Feeling stressed? Blame it on that evil grin

Smiling like a Cheshire cat can change the heart rate or stress hormones of the person in front of you for better or worse‚ a new study shows.

Three main types of smiles have been defined by US psychology professor Paula Niedenthal: “Dominance [meant to convey status]‚ affiliation [communicates a bond and shows you’re not a threat] and reward [the sort of beaming‚ toothy smile you'd give someone to let them know they're making you happy]”.

The latest research demonstrates how people’s bodies react to the emotional intention behind a smile.

Jared Martin‚ from the University of Wisconsin-Madison psychology department‚ did a trial which found that dominance smiles led to a spike in stress hormones. Friendly smiles‚ signalling a reward‚ buffered people against stress.

“Facial expressions really do regulate the world. We have that intuition‚ but there hasn’t been a lot of science behind it‚” said Martin‚ who co-authored the study‚ published in the journal Scientific Reports on Thursday.

“Our results show that subtle differences in the way you make facial expressions while someone is talking to you can fundamentally change their experience‚ their body‚ and the way they feel like you're evaluating them.”

For the experiment‚ 90 male college students had to deliver short‚ impromptu speeches and be judged over a webcam by a fellow student‚ who was in on the study.

“Throughout their speeches‚ the participants saw brief video clips they believed were their judges’ reactions. In fact‚ each video was a prerecorded version of a single type of smile — reward‚ affiliation or dominance.”

During the exercise the speakers’ heart rates and the levels of cortisol (a hormone linked to stress) in their saliva were measured regularly.

Niedenthal said: “If they received dominance smiles‚ which they would interpret as negative and critical‚ they felt more stress and their cortisol went up and stayed up longer after their speech.

“If they received reward smiles‚ they reacted to that as approval‚ and it kept them from feeling as much stress.” The impact of affiliative smiles seemed similar to that of reward smiles.

The volunteers showing stronger reactions to the different smiles had a higher heart-rate variability.

Niedenthal said people’s parasympathetic nervous system – which controls heart rate and breathing – influences how they absorb and process information.

This meant differences in people’s bodies influenced the “very sensitive and personal ways” they perceived a smile.