Psychology has not kept pace with science because symptom-based diagnosis stopped being used 50 years ago in medicine yet psychologists still rely on it. As evidence,the Diagnostic and Statistical Manual of Mental Disorders (DSM) has become 'The Bible' of psychiatry because it diagnoses depression when patients tick off a certain number of symptoms on the DSM checklist.
But the National Institutes of Mental Health no longer use it as anything more than a glossary because of controversy and maneuvering (e.g. a desire to get more and more disorders covered by insurance), 

A large-scale quantitative study coordinated at KU Leuven, Belgium, undermines it even more, showing that it is not even valid if symptom-based medicine were rigorous; the symptoms listed in DSM may not even be the most useful ones.

To diagnose depression, psychiatrists typically tally up the number of depression symptoms that patients report in questionnaires. It does not matter which of the symptoms these patients have, as long as they have a certain number of them.

A new study challenges that approach. "We need to stop thinking of depression as a disease that causes a number of interchangeable symptoms," says lead author Dr Eiko Fried from the KU Leuven Faculty of Psychology and Educational Sciences. "Depression is a complex, extremely heterogeneous system of interacting symptoms. And some of these symptoms may be far more important than others."

The researchers analyzed data on 28 symptoms provided by 3463 depressed patients. They then examined the connections between these symptoms. Their network analysis shows that some symptoms are more 'central' - more connected - than others. As a result, these symptoms have a much bigger impact on the depressive process.

"If you think of depression as a network of interacting symptoms, one symptom can cause another," Fried clarifies. "For instance, insomnia may lead to fatigue, which in turn may cause concentration problems that feed back into insomnia. This example of a vicious circle shows that the specific symptoms patients report, and their interactions, can be of crucial clinical importance.

"Depression is not like, say, measles. When you have measles, your symptoms help the doctor figure out what underlying disease you have. But once you are diagnosed, it doesn't really matter which of the possible symptoms you did or didn't get. Treating the disease itself makes all your symptoms disappear. Depression is more complicated. It is not an infection or a specific brain disease. There is no easy cure, no drug that makes all symptoms go away. Instead, we may want to focus treatment efforts on the symptoms driving a patient's depression."

In the study, the two main DSM symptoms - sad mood and decreased interest or pleasure - ranked among the top 5 in terms of centrality. But the researchers also found that DSM symptoms such as hypersomnia, agitation, and weight change are not more central than other common depression symptoms such as pessimism and anxiety.

So where does that leave psychiatry? Has the time come to chuck the DSM checklist of depression symptoms? "Ideally, the list of depression symptoms should become more comprehensive to do justice to the heterogeneity of depression. I also think both clinicians and researchers can learn a lot from paying more attention to individual symptoms and their interactions. But of course, we need much more research before we can actually replace the DSM diagnosis of depression with something else."