The idea that we might force emotions by expressing them is as old as psychology. It's been supported, then debunked, and supported again (with caveats). Now, we might have to concede there's something to "turning that frown upside down" after all.

Pooling together 138 studies on the effect our facial expressions have on our mood, researchers from the University of Tennessee and Texas A&M University took a broad look at the evidence and found a small but not insignificant effect under some circumstances.

"Conventional wisdom tells us that we can feel a little happier if we simply smile. Or that we can get ourselves in a more serious mood if we scowl," says social psychologist Nicholas Coles from the University of Tennessee.

"But psychologists have actually disagreed about this idea for over 100 years."

Even good old Charles Darwin had an opinion on the topic. "The free expression by outward signs of an emotion intensifies it," the naturalist wrote in his less famous book, The Expression of the Emotions in Man and Animals.

This 'facial feedback hypothesis', as it came to be known, has received a regular kicking ever since. A range of experiments have tested whether performing a facial expression to achieve a mood change will flip the usual order, where emotion is followed by a physical reaction.

One of the better known examples in recent history involved asking volunteers to evaluate humorous cartoons while gripping a pen lengthways in their mouth, pushing their lips into a rather awkward grin.

The results suggested even a strained grimace could be enough to make people feel in the mood for a laugh, sparking a new generation of discount bins full of guides on how to laugh your way to a better life.

Nearly thirty years later, a famous 1988 experiment drew controversy when a coordinated replication study run by the original researchers using more than 1,800 participants across 17 labs found absolutely nothing.

For those sick of being told to smile because it'll make you feel better, the result was a pleasant one.

"But we can't focus on the results of any one study," says Coles. "Psychologists have been testing this idea since the early 1970s, so we wanted to look at all the evidence."

Such psychology experiments can be notoriously finicky, with all sorts of variables potentially skewing results. There are a variety of facial feedback hypotheses and a range of disagreements over their effects.

So Coles and his colleagues, Jeff Larsen and Texas A&M University psychologist Heather Lench, conducted a statistical meta-analysis to dig deeper into the variety of methods to push through some common biases and see if there was more to it.

The effect wasn't exactly huge. But it was there, suggesting we can feel a bit brighter by smiling, or experience a degree of disgust if we feign a scowl. On the other hand, acting out the emotions of fear or surprise won't do much for you.

They also found these effects were different for certain evocative stimuli, such as cartoons versus sentences. Being forced to smile while looking at a Garfield comic strip simply isn't going to make it funny, no matter how big that grin is.

The research will no doubt keep the field of facial feedback research alive for a while to come. The cause-effect relationship between our mood and our body's reactions is no doubt a complicated one that will take some serious work to untangle.

"We still have a lot to learn about these facial feedback effects, but this meta-analysis put us a little closer to understanding how emotions work," says Coles.

The researchers do make it clear that they don't think it means we can simply smile our depression away.

Still, we'll probably need to put up with being told to smile by well-meaning life coaches a little longer. Just try to grin and bear it.

This research was published in Psychological Bulletin.