There are many studies. Here's one:

http://jama.jamanetwork.com/article.aspx?articleid=185157

This study found that SSRIs provided little to no benefit over placebo for patients suffering from mild to moderate depression. SSRIs did provide a benefit to patients suffering from severe depression.

This raises the question, how many of the 30+ million Americans currently prescribed anti-depressants actually suffer from severe depression and benefit from the drugs? How many are taking it unnecessarily, subjecting themselves to unintended side effects and eventual withdrawal symptoms.

You ignore people who take it for anxiety, where SSRI are uncontrovertibly effective.