There’s one at every comedy club: the guy sitting there stone-faced, while everyone around him is laughing. There are many possible explanations: He was dragged there by his girlfriend, doesn’t like the stand-up’s style, or is simply having a bad day.

But if his humorlessness is chronic, the underlying issue may be more basic: He just isn’t honest with himself. According to newly published research, self-deception inhibits laughter.

“Humor deals with the absurdities of life,” Rutgers University anthropologists Robert Lynch and Robert Trivers write in the journal Personality and Individual Differences. “The less you are in tune with reality, the less likely you are to see the absurdities.”

The researchers describe an experiment featuring 59 undergraduates, who were videotaped as they watched a performance by stand-up comedian Bill Burr. They watched the routine alone so they wouldn’t be influenced by others’ reactions. After the program, the students rated how funny they found it.

For eight minutes of the half-hour program, their facial expressions were closely monitored. Particular attention was given to specific motions that indicate laughter: a tightening of the ring muscles around the eyes, and the raising and tightening of the outer corners of the lips.

Either before or after watching Burr’s comedy act, participants filled out a questionnaire designed to measure self-deception. They responded to potentially uncomfortable statements such as “I could never enjoy being cruel” and “I must admit revenge can be sweet,” expressing their level of agreement on a one-to-seven scale (one denoting “not true at all,” seven signifying “very true”).

Responses at the extremes were scored as one point if admitted something distasteful, the researchers explain. For instance, anyone who responded to the statement, “I can’t think of anyone I hate deeply,” received a point if they emphatically agreed with the statement by responding with a 6 or a 7.

“The more points a participant received, the higher in self-deception he or she was perceived to be,” they write.

The researchers found a link between a lack of laughter and denial of one’s darker impulses. “Participants who scored higher on a self-deception questionnaire laughed less, and reported less enjoyment, in response to a stand-up comedian than those who scored lower,” Lynch and Trivers report.

They cite several possible explanations for this humor impairment. “Because self-deception interfered with one’s ability to accurately perceive reality, it may hinder one’s ability to recognize incongruities, thus reducing laughter,” they write.

“Humor also often involves seeing something from a novel angle, with surprising and pleasing effects. But if you are practicing self-deception and blocking out certain angles, you will, when these angles are exposed, fail to see the absurdity and fail to enjoy the humor.”

So if you want to really get to know someone share something amusing with them, and watch for unforced smiles, chuckles or guffaws. A lack of laughter suggests they may be hiding something—from themselves.