People smile for different reasons, in various contexts, but only certain kinds of smile generally pass as an honest mark of happiness. Is this what they really are, though?

Although smiles are generally taken as signs of contentment, humans actually smile for many different reasons.

Sometimes we do smile simply because we are happy, but we also smile for social reasons and to put people at ease, as well as to show more complex emotions, such as resignation.

One smile type that people tend to perceive as a genuine mark of happiness is the Duchenne smile, wherein different sets of facial muscles are activated at the same time.

In the Duchenne smile, the person smiles with their mouth as well as their eyes. In popular culture, this action is sometimes referred to as "smizing."

Are these kinds of smile always what they appear to be? What really does make us smile? These are questions that researchers from the Brighton and Sussex Medical School in the United Kingdom have recently strived to answer.

Dr. Harry Witchel and colleagues have conducted a study aiming to learn when participants tended to smile in an experimental context, and why that might be.

They presented the new findings at the European Conference on Cognitive Ergonomics, held in Utrecht, the Netherlands.

"According to some researchers, a genuine smile reflects the inner state of cheerfulness or amusement," says Dr. Witchel.

"However, Behavioral Ecology Theory suggests that all smiles are tools used in social interactions; that theory claims that cheerfulness is neither necessary nor sufficient for smiling."

'Smiling is not driven by happiness'

The researchers worked with a cohort of 44 healthy participants, 26 of whom were women, aged 18–35.

As part of the experiment, the participants had to answer a fairly difficult quiz — presented on a computer — that lasted only 175 seconds. The level of difficulty, as well as the short duration, ensured that the volunteers often provided the wrong answers.

Each participant was seated and left alone with the computer. Their facial expressions were recorded using special facial recognition software.

Then, the researchers assessed the correspondence between the participants' various moods and the times at which they smiled using a two-way approach.

On the one hand, each participant rated their own experience of the quiz on a scale of 12 possible moods, such as "bored," "interested," or "frustrated." On the other hand, the researchers used the facial recognition software to see how often the participants smiled.

"Our study showed," Dr. Witchel says, "that in these Human-Computer Interaction experiments, smiling is not driven by happiness; it is associated with subjective engagement, which acts like a social fuel for smiling, even when socializing with a computer on your own."

The researchers found that, all in all, the participants were not likely to smile when they were trying to answer the questions on the quiz. Instead, they were likely to smile after they had answered the questions, as the computer would confirm whether they had been right or wrong.

Most notably, however, the participants seemed to smile most often when they found out that they had delivered the wrong answer.

After analyzing the data, the researchers concluded that the mood that seemed to be associated with smiling most often was, simply, "engagement." This suggests that smiles might sometimes appear as an unconscious social reaction.

"During these computerized quizzes," explains Dr. Witchel, "smiling was radically enhanced just after answering questions incorrectly."

"This behavior could be explained by self-ratings of engagement, rather than by ratings of happiness or frustration," he adds.