Not all smiles express warmth and joy, but they can also be mean. Human bodies tend to react differently depending on the message a smile is meant to send, say US psychologists.

The study showed that smiles meant to convey dominance are associated with a physical reaction "a spike in stress hormones" in their targets. On the other hand, smiles intended as a reward, to reinforce behaviour, appear to physically buffer recipients against stress. Researchers established three major types of smiles as dominance -- meant to convey status --, affiliation -- which communicates a bond and shows you are not a threat --, and reward -- the sort of beaming, toothy smile you would give someone to let them know they are making you happy.

"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 are evaluating them," said the study led by Jared Martin, a psychology graduate student at the University of Wisconsin Madison in the US.

For the study, published in the journal Scientific Reports, the team stressed out male college students by giving them a series of short, impromptu speaking assignments judged over a webcam by a fellow student. The speakers' heart rates and levels of cortisol -- a hormone associated with stress were measured. "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," said co-author Paula Niedenthal, Professor at the varsity.

"If they received reward smiles, they reacted to that as approval, and it kept them from feeling as much stress and producing as much cortisol," Niedenthal added. Participants with high heart-rate variability have shown stronger physiological reactions to the different smiles, while the variability is not innate and unalterable. However, certain disorders like obesity, cardiovascular disease, anxiety and depression can drag down the heart-rate variability, which can make it difficult to recognise and react to social signals such as dominance and reward smiles, Martin said.