Psychologist Jürgen Margraf and his coauthors looked at 506 people who had plastic surgery, and 163 who wanted it but had not yet gotten it (the most common type of surgery sought was breast enlargement, followed by liposuction). They measured both groups' life satisfaction, feelings of attractiveness, and levels of mental health problems at regular intervals, up to a year after those in the surgery group were operated on.
The chart above shows the average scores for depression, anxiety, and social phobia (all scored on a 0-to-18 scale) and body dysmorphia (scored 0 to 21) for the surgery and non-surgery groups after one year. The differences for depression and social phobia weren't significant, but those for anxiety and body dysmorphia were. The group who had surgery also scored significantly better than the group who didn't on measures of self-esteem, positive attitude, life satisfaction, and general feelings of attractiveness. And they were significantly happier with the specific body part they got surgery on than the group who never got surgery were with their "problem" body parts.
Previous research has bolstered the idea that the desire for plastic surgery is just an expression of underlying psychological problems, which the surgery doesn't fix — but Margraf's research implies that getting the surgery they want might actually make some people feel better.