Children with autism have difficulty understanding the meaning behind a smile, a study published Friday in the British Journal of Developmental Psychology finds.

Researchers presented photos of people with neutral expressions, fake smiles and those with real smiles to a group of boys age 5 to 15 with autism and a control group of boys without autism. The boys in both groups were asked if the person in the photograph was “feeling happy on the inside” and if the person was “looking happy.”

The boys with autism were unable to determine from the photos if a smile was fake or real, while the boys without autism were quite successful at distinguishing between the two.

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“The difference between the boys with and without the disorder was pronounced, especially when we note that the boys without the disorder were as sensitive to the difference between smile types as were adults in our previous research,” says Lucy Johnston, an author of the study, and a professor at the University of Canterbury in New Zealand. “Being sensitive to the differences between enjoyment and faked smiles is essential for effective social interaction.”