Publication bias strikes again: because only positive results are published in scientific journals, medical literature greatly overstates the benefits of talk therapy for depression.

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Doctors have long known that journal articles significantly overstate the benefits of drugs due to publication bias; basically, if you’d look at the published articles in journals, you’d think that all drug experiments work because only the drugs that work actually get published. The same thing seems to happen for speech therapy, although it is far less obvious. The new review, in the journal PLOS One, found that while treatments like speech therapy and behavioral therapy are indeed effective, they are 25% less than previously thought. This study, while won’t settle the debate around psychology’s relative merits, will go a long way to providing guidelines for more effective depression treatment.

Over 5 million Americans are treated for depression each year, and many of them are also on antidepressants, which often have many negative side effects. For most people, simply consulting a doctor and talking about the problem helps a lot, and according to the new analysis, engaging in well established psychotherapy treatment gives them an extra 20% chance to achieve long-lasting improvements or even overcoming depression. But according to previous analysis, that figure was closer to 30% – quite a difference.

“That seems to be the magic number, a quarter — about the same as you see in the pharma trials” of antidepressants, said a co-author, Dr. Erick Turner, an associate professor of psychiatry at Oregon Health & Science University and the lead author of the 2008 paper detailing bias in those drug trials.

This raises once again the problem of publishing: studies that test out a hypothesis or a treatment and don’t come out positive simply don’t get published by journals, and this is a problem. First of all, other people won’t know that that was tested and may try to do it again, wasting valuable time and resources, and second of all, it creates biases.

“We need to seriously consider publishing all completed studies,” whether encouraging or not, said Jelte Wicherts, an associate professor in the department of methodology and statistics at Tilburg University in the Netherlands, who was not involved in the study.

But while the general reviews were positive for this paper, some researchers warn to take it with a grain of salt.

“The number of trials they looked at was fairly small, and the different psychotherapy approaches were all pooled together,” said Stefan Hofmann, a professor of psychology at Boston University.

Either way, it’s both a positive and a negative signal for psychotherapy – it shows that while clearly effective when conducted properly, the technique’s effects are often not as strong as anticipated.