Michelle Obama spoke movingly last week at a press conference about how parents agonize over the pain bullies inflict on children. Maybe she should talk to Casey Heynes about that. Heynes is a 16-year-old Australian fat kid who according to his father has been bullied for years by classmates about his weight. A few days ago, some of them decided to record their latest attack on a camera phone.

The first lady would, no doubt, be horrified by the suggestion that her Let’s Move campaign, which is dedicated to trying to create an America without any fat kids, is itself a particularly invidious form of bullying. But practically speaking, that’s exactly what it is. The campaign is in effect arguing that the way to stop the bullying of fat kids is to get rid of fat kids.

The whole Let’s Move campaign is like a Tea Partier’s fever dream of wrongheaded government activism. Now, as a liberal, I believe that government activism is often justified. For more than a generation, this idea has been attacked relentlessly by conservatives, and now the Tea Party movement is subjecting it to fresh assaults. Given our political climate, it’s more important than ever for liberals not to assume that a particular government initiative to stop something from happening is a good idea. Rather, we need to be reasonably certain that a) the something in question is actually happening; b) we know why it’s happening; c) we know how to stop it from happening; and d) the benefits of stopping it from happening are worth the costs.

Any time liberals support an ambitious government program that fails to meet this test, we are empowering the successors of Ronald Reagan, who famously declared that “the nine most terrifying words in the English language are ‘I’m from the government and I’m here to help.’”

The Let’s Move campaign fails this test spectacularly. It has had one notable success, however: According to a Pew Foundation poll, nearly three in five Americans now believe that the government should have “a significant role in reducing childhood obesity.”

Predictably, the prevalence of this belief tends to split along partisan lines: 80 percent of liberal Democrats compared to only 37 percent of conservative Republicans and 33 percent of self-described Tea Partiers.

Fat kids have enough problems without government-approved pseudo-scientific garbage about how they could be thin if they just ate their vegetables and played outside more often.

New York Times columnist Charles Blow sees the poll results as evidence that conservatives will oppose anything proposed by Mrs. Obama or her husband, “no matter how innocuous or admirable.” But there’s nothing innocuous or admirable about this crusade. The “childhood obesity epidemic,” to the extent that concept ever made any sense, may well be over. As Australian scholar Michael Gard points out in his new book, The End of the Obesity Epidemic, over the last decade obesity rates among both adults and children have leveled off or declined all over the world, including in the United States. Contrary to alarmist predictions from just a few years ago that by the middle of this century all Americans would be overweight or obese, the “obesity epidemic” has, for the time being at least, stopped. Americans weigh no more than they did a decade ago.

The fact that Americans did not gain weight in the 2000s merely highlights that we don’t know why body mass levels increased in the 1980s and 1990s, or indeed why they remained basically stable in the 1960s and 1970s. We don’t know if adults or children consume more calories today than they did forty years ago: Even weakly reliable statistics regarding this question don’t exist. Similarly, we don’t know if people today are less active than they were a generation ago. Nor do we know if caloric intake and activity levels have changed over the past 10 years, when the “obesity epidemic” apparently ended.

In the face of all this, public health authorities invoke what people always invoke when they don’t have any good data: “common sense.” They argue that it’s just common sense that Americans got fatter in the 1980s and 1990s because they ate more, or were less active, or both. But these are far from the only explanations for weight gain in populations. For instance dieters tend to gain more weight over time than non-dieters, non-smokers gain more weight than smokers, and people generally gain weight as they age. Since the 1960s, smoking rates have plummeted, the median age of the population has gone up by nearly 10 years, and dieting has become much more common. In addition, even if we assume that weight gain in the 1980s and 1990s was caused exclusively by changes in caloric consumption and/or activity levels, it’s crucial, from a public policy perspective, to have a good idea what the relative contribution of these factors was. If Americans aren’t eating more than they were a generation ago, attempts to get them to eat less are especially likely to fail. But we simply don’t know whether this is the case.

