Just like yawning, smiling and laughing can be contagious. It's the reason comedy actors dread getting the giggles on stage, and why so many of us love to play with happy babies. (Just try not to smile while watching this little one laugh)

Behaviour researchers call our instinct to mimic the moods of those around us "emotional mirroring," and a new study this month finds it happens constantly, even without our realizing it.

The review authors looked at hundreds of studies on facial mimicry and concluded that we tend to smile when others smile because it allows us to experience the other person's feelings for a moment. When we see someone smile, we "try on" that smile -- all in a matter of a few milliseconds, without realizing we're doing it.

But while some smiling is done internally or unconsciously, it's also something we can choose to do. And there's been plenty of research demonstrating why something as simple as a grin can lead to powerful changes both within us and those around us.

Here are four science-backed reasons why smiling is good for you.

1. It can help you through stress

The old adages of "grin and bear it," and "fake it 'til you make it" might actually be true when it comes to using a smile to get through stress. A recent study found that people who are told to hold a smile through difficult exercises recover from the stress quicker.

The study had volunteers hold chopsticks in their mouths either downward with no smile, downward with a half-smile to expose their teeth, or biting the chopsticks sideways to force their mouths into a wide smile.

(Tara Kraft)

They then had to trace a star using their non-dominant hand, looking only at a mirror as they worked, and then submerge their hands in ice water.

While all the participants showed increased heart rates throughout the stress tests, those who had to hold big smiles during the tasks saw their heart rates recover much faster than those who did not smile. The researchers say that the actual activation of the muscles involved in smiling -- even if it's not a smile that came naturally -- seems to affect our moods unconsciously, making us better able to handle stress.

2. Smiling could improve your memory

Smiling and laughing are great stress-relievers, which may be why research has found a bit of laughter might help improve our memories.

California researchers tested the memory-boosting effects of smiling by showing a group of seniors a funny, 20-minute video. They then asked them to do a memory test that measured their recall and sight recognition. A second group of seniors did the same test but did not watch the funny video first.

The researchers also measured the cortisol levels of both groups using spit tests at the beginning and end of the experiment. Cortisol is a hormone that soars when we are under stress but drops when we feel happiness.

The group who watched the funny video enjoyed a significant decrease in cortisol afterwards and also performed much better in all areas of the memory test compared to the control group.

The researchers say cortisol appears to inhibit the brain's ability to learn and remember, but that smiling and laughter help to relieve that stress. So a little bit of happy thoughts and smiling may prime your brain for better learning.

(AP/The Flint Journal-MLive.com, Sam Owens)

3. It may help you live longer

The feel-good benefits of smiling could even have an effect on how long you live. One study a few years ago found that athletes who offered full, genuine smiles in photos were more likely to live longer than others who refused to smile.

The study looked at the player cards of 230 Major League Baseball players from 1952 and assessed whether they displayed a full smile, a partial smile or no smile. Of the players who had died by 2009, those who didn't smile in their photos lived an average of 72.9 years; those with partial grins lived an average of 75 years; and those with big, authentic smiles lived an average of 79.9 years.

The differences may appear small, but the researchers say they were statistically significant. Of course, it's unlikely that smiling in one photo caused these players to live longer, but the researchers say the big smiles may have been indicators that those players were basically happier than others with less intense smiles "and hence may be more predisposed to benefit from the effects of positive emotionality."

(AP/The Journal-Star, Eric Gregory)

4. It makes others feel good

Smiling not only benefits you; it makes others feel good about themselves too. One recent study found that people who have been smiled at by a stranger feel good and more socially connected to others than people who have been deliberately ignored.

The study had a research assistant walk along a path on a university campus, catching the eyes of certain people and smiling at them. Other times, the researcher would catch the eyes of someone they passed but then appear to look right through them. After they passed, another researcher would swoop in, and ask the person: "Within the last minute, how disconnected do you feel from others?"

Those who had gotten eye contact and a smile felt less disconnected than people who had been looked at as if they weren't there. The researchers say we humans are continually looking for clues about whether we belong or have been socially excluded. So a little eye contact and a smile can go a long way into making others feel good, if even for a little while.