For example, schizophrenia. It doesn't generally show itself until one's early 20's: there are many college students who find their studies halted because of it. It is really easy to have children before this. Depression can render itself after having children, especially in women. We treat it now.
Lots of non-psychiatric disorders are similar: no problems until early-to-mid adulthood. Plenty of time to have children beforehand.
The difference now isn't that genes aren't passed along to children, but more that we try to give folks a more normal life and less misery. And we frown upon the abusive practices that were so commonplace in asylums of yesteryear.
There seems to be a bit of replying to the wrong comment going on, but I'll comment here in response to you and the parent...
This line of argument is flawed because:
1) Plenty of depression, anxiety and of course ADHD occurs in childhood, while bipolar and schizophrenia more commonly occurs in late adolescence or early adulthood. Plenty of reproduction happens - particularly by men of course - at ages later than these when these illnesses can manifest (we can even see how some men with ADHD or bipolar tendencies have reproduced at higher rates than other men, and rationalise why this would have been the case through evolutionary history).
2) The suggestion that such widespread tendencies are in the gene pool only as an undesirable accident doesn't pass Occam's razer. We must assume they are there because they have been selected for, unless there is evidence to the contrary, which nobody in this whole thread has been able to offer.
3) We can easily see reasons why it has been beneficial to have some level of propensity for tendencies like depression, anxiety, ADHD, bipolar and even some schizophrenia in the gene pool.
I don't think this is right. I understand the basis for thinking it; I've spent much of my adult life accepting or assuming this is how it works. But the more I learn (and I spent a whole lot more time reading scientific papers on the topic this afternoon), the less it holds.
First, the logic assumes the genetic coding for these conditions is never helpful to survival/replication, which as I've pointed out in several other comments, is demonstrably false.
But even if that coding really was never ever helpful, the theory relies on the notion that unhelpful DNA just keeps hanging around on the genome anyway.
Though I can't find a big amount of research on this (particularly regarding humans), there is evidence of "genome streamlining" where genes that serve no purpose get stripped out of the genome. This is necessary because it there is a cost in retaining DNA (continuing to transcribe it and modulate it), so it's far more efficient and thus helpful for survival to remove it.
The main research I've found on this pertains to birds and bats [1] (in which it is particularly important due to the metabolic cost of flying), but it would be reasonable to expect it also happens in primates, including humans, at least to some degree.
I concede this is speculative, but it's compelling.
Even still, the validity of my main point doesn't rely on this being true. We know that most mental "illness" tendencies are sometimes useful for survival/replication. That's what matters most here.
For example, schizophrenia. It doesn't generally show itself until one's early 20's: there are many college students who find their studies halted because of it. It is really easy to have children before this. Depression can render itself after having children, especially in women. We treat it now.
Lots of non-psychiatric disorders are similar: no problems until early-to-mid adulthood. Plenty of time to have children beforehand.
The difference now isn't that genes aren't passed along to children, but more that we try to give folks a more normal life and less misery. And we frown upon the abusive practices that were so commonplace in asylums of yesteryear.