Remarkably, debates about whether the government ought to have a role in making American children thinner almost never acknowledge that we have no idea how to do this. Consider the first lady’s major policy goals: She wants children to eat a healthy balance of nutritious food, both in their homes and at school, and she advocates various reforms that will make it easier for kids to be physically active. These are laudable goals in themselves, but there is no evidence that achieving them would result in a thinner population. Indeed ambitious, resource-intensive versions of Mrs. Obama’s initiatives have been implemented on a smaller scale, for example by the Johns Hopkins University Pathways program, which attempted to improve the diets and increase the activity levels of Native American children in three states, while educating their families about health and nutrition. The program had some success in all these areas, but it produced no weight loss among the children as a group. The same basic results, improved health habits but no weight loss, were obtained in the Child and Adolescent Trial for Cardiovascular Health, a similar program involving thousands of ethnically diverse children in four states. Pursuing comparable initiatives at a national level might be worthwhile-these programs did, after all, result in improved health habits among the children who participated-but there is no reason to think the kinds of reforms Mrs. Obama is advocating will make American children thinner. The perverse result could be that an initiative that might have been judged a success had its primary focus been on producing healthier children will instead end up being used as another example of a failed Big Government program, simply because it did not produce thinner ones.

For the sake of argument, let’s assume there’s actually an ongoing childhood obesity epidemic, that we understand what is causing it, and that we know how to stop it. Even assuming all this, does it make sense to try to make American children thinner, as opposed to merely healthier? Why, after all, is such a goal so important in an age of increasingly scarce public health resources? At this point, we need to consider how the concept of “childhood obesity” got defined in the first place. The Centers for Disease Control website offers these definitions of “overweight” and “obesity” in children:

· Overweight is defined as a BMI at or above the 85th percentile and lower than the 95th percentile for children of the same age and sex. · Obesity is defined as a BMI at or above the 95th percentile for children of the same age and sex.

These definitions raise a couple of obvious questions in a nation that has been bombarded with claims that childhood obesity is skyrocketing. After all, by this standard, aren’t exactly 10 percent of children always overweight by definition, while another 5 percent are obese? And what’s the justification for these statistical cut-points, anyway?

The definitions were created by an expert committee chaired by William Dietz, a CDC bureaucrat who has made a career out of fomenting fat panic. The committee decided that the cut-points for defining “overweight” and “obesity” in children would be determined by height-weight growth chart statistics drawn from the 1960s and 1970s, when children were smaller and childhood malnutrition was more common. The upshot was that the 95th percentile on those charts a generation ago is about the 80th percentile today-hence, the “childhood obesity epidemic.”

These definitions are completely arbitrary. The committee members chose them not on the basis of any demonstrated correlation between the statistical cut-points and increased health risk, but rather because there was no standard definition of overweight and obesity in children, and so they invented one. In other words, the “childhood obesity epidemic” was conjured up by bureaucratic fiat.

The committee did this despite Americans being healthier, by every objective measure, than they’ve ever been: Life expectancy is at an all-time high, and demographers predict it will continue to climb steadily. This isn’t surprising given that mortality rates from the nation’s two biggest killers, heart disease and cancer, are at historical lows and keep declining, while infectious diseases are under better control than ever. There’s no reason to think that today’s children won’t be healthier as adults than their parents, just as today their parents are healthier than their own parents were at the same age, continuing a pattern that has prevailed since public health records began to be kept in the 19th century. (Tellingly, 50 years ago government officials were issuing dire warnings that a post-World War II explosion of fatness among both American adults and children was going to cause a public health calamity).

And none of this even touches on a subtler and more invidious cost to the Let’s Move campaign: the profound shaming and stigmatization of fat children that is an inevitable product of the campaign’s absurd premise that the bodies of heavier than average children are by definition defective, and that this “defect” can be cured through lifestyle changes. As Casey Heynes’ desperate act of self-defense illustrates, fat kids have enough problems without the additional burden of being subjected to government-approved pseudo-scientific garbage about how they could be thin if they just ate their vegetables and played outside more often.

Michelle Obama’s campaign against childhood obesity is exactly the sort of crusade that liberals who don’t want to give ammunition to conservative critiques of government activism should oppose. It is a deeply misguided attempt to solve an imaginary health crisis by employing unnecessary cures that in any case don’t work. As such, it is almost a parody of activist government at its most clueless. Politically speaking, it deserves the same treatment Heynes gave his tormentors.

Paul Campos is a professor of law at the University of Colorado at Boulder.