> I don't think there is any solid basis to say this.
The history of eugenic programs as well the blocking of male/female sex hormones to inhibit sexual development or for transitional purposes should reveal a solid basis. Brain/body are capable of recovering close to a genetic baseline, at least up to some age.
> developmental signals have widespread cascades of impact throughout the body.
I read too little about that in edge cases, but in brains and bodies that developed normally, these changes are ever so slight that, as is commonly known, radically changing diets and lifestyle can recover the genetic baseline. Physical and psychological manifestations and changes are bio-chemistry and neuro-chemistry, and I do mean the one-to-one match as much as the correlates.
Bad posture ruins the kinetic chain as much as it impedes systemic metabolic logistics (blood circulation, lymphatic transport, ...). And if it happens too early and goes on for too long, the developmental differences to the genetic baseline become pervasive.
But bad posture isn't ADHD or anxiety or temporal lobe epilepsy, for example, for which people with ADHD have a higher susceptibility for, all due to Neuroplasticity aka neural adaptation.
But the intensity of any phenomenon depends to at least some more than "just barely" (3-5%) relevant degree on lifestyle and the psycho-social environment. Psychological traits are more nurture-dependent than physical traits but physical traits (can) have a massive influence on psychology, a lot of which can be attributed to how the environment perceives qualities and behaviors. For people with different sub-types of ADHD, this can either mean worlds-apart to their genetic baseline or just slightly off enough to be a tad bit bitter.
This was a lot but here's what I'm getting at:
> Seems unlikely that any decrease anywhere, even with increases elsewhere, post-development, would be uniformly helpful.
Any increase towards baseline is beneficial throughout the entire affected graph of neural structures, as well as proximate ones.
I assume there are fMRI studies on specific cognitive performance during ischemia and post-ischemia. Their findings should confirm that recovery from damage due to long-term hypodensity and decreased perfusion but that is also quite normal as any recovery from injury comes with a restoration of functionality.
The reason I am mentioning ischemia specifically is that brains don't stop working just because some part suffers from a reduction in blood flow. But ischemia can last for a very long time, especially if undiagnosed.
Developmental signals and ischemia are, of course, two entirely different things but the connection I see is the part where the brain reroutes neural signals simply because it's an innate mechanism, not an adaptive response. [citation needed, but I believe I read something about earlier this year. I am not uncertain.]
While ADHD is "a genetic thing", it can also be the consequence of lifestyle and "psycho-social" environment aka nurture, without any part of the essential genetic component. But the intensity or severity of positive and or negative ADHD symptoms is mostly the result of lifestyle and nurture, for which the impact(s) of drugs, diet and environment are proof.
So while it's absolutely true, that
> developmental signals have widespread cascades of impact throughout the body,
there is definitely
> functional-physical causation,
that is significant.