Edward Shorter, author of How everyone became depressed: The rise and fall of the nervous breakdown, a medical historian who specializes in psychiatry, offers some thoughts on the rise of popular psychiatric diagnoses—thoughts that provide a useful analogy to the pop science theories we skewer daily here.
The problem isn’t that these theories exist, in either case, but that they are taken seriously, made into beliefs about life in general, and acted on—and in some case signed into law. 
Anyway, here’s Shorter:
Historians are well situated to appreciate how fragile current diagnoses are because you can see, over a long period, what psychiatry has considered to be the main diseases and also you get a sense over the centuries of the treatments that actually worked and were effective.
By contrast, you look at what’s happening in psychiatry today, and many of the diagnoses were created out of whole cloth in 1980 with the famous DSM III [Diagnostic and Statistical Manual III, meaning third edition].
I went back through the archives of the American Psychiatric Association, and looked at all the correspondence surrounding the DSM III. I was just amazed at how unscientific they were. The ideas were based on consensus, which implicitly is unscientific. Also, the whim of one man, Robert Spitzer, played an overpowering role in the creation of diagnoses such as bipolar disorder and major depression. [colour added]
Over the years, psychiatry has gained an enormously powerful sense of what real illness is, and the DSM III flew in the face of that consensus. Major depression, for example — which is probably the most popular diagnosis in the DSM III — was simply put together by collapsing two very different depressive illnesses psychiatry always recognized: melancholic illness and non-melancholic illness. There is a lot of scientific evidence to back up that differentiation.
They flicked this aside in 1980, and said one kind of depression is the same as any other, so why make this distinction? It was a completely arbitrary, ruthless way of coming up with a diagnostic category. We’re paying the price for that today.
This is a concrete illustration of how knowledge of the history of something like diagnosis can give us an effective critique of current concepts. More.
One could say the same for a history of Darwinism, the multiverse, evolutionary psychology, origin of life studies … etc. …
See also: The psychiatric Bible?