What if there was a single activity that had been shown to lead to poor performance on tests, getting less exercise, reduced participation in school and extracurricular activities, and even unhealthy eating. You would want to make sure your children never did this activity, wouldn't you?

Well there is. And while it may sound like I'm talking about alcohol or illegal drug use, it's something much more insidious and universal--in fact it's something every single young girl has done: Worrying that you are ugly and/or fat.

Can fretting over your appearance truly cause this much harm? Yes, according to a thought-provoking TED Talk by self-esteem expert Meaghan Ramsey, who was global director of the Dove self-esteem project, among other things. She cites some frightening statistics: 60 percent of girls choose not to participate in activities because they think they don't look good enough; 31 percent withdraw from classroom debate so as not to draw attention to themselves if they believe they look bad; 20 percent skip class altogether when they feel unattractive; and research shows that girls who think they're fat get lower grades on tests that those who don't worry about this.

And, Ramsey explains, the adverse effects go beyond academics. Girls with low body confidence are less likely to get regular physical exercise, less likely to eat fruits and vegetables, more likely to suffer depression and, not surprisingly, more likely to develop a debilitating eating disorder such anorexia or bulimia. And you can't solve the problem by simply making sure your daughter stays at a healthy weight--a thin girl is just as capable of feeling fat and ugly as a girl who is genuinely overweight.

Sadly, these effects are not limited to girls--both boys and adult women are subject to the negative effects of low body confidence as well. But girls, who live in a fishbowl where they often feel judged on looks alone, are particularly vulnerable.

What can you do to help? Talking about the problem isn't enough, Ramsey says--we have to educate our daughters on this issue and help build their self-esteem. "The good news is that there are many programs out there available to do this. The bad news is that most of them don't work," she adds. In fact, she says, some well-intentioned programs actually make the problem worse.

To be effective, Ramsey recommends that a program address some key areas. And while you may or may not think a formal program is the right choice for your daughter, the issues Ramsey identifies are ones that all parents should actively address if they want to keep negative body image from affecting their daughters' academic performance and health:

1. Talking about their bodies with family and friends.

Your first step should obviously be to address how you yourself and others in your family talk to your daughters about their appearance and weight. But you also need to know how friends, teachers, coaches, and other influencers in their lives are talking about these issues with them. You won't be able to control what others say to them, and their peers will likely be struggling with their own body image issues that can come out in all kinds of destructive ways. But giving your daughters some perspective for how to think about these issues and what's truly important is a start.

2. Dealing with media and cultural body image messages.

Just yesterday, I found myself watching a television ad that was supposed to be all about individuality--a diverse group of female models wore different quirky outfits while the voice-over told viewers to "most important, be you." No one considered it ironic that all the models had exactly the same bodies: Tall and thin, and most likely digitally enhanced.

With airbrushed, size-zero people presented as the standard for normal humanity in television, movies, print, and online media, it's practically impossible for your daughters (and everyone else) to avoid developing a negative body image. But talking to them about it and making sure they understand that the people they see in the media don't really look like that is good first step.

3. Handling teasing and bullying.

The Internet is truly disturbing place if you have a negative body image. According to Ramsey, 10,000 people Google the phrase "Am I ugly?" every month. And some of them post images or videos of themselves online and ask the Internet at large to decide. Predictably, the Internet can be very unkind in such situations and one hapless--and attractive--Denver teen got 13,000 comments to her YouTube video asking if she was ugly. Most of those comments were horridly cruel.

Every teenage girl, no matter how lovely, will get teased or bullied about her appearance at some point. Just telling them they're beautiful won't solve the problem. They have to learn some ways of coping with the negative comments and bullying they will inevitably face out in the world.

4. Dealing with competition over appearance.

Who's prettier than who? Who's thinner than who? Teenagers tend to be competitive, and they will compare each others' looks just as much as anything else, but it can be particularly harmful when it happens. Giving girls the emotional skills not to be drawn in to competitions over their appearance or weight may help them avoid falling into this trap.

5. Avoiding fat talk.

"Oh my God, I'm so fat!" You've heard this thousands of time and probably said it yourself. It's often met with commiseration ("Me too!") or contradiction ("What are you talking about? You look great!"). Even so, fat talk is dangerous and tends to make both speaker and listeners feel bad about their bodies. Research shows it has been linked with increased shame over one's body and even eating disorder behavior. Getting your daughters to avoid doing this to themselves, and resist the compulsion to engage with fat talk when they hear others doing it can help strengthen their self-esteem.

6. Respecting and caring for their bodies.

We live in a world where body image has gone so completely wrong that the Web is full of "pro-ana" sites, blogs, and support groups that teach girls how to starve themselves into emaciation. Teenagers flock to these sites. If I know about them, it's likely your daughters do too. In a culture where all of us are constantly seeing images of skinny models interspersed with ads for food and soda...well, taking good care of our bodies and respecting those bodies can be a real challenge. Teaching your daughters about good nutrition and good health--and that good health is always beautiful--is one way to fight back.