That tend to hold "on average" for a population but often don't hold for the individual within a population. This is the ecological fallacy [0], just one of the fallacies underlying psychiatry.

My argument isn't that psychiatric symptoms don't exist or aren't real and there is no real underlying phenomenon. My argument is simply that we've drawn the lines between the units of study too high up and we should be more granular. This level of nosology was chosen in 1952. Do you really think they got it 100% right almost 75 years ago? And what is the mechanism for defining and maintaining these categories? A bunch of committees get together every few years and decide on them, then they tell us all what's "true". Bullshit. What are the odds that a committee will define itself out of existence? Pretty slim. [1]

I have traits that could be considered as autism, ADHD, obsessive compulsive personality disorder, PTSD, bipolar II, social anxiety disorder, and probably a dozen more disorders. But by quantizing the disorder at the current level, by necessity, the other traits are cropped out of view. Relevant information is lost and irrelevant information is blurred together. And the level of overlap between disorders is absurd. They cannot possibly be "real" because the lines between them aren't even distinct.

The useful unit to study is the individual trait, not the cluster of traits that is different in each individual. The traits are more granular and map more closely map to underlying biology anyway. The current model is akin to what the geocentric model was in astronomy. It's outdated, wrong, and holding us back from a more accurate, detailed view.

[0] https://en.wikipedia.org/wiki/Ecological_fallacy

[1] https://en.wikipedia.org/wiki/Conway%27s_law