Do kids get fat because their moms work?

The economic mysteries of daily life.
Sept. 30 2006 8:03 AM

When Moms Work, Kids Get Fat

An incendiary new explanation for childhood obesity.

The Western world is, famously, full of fat kids. It is not clear why. Could it be because of the insidious power of advertising? Or the fear of traffic and kidnappers, which persuades parents to keep their children indoors? Or should we just blame the steady spread of fast food?

All three, it seems, are guilty—or so say economists, who in recent years have started publishing a bewildering array of explanations for the obesity epidemic.

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Shin-Yi Chou, Inas Rashad, and Michael Grossman have recently published research pointing at the effect of fast-food advertising. The difficulty of any such research project is the tangle of causal factors. A child who is watching Ronald McDonald cavort around with Hamburglar on television is a child in easy reach of snack food, a child who is not playing football in the street, and, perhaps, a child with parents who lack the inclination or time to help the him stay healthy. Or perhaps the child is simply watching television in the first place because he's too fat to enjoy playing outside.

Chou, Rashad, and Grossman nevertheless think they have found a clear effect by looking at local variations in advertising across the United States. They believe that if a given child watches an extra 30 minutes of fast-food advertisements a week, he or she will get fatter, with an increase in body mass index of about 1 percent. For adolescents the effect is twice as big, which sounds plausible given that they are likely to have more control over what they eat than younger children do.

That is all very interesting, but it does beg the question of why things are getting worse. Another trio of economists—Patricia Anderson, Kristin Butcher, and Phillip Levine—has suggested that two-income families may be producing the problem. They find that children are fatter if their mothers work longer hours. This is true even within families: The sibling who spent more time as a latchkey child will tend to be the fatter one, perhaps because the mother is less able to supervise outdoor play or has less time to cook and therefore buys more fast food. Unfortunately for working mothers who are already struck by guilt, the effects are pretty substantial. A mere 10 hours at work raises the chance of childhood obesity by 1.3 percentage points, which is about 10 percent.

Despite all the concerns about childhood obesity, most of the fat people in the world are old enough to look after themselves. So, what's going on? Here, traditional economics seems to offer a perfectly straightforward pair of explanations. First, the cost of exercise has risen: Most of us used to be paid to burn off calories in physically demanding jobs, after all. It is hard to undercut a form of exercise that pays you, and modern gyms haven't tried.

Second, food technology has tipped the balance in favor of more snacking. Think of the humble potato, once consumed in bland form, boiled or in stews: It was messy and time-consuming to make fries. But industrial processing, freezing, and vacuum-packing now make fries and chips easy to enjoy at home or in a fast-food joint. It is not just that potato chips are more calorific than boiled spuds, but that they can be conveniently eaten at any time of day. Despite the attention devoted to "supersize" portions, the calories consumed at main meals have actually declined.

Three Harvard economists, David Cutler, Edward Glaeser, and Jesse Shapiro, argue that food technology has dramatically lowered the cost in time and money of grazing on junk food all day. Judging by the ever-expanding waistlines of the developed world, that seems to be an opportunity many of us have seized upon hungrily.

Tim Harford is a Financial Times columnist. His latest book, The Logic of Life, will be published in paperback on Feb. 10.

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