Enhancing the upper lip only is more likely to appear artificial to observers than enhancing both the upper and lower lips, researchers found.

Writing in JAMA Facial Plastic Surgery, the authors surveyed 98 participants — most of whom were women. They gave them photographs of a woman model’s lips that showed alterations to the upper, lower, upper and lower lips, as well as the shape of the Cupid’s bow.

Participants were asked to judge whether the model in each photo had received cosmetic treatment, as well as whether the lips looked “attractive and natural” or “artificial and unnatural.”

The overall lowest margin for fake-ness was in changes to the Cupid’s bow. The study authors concluded that facial plastic surgery requires “technical and artistic prowess,” though the perceptual threshold for artificial appearance is different for each area of the lips.