Now you see sadness, now you don’t.

A new study has found that removing just the tears out of pictures of people crying reduces the sadness that viewers perceive in the photos, even though the rest of the expression remains intact. The research subjects said when the tears were digitally erased, the faces’ emotional content became ambiguous, ranging from awe-filled to puzzlement.

“One of the startling things is that the faces not only look less sad but they don’t look sad at all. They look neutral,” said Robert Provine, the University of Maryland-Baltimore County neuroscientist who led the work. “Any photograph you see, you can put your finger on the screen and block out the tears. It’s like the face is transformed.”

Scientists have spent plenty of time thinking about how humans communicate emotion in non-verbal ways, the signals that we’ve evolved for other members of the species. Paul Ekman of the University of San Francisco — and other researchers — have found that certain facial and bodily expressions mean the same thing from Bogota to Beijing. As described by Malcolm Gladwell in a 2002 profile, “Ekman had established that expressions were the universal products of evolution.”

But Provine argues in an article published in the open-access journal Evolutionary Psychology that Ekman left out a key component of basic human emotional signaling (PDF) — the tear effect, which appears unique to our species.

“With tears, you increase the richness of the face as an instrument for communication,” Provine said.

Eighty-three subjects evaluated 200 photographs showing 100 random facial expressions and 100 pairs like the photos above. On a seven-point scale ranging from “Not Sad at All” to “Extremely Sad,” erasing the tears dropped the rating by about 1.25 points. The study suggests that the way humans read other people’s emotions is markedly impacted by the presence (or absence) of tears.

“The Ekman expressions are not the whole story. When you add tears, you’re getting other combinations,” Provine said.

For example, Provine writes in the paper, “Does a happy face with tears appear more or less joyous, or something in between, perhaps described as ‘bittersweet?’”

While we might have an intuitive sense of the answer, the neuroscientist said the phenomenon deserves scientific scrutiny.