CONCORD, N.H. — It's not just fast food restaurants that have supersized the way Americans eat — cookbooks share the blame.

So-called portion distortion, the trend of eating larger and larger servings, is as much a problem with recipes as it is restaurants, and has been going on even longer, a study published this week in the Annals of Internal Medicine found.

The study, which looked at how classic recipes have changed during the past 70 years, found a nearly 40 percent increase in calories per serving for nearly every recipe reviewed, about an extra 77 calories.

"So much finger-pointing is going on at away-from-home dining it really takes the focus off where we could probably have the most immediate influence," said Cornell University marketing professor Brian Wansink, who directed the study.

The study identified the trend in numerous cookbooks, but it focused on the "Joy of Cooking," first published during the 1930s and regularly updated with new editions since then, most recently in 2006.

Those editions gave researchers a continuity of recipes from which to draw their data, Wansink said.

Of the 18 recipes published in all seven editions, 17 increased in calories per serving. That can be attributed partly to a jump in total calories per recipe (about 567 calories), but also to larger portion sizes.

Only the chili con carne recipe remained unchanged through the years. The chicken gumbo, however, went from making 14 servings at 228 calories each in the 1936 edition, to making 10 servings at 576 calories each in the 2006 version.

Calls to Scribner, publisher of "Joy of Cooking," were not immediately returned.

Most excess calories in the American diet still come from food eaten outside the home, said Marion Nestle, professor of nutrition and food studies at New York University. But she said the study is yet another illustration of how accustomed people are to eating ever increasing quantities of food.

And changes in "Joy of Cooking" have been going on for a while. Increases in overall calories per recipe have been gradual, but portion sizes tended to jump, first during the '40s, again during the '60s, and with the largest jump in the 2006 edition.

The first significant signs of restaurant portion inflation didn't show up until the late '70s, said Wansink.

Lisa Young, an adjunct nutrition professor at New York University, had similar findings in a 2002 study that compared the book's brownie recipe from the '60s and '70s editions to the recipe from the 1997 edition.

"Same recipe. Same pan. But in the '60s and '70s it yielded 30 brownies," she said. "In the 1997 edition it yielded 15."

She also was able to trace the trend to other recipe sources. For example, a popular chocolate chip cookie recipe that decades before produced 100 cookies, made only 60 during the '80s, though no ingredients had changed.

Wansink said he is more concerned by the increase in overall calories per recipe — what experts call caloric density — than in the portion size increases, which is a more easily recognized phenomenon.

"That (calorie increases) is more insidious because that's the sort if thing the average person wouldn't notice, wouldn't even think would have happened over the years," said Wansink, author of "Mindless Eating," an examination of why people overeat.

Much of the change can be attributed to money. Relative to household income, food is cheaper than during the '30s. So recipes once padded with less expensive (and lower-calorie) ingredients like beans, now often have more meat, Wansink said.

The scope of Wansink's study is limited. It measures the recipes only as written, not as eaten. Because people may eat more or less than the suggested serving, estimating the effect on the typical diet is challenging.

But a 40 percent increase is significant. A change of even 10 percent can affect weight, especially when dealing with high calorie foods, said Wansink.

His solution? Don't let a full portion get anywhere near your plate.

"It's not enough to just be aware," Wansink said of the recipes once intended to serve nearly twice as many people are they do today. "Put half of it away as soon as it's cooked."