Does faking a smile make you feel happier? That question has been debated since famed naturalist Charles Darwin published a book on the subject in 1872.

"The free expression by outward signs of an emotion intensifies it… Even the simulation of an emotion tends to arouse it in our minds," Darwin wrote.

Popular culture has perpetuated the idea. For example, Nat King Cole's 1954 hit song "Smile" contained the lyrics: "Smile though your heart is aching […] You'll find that life is still worthwhile/If you just smile."

But does this popular notion stand up to scientific scrutiny? A 1988 study asked people to hold a pen with their teeth to simulate smiling or with their lips to simulate a neutral expression.

The study found that smiling without being aware of it made people feel more amused when shown a cartoon.

However, a 2016 meta-analysis that compiled data from 17 studies using the pen-in-mouth trick found they could not replicate the original findings.

Researchers have carried out other studies with various methods over the years to examine how what's written on our faces affects how we feel. A 2019 review of 138 studies found that, indeed, smiling influences people's emotions, but the effect was only small.

Now, researchers have recruited thousands of people from all over the world to painstakingly put this smiling effect to the test – again.

In the study, published in Nature Human Behavior, around 3,800 volunteers from 19 countries were asked to smile or maintain a neutral expression using several different prompts and then rate their happiness.

If volunteers knew what the researchers were studying, that could influence how they rated the smiling interventions. So, the researchers invented some decoy experiments to throw volunteers off the scent.

They pretended to be studying how small movements and distractions affected math-solving abilities and issued decoy instructions, such as "Place your left hand behind your head and blink your eyes once per second for 5 seconds."

Three different 5-second smiling interventions were mixed in with the decoy tasks in random order.

For one of these tasks, volunteers had to put a pen between their teeth or hold it with their lips. This was a copy of the 1988 study with some slight adjustments: a cartoon wasn't used, and happiness was measured instead of amusement at the end of the task.

In a second task, volunteers mimicked a photo of an actor smiling or maintaining a blank expression.

In a third task, researchers asked participants to put on a happy expression by either moving the corners of their lips toward their ears and elevating their cheeks or maintaining a blank facial posture.

After each task (including the decoy tasks), participants completed a simple math problem, a happiness and anxiety questionnaire, and an anger, tiredness, and confusion survey to "obscure the purpose of the study".

Feelings of happiness somewhat increased in every smiling intervention, but the effect was greater in the mimicry and facial action tasks than in the pen-in-the-mouth task.

"Consistent with a previous meta-analysis, these results suggest that facial feedback can not only amplify ongoing feelings of happiness but also initiate feelings of happiness in otherwise neutral contexts," the researchers write.

It could be that an active task (such as mimicking a facial expression) was simply less boring than a passive task (such as maintaining a blank stare), influencing study participants' happiness.

To control for this effect, the researchers compared the neural expression tasks against the active decoy tasks. This showed that smiling had more of an impact on happiness than other simple activities involving muscle movement.

Half of the participants viewed a series of upbeat images for each smiling task during the experiment. This tested whether the effect of smiling on happiness was greater in the presence of positive stimuli.

The results showed that the happiness effect emerged in both the presence and absence of emotional stimuli.

The researchers write that faking a smile could influence our mood because people infer that they are happy because they are smiling or because smiling automatically activates biological processes associated with emotion.

So, can you improve your mood by smiling in the mirror for five seconds every morning? Well, that's still debatable.

"It is possible that relatively small facial feedback effects could accumulate into meaningful changes in well-being over time," the researchers write.

"However, given that the similar-sized effect of positive images on happiness has not emerged as a serious well-being intervention, many (but not all) authors of this paper find it unlikely that facial feedback interventions will either."

This paper was published in Nature Human Behavior.