Numerous studies have found that when a depressed person’s mood lifts, his or her personality changes, and it was assumed that these personality changes in a person on antidepressants were an indirect result of mood elevation. According to a new study reported by the National Institute of Mental Health (NIMH), however, it looks like antidepressants might directly change personality and that this, in turn, might help alleviate depression.

As defined by psychologists, an individual’s personality can be broadly broken down into several categories. One of these categories is called neuroticism, or the degree to which a person has a tendency toward negative emotions and moodiness. People who suffer from depression are more likely to be neurotic, and studies of twins indicate that the two conditions might have a genetic link. Previous research has found that when a treatment effectively treats depression, it also usually decreases a person’s neuroticism.

In the new study, researchers at the University of Illinois randomized 240 depressed patients to receive either the selective serotonin reuptake inhibitor (SSRI) Paxil (paroxetine), a placebo or cognitive behavior therapy. Researchers then measured improvements in both depressive symptoms and neuroticism.

The researchers found that the drop in neuroticism was as much as eight times larger in those who received Paxil than in those receiving a placebo. People who received cognitive behavior therapy also had greater decreases in neuroticism compared with those on a placebo. What was most interesting, however, was that when the researchers controlled for improvements in depressive symptoms, decreases in neuroticism continued to be significant in those on Paxil, but not in those on placebo or cognitive behavior therapy. Moreover, those on Paxil with the greatest degree of change in neuroticism were the least likely to have a relapse of their depression.

“These results contradict the prevailing assumption that changes seen in personality traits in patients taking SSRIs are a result of the drugs’ effects on depression,” the authors concluded. “SSRIs may alter personality directly—and thus lift depression—or may act on a third factor that underlies both.

“Continued research on how these treatments work,” they continued, “can provide a clearer understanding of the mechanism of action of SSRIs and how treatment can be best used to reduce depression and minimize relapse.”