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,” says Cornell University marketing professor Brian Wansink, who directed the study.

The study identified the trend in numerous cookbooks, but it focused on American kitchen icon “Joy of Cooking,” first published during the ’30s 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 says.

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.