have jumped 63 percent in the past 70 years.

The study, which is published in the Feb. 17 issue of the Annals of Internal Medicine (150:3), looks at recipes ranging from macaroni and cheese, beef stroganoff, Spanish rice and goulash to brownies, sugar cookies and apple pie.

"This jump in calories was influenced by both changes in ingredients -- usually increases in fat and sugar -- and changes in serving size," said Wansink, Cornell's John S. Dyson Professor of Marketing. Wansink recently returned to Cornell after a 14-month stint as executive director of the U.S. Department of Agriculture's Center for Nutrition Policy and Promotion. He co-authored the study with Collin Payne, a former Cornell postdoctoral researcher, now an assistant professor of marketing at New Mexico State University -Las Cruces.

Wansink and Payne found that the average number of calories per recipe in 1936 was 2,124, about 268 calories per serving. In 2006, the average number of calories per recipe was 3,052 calories, about 436 calories a serving.

"What served four people in 1986 would have served almost seven people by 1936 standards," Wansink noted.

In analyzing just the calorie density of the recipes -- the amount of calories in the food, regardless of serving size -- the foods in the 2006 edition had 37 percent more calories than the 18 recipes did in the 1936 edition.

"The Joy of Cooking," which has sold more than 18 million copies , is one of the country's best-known cookbooks and is considered a backbone cookbook by many home cooks, said Wansink. He said he suspected that analyses of other long-published cookbooks would yield similar results.

"People often blame eating out as being one of the big culprits for gaining weight, but this study suggests that what we do in our own homes may be equally bad or even worse," he said. "Family size has gotten smaller, but calorie content and portion sizes have gotten bigger."

What are considered "normal" portion sizes have gradually grown, Wansink said, perhaps because Americans have increasingly grown larger, the amount of income spent on food has grown proportionally smaller over the years so we can afford more food, or because we have just gotten used to the larger portions that many restaurants serve.

"To prevent overeating, I'd recommend making a recipe that says it serves four, and freezing half of it immediately for future use [assuming you are feeding four people], and only serve half of what you prepared," said Wansink, author of Mindless Eating: Why We Eat More Than We Think" (Bantam, 2006).

In other words, families need to be aware that serving size and calorie composition of classic recipes "should be downsized to counteract growing waistlines," Wansink said.