Friends and family of people with depression may feel that their loved one has been replaced by a gloomy doppelgänger. According to recent research, however, it may be the treatment of depression that actually causes personality changes in people with the disorder.

Experts have long known that the placebo effect explains much of the mood lift patients report after going on antidepressants. This was the case in the new study, published in the journal Archives of General Psychiatry—patients with major depressive disorder who were given a placebo saw their symptoms im­prove about three quarters as much as those given paroxetine, an antidepressant also known as Paxil. But only the patients who took parox­etine displayed personality changes in two key areas of the widely used five-factor model of personality: they scored lower on neuroti­cism, the tendency to experience negative emotions such as guilt and anxiety, and they scored higher on extroversion, which includes traits such as talkativeness and assertiveness.

Personality traits are thought to be relatively stable over a person’s life—even the onset of depression, which comes with unusually low moods, should not alter a person’s fundamental traits. Personality can affect a person’s risk for mental illness, however—past research has established neuroticism as a key risk factor for depression, explains Tony Tang of Northwestern University, the lead author of the study. Tang and his colleagues found that the more a patient’s neuroticism dropped while taking paroxetine, the smaller the chance that his or her depression would return after they stopped taking the drug.

The study “proves in an elegant way that antidepressant medications and placebo have different actions in many cases,” says Andrew Leuchter, a depression researcher at the Univer­sity of California, Los Angeles, who was not a part of the study. “This may explain in part some of the ways that antidepressants have a thera­peutic benefit for some patients.”