Have you ever noticed how infectious the giggles are? Even if you enter the conversation late and have no idea what the guffawing is about, you probably at least break into a smile. You just can't help it. The clatter of all that hilarity tickles the funny bone.

It turns out this is not just your imagination. A study conducted by researchers Nicholas A. Christakis and James H. Fowler, which charted the emotions of 4,739 people from 1983 to 2003, found that the happiness of other people does in fact have a strong effect on our own feelings of happiness.

Just how communicable is glee? It's powerful enough to spread through people who don't even know each other. Between two people who know each other, a happy person increases the other's happiness by 15 percent. A friend of a friend will increase happiness by 10 percent, and a third-degree connection can increase happiness by 6 percent. Mutual friends, or friends that name each other as close friends, seem to have the greatest affect on each others' bliss.

The study researched the contagion of happiness within traditional social networks. The question is: Is happiness just as catching in online social networks as it is in real-life social connections? In a separate but related study, Christakis observed 1,700 Facebook profiles. He determined that a person who posts a picture of him or herself smiling on Facebook is more likely to have online friends who are also smiling in their profile pictures.

But before we get to grinning profile pictures, let's take a look at what happiness is in the first place. What exactly does it mean to be happy?