Facial expressions might not be reliable indicators of emotion, say researchers, adding that, people should never trust a person's face.
According to the study, some businesses are even working on technology to determine customer satisfaction through facial expressions.
"The question we really asked is: 'Can we truly detect emotion from facial articulations?' And the basic conclusion is, no, you can't," said study researcher Aleix Martinez from Ohio State University in the US.
For the findings, the researchers focused on building computer algorithms that analyse facial expressions.
The researchers analysed the kinetics of muscle movement in the human face and compared those muscle movements with a person's emotions.
They found that attempts to detect or define emotions based on a person's facial expressions were almost always wrong.
"Everyone makes different facial expressions based on context and cultural background," Martinez said.
"And it's important to realize that not everyone who smiles is happy. Not everyone who is happy smiles. I would even go to the extreme of saying most people who do not smile are not necessarily unhappy," Martinez added.
It is also true, that sometimes, people smile out of an obligation to the social norms, the researchers said.
This would not inherently be a problem, he said -- people are certainly entitled to put on a smile for the rest of the world -- but some companies have begun developing technology to recognize facial muscle movements and assign emotion or intent to those movements.
The research group analyzed some of those technologies and, Martinez said, largely found them lacking.
"Some claim they can detect whether someone is guilty of a crime or not, or whether a student is paying attention in class, or whether a customer is satisfied after a purchase," he said. " What our research showed is that those claims are complete baloney. There's no way you can determine those things. And worse, it can be dangerous," he added.
After analysing data about facial expressions and emotion, the research team concluded that it takes more than expressions to correctly detect emotion.
"What we showed is that when you experience emotion, your brain releases peptides -- mostly hormones -- that change the blood flow and blood composition, and because the face is inundated with these peptides, it changes colour," Martinez said.
According to the researchers, facial colour, for example, can help provide clues.
In one experiment, the researchers showed study participants a picture cropped to display just a man's face. The man's mouth is open in an apparent scream; his face is bright red.