Do you know why she feels happy when you are in jolly mood? The answer lies in your sweat.
When happy, our bodies produce chemical signals that are detectable by others who smell our sweat and instantly share our happiness, researchers report.
While previous research has shown that negative emotions related to fear and disgust are communicated via chemical composition of sweat, a few studies have examined whether the same communicative function holds for positive emotions.
"The findings show that being exposed to sweat produced under happiness induces a simulacrum of happiness in receivers, and induces a contagion of the emotional state," explained psychological scientist Gun Semin of the Utrecht University in the Netherlands.
It means that somebody who is happy will infuse others in their vicinity with happiness.
"In a way, happiness sweat is somewhat like smiling – it is infectious," he added.
The research indicates that we produce chemical compounds, or chemosignals, when we experience happiness that are detectable by others who smell our sweat.
To reach this conclusion, Semin and colleagues examined whether sweat taken from people in a happy state would influence the behaviour, perception and emotional state of people exposed to the sweat.
The researchers recruited 12 males who were prohibited from alcohol use, sexual activity, consumption of smelly food or excessive exercise during the study.
The sweat donors came to the lab, rinsed and dried their armpits, and had absorbent pads attached to each armpit.
They watched a video clip intended to induce a particular emotional state (fear, happiness, neutral). The sweat pads were then removed and stored in vials.
For the second part of the study, the team recruited 36 women as women generally have both a better sense of smell and a greater sensitivity to emotional signals than men do.
The women were exposed to a sweat sample of each type (fear, happiness, neutral), with a five-minute break in between samples.
Initial analyses confirmed that the videos did influence the emotional states of the male participants.
But were these emotions conveyed to the female participants? The results suggest the answer is "yes".
Facial expression data revealed that women who were exposed to "fear sweat" showed greater activity in the medial frontalis muscle, a common feature of fear expressions.
"Women who were exposed to ‘happy sweat’ showed more facial muscle activity indicative of a Duchenne smile, a common component of happiness expressions.," the authors noted.
"Happiness may be communicated chemically could be of particular interest to the ‘odour industry," Semin concluded.
The study was published in the journal Psychological Science.