It's no secret that your friends influence your taste in music and clothes, but a growing body of research suggests that their words and actions might hold sway over your weight as well. In our new book, "Thinfluence," I and my co-authors, Dr. Walter Willett and Dr. Malissa Wood, explore the studies that make this connection. And what we found about the ways your friends could be affecting your weight-related behaviors might surprise you.

1. They have friends who are at a healthy (or unhealthy) weight. You might find it surprising enough that research shows having a close friend who is obese increases your chances of being obese by 57 percent. But researchers Dr. Nicholas Christakis and James Fowler also found that if your friends' friends are obese, you face a 20 percent higher risk of being obese yourself. If your friends' friends' friends are obese? You are about 10 percent more likely to be obese yourself. All told, this influence reaches out three degrees. The reason for this is a concept known as social contagion: the idea that habits and behaviors can be spread from person to person within networks of friends and acquaintances. The good news is that researchers believe that not only do unhealthy habits spread from person to person, but healthy habits as well: habits like exercise and smart food choices.

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2. They remember your birthday … and like to bake. You might be one of those people who looks forward to that special day every year to your friends and coworkers singing "Happy Birthday to You" in your honor, or you maybe you loathe the prospect so much that you hide in your cubicle. Either way, if your friends at work regularly bring cakes, cookies and other treats to work to celebrate (your office may even have a desk or a countertop as the designated "cake space"), their generosity and baking skill could be doing a tune on your waistline. Think about it: If you work in an office with just 11 coworkers, that's at least one "special" occasion per month that could be a real calorie bomb if you aren't careful. Fortunately, a simple change in office policy that encourages healthier celebrations would go a long way in keeping you and your colleagues at a healthier weight.

3. They are faster (or slower) with their fork than you are. How many bites do you take in a minute? It's a question that you have probably never considered, which is why you might be surprised to learn that researchers have actually studied how quickly pairs of people who are enjoying lunch together eat their meals. What they found was that we tend to "sync up" with our friends when it comes to the rate at which our forks go from our plates to our mouths and back. So if you have a regular lunch partner who is too quick with the fork, it is likely that you are as well.

4. They are courteous hosts. Maybe even too courteous. You know these friends. They live for Super Bowl Sunday and charades night. And when you arrive at their home, you are greeted with bowls of cheese puffs and plates of nachos. This gets especially interesting if you happen to be a "people pleaser" by nature. Researchers have found that, in these kinds of gatherings, people who identify themselves as wanting to please others are far more likely to accept snacks and drinks if offered, even if they don't really feel like having them, and even if they are laden with sugar, salt and calories.

5. They are trying to get to a healthier weight themselves. If you are trying to manage your weight, perhaps one of the best things you can do for yourself is to find a friend who shares your goals. A growing body of research points to how social contagion can serve as a basis for the transmission of healthy lifestyle habits. The more time you spend reinforcing each other's efforts, whether at the table of a restaurant or along the second mile of a walking trail, the higher the chance that you will both be able to achieve your goals. That's the heart of "Thinfluence," and it could be your key to a better weight and a healthier life.

Dan Childs is managing editor of the ABC News Medical Unit.