Losing weight. Quitting smoking. Becoming a "yes, and" kind of person.

We've all made promises to ourselves on New Year's. And yet somehow, by mid-February, we're decidedly less resolved. We blame willpower, we blame distractions, we blame all kinds of things. But this year, consider blaming your co-worker's cousin's friend.

It's easy to think about contagion in terms of physical illness – we're smack dab in the middle of flu season now, and you might be running for cover when a sniffling co-worker approaches.

But you might not think about moods and mental states as having the same kind of power — meaning that what you weigh, or how even how happy you are, is contagious.

"We have been able to show that seemingly very personal things, like your emotional state, or your body size, or how kind you are, or whether you vote, depends on whether other people around you do that, and even other people you don't know," explains Nicholas Christakis, professor at Yale and co-author of Connected: The Surprising Power of Our Social Networks.

True, it's pretty obvious that we change our behavior based on what our friends are doing. Just ask any 6th grade girl. But this is different, argues Christakis.

It's peer pressure on steroids. Now what we're showing is that it's not just you take up smoking in high school because your friends take up smoking. It's you take up smoking because your friends' friends' friends, people you don't even know, take up smoking.

The same phenomena can happen in all kinds of realms. Take weight, and the curious way that a place like Hollywood is much thinner than the average American city. "There's a spread through the social network of normative ideas about body size. And that norm then can cascade and spread ultimately to you."

It isn't all negative though, Christakis is quick to point out. "One of the big ideas about social networks is that networks magnify whatever they are seeded with. But they must be seeded. Networks will magnify ebola, fascism, violence and criminal behavior; but equally they will magnify happiness, and kindness, and love, and ideas."

Criticism of Christakis' research has pointed out that he could be undermining the power of free will — something we hold near and dear.

Our work does show that…But even if that's the case, our work also shows that your actions affect others. When you make a positive change in your life – when you act kindly towards others, when you vote, when you express joy and happiness, it doesn't just benefit you, it benefits those people and in fact ripples out and could affect many, many other people.

And that's something the professor has taken to heart.

"I've become much more aware about how my behaviors affect other people."