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Fat ChanceObesity, genetics, and responsibility.

By William Saletan
Posted Friday, Feb. 15, 2008, at 8:07 AM ET

Read Daniel Engber's "Science" article about the true cost of obesity.

(Continued from page 1)

In other words, environmental changes can significantly raise or lower average weight—as, in fact, they have—even if genes largely determine your weight relative to your peers.

Third, not all environmental factors fit into the categories evaluated in heritability studies. Non-shared environment is specific to you. Shared environment is specific to your household or some other setting. But any environmental factor that affects the whole study population is too big to show up in the analysis. I'm not talking about things each family could do. I'm talking about things that affect nearly all families simultaneously: urbanization, pollution, the arrival of television, and the proliferation of fast food.

Fourth, just because something is genetically caused doesn't mean it can't be behaviorally controlled. As an example, the authors cite phenylketonuria, a genetic disease whose symptoms are commonly averted by altering the patient's diet.

Fifth, diet and lifestyle can themselves be genetic pathways. In the case of hereditary overweight, the authors observe, "Part of the genetic effect may well be due to variations in appetite and satiety." Fidgeting has been shown to burn lots of calories; it's highly plausible that fidgeting is genetically influenced and that it drives people to exercise. To the extent that genetics overlaps with fattening behavior, old dichotomies have to be chucked. You can't just blame fat people for eating too much. Nor can you assume that because fat is hereditary, there's nothing they or society can do about it.

Accordingly, the authors propose a "behavioral, genetic model" of overweight. They embrace the liberal idea of "creating healthier external environments," along with the conservative idea of "teaching vulnerable persons to adopt life-long prudent habits." Note the latter formulation. Not everyone needs good habits. Only "vulnerable persons" do.

That's my personal takeaway from the study: Those of us who don't get fat should stifle our piety. Our relative thinness is 77 percent hereditary. I should know: I eat like a horse and can't gain weight. We need to think of obesity the way we think of alcoholism or allergies: as an unevenly distributed biological predisposition to seek or suffer harm from common environmental factors. Yes, we should struggle against it. But it's more of a struggle for some than for others.

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William Saletan is Slate's national correspondent and author of Bearing Right: How Conservatives Won the Abortion War.
Photograph of potato wedges by George Doyle.
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