[golover721]

> The whole point of evolution is that only genes that promote survival and replication can make it through

While this has been true in our past, we as a species have completely diverged from this. We do everything in our power to save every life no matter what the disorder or weakness. So it is no longer survival of the fittest in the traditional sense.

Whereas in the past someone with schizophrenia or major depression would not likely live a “normal” life and have children, now obviously it is very likely.

[_ywdj]

These genes didn't just appear in the last couple of hundred or even few thousand years.

And the fact that they're widespread means they're selected for, not just not selected out.

[dodobirdlord]

Genetic evidence suggest that the human population was bottlenecked to 3,000–10,000 individuals about 70,000 years ago. Not all that much time has passed since then, evolutionarily speaking, and whatever genetic predispositions were present in that tiny population are probably mostly still kicking around the gene pool. A disorder that doesn't cause problems before sexual maturity or that wouldn't pose an issue for a hunter-gatherer in the environment of evolutionary adaptation isn't really maladaptive, and there's no reason to suspect some number of such traits wouldn't be widespread.

It can also mean that the normal state of the brain exists near a small number of other stable modes, and that a wide variety of small nudges would have similar effects. The article mentions that a study of schizophrenia found that it was characterized in part by extremely rare mutations, suggesting that the brain is at an unstable local maximum, vulnerable to degrading into schizophrenia if unbalanced in any direction. Schizophrenia may in a sense be a much hardier fallback mode of brain operation.

[_ywdj]

Many of these "disorders" manifest before reproductive age; depression, anxiety, ADHD, bipolar, even schizophrenia occasionally.

Think about the condition we currently call the "disorder" ADHD.

We can easily imagine how the behavioural tendencies ascribed to it would have been beneficial in hunter-gatherer times; i.e., it would be beneficial to the tribe for a certain number of people to be restless and explorative, rather than being conformist and content doing repetitive tasks. Same goes for bipolar; think about how many of our leading writers, inventors, musicians are diagnosed bipolar, and how useful it would always have been to have had a few people with that kind of creativity and propensity for bursts of novel productivity in the tribe.

And I can easily think of ways depression and anxiety are adaptive and supportive of survival, rather than not (for depression it's the ability to contemplate/reform a troubled life, and anxiety it's the avoidance of danger). Even schizophrenia in certain circumstances.

And of course when we invoke Occam's razor, it makes far more sense that these genes are there because they're beneficial, rather than being there due to a historical accident.

[gerbilly]

Some cultures have been much more accepting of mental illness than western culture is today.

See: https://en.wikipedia.org/wiki/Heyoka

[Broken_Hippo]

That's not really correct, though.

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.

[golover721]

Yes 100% agree. That was what I was saying. Maybe you meant to reply to the parent.

[_ywdj]

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.

[watwut]

> We must assume they are there because they have been selected for

It is enough for them to not be much of obstacle or to not manifest symptoms all the time. That is enough for them to stay.

[_ywdj]

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.

[1] https://www.sciencedaily.com/releases/2017/02/170206155949.h...