[klmr]

> There seems to be an assumption underlying medicine that things must be true at the population level for them to also be true at the individual level.

That really isn’t true: medicine as a field does not have this assumption, and rare disease research is a thing (and it’s commonly acknowledged that “rare diseases”, taken together, aren’t rare at all). In fact, rare disease research is providing acute clinical diagnosis and sometimes cures for highly individual cases. Personalised medicine is probably the fastest growing subfield in medical research.

Your example of antidepressants underlines this: it’s in fact widely acknowledged that, while antidepressants on average only have a very mild effect, they can be very effective (or, conversely, not effective at all) for an individual, and they are actively prescribed with this in mind. Contrary to what you’ve claimed, no expert assumes that antidepressants have a uniform effect across subpopulations.

However, treating each disease as a completely individual case just isn’t feasible at scale, and is usually also not appropriate. So the first anamnesis will always focus on common diseases, and rightly so.