I think we have biological evidence that changes to a creature's environment, be it food scarcity, stress, overpopulation, food surplus, lack of predators, or what have you, affect the frequency and quantity of children those creatures have. Humans living in hunter-gatherer societies tend to have between 2-3 children on average it seems, whereas humans living in post-industrial agrarian societies have 4-10 children on average. The large family helps on a farm, but not for the hunter-gatherer.
The mouse utopia experiment shows how overpopulation and stress can result in a lack of offspring and even mistreatment of existing offspring. I think we are seeing this now in humans. Something tripped that said there are too many people and now we are having fewer kids.
This is similar to how grasshoppers become locusts when they are overpopulated. Sort of a biological failsafe device to prevent a species from killing itself by overpopulating.
People affected in this way were not born with this disorder to their normal functioning like a person with autism is born with autism. Rather they gained this disorder from environmental factors interacting with their system, like a person that gained depression.
This system is not something we could ever ethically test for, no IRB would allow it, but I believe it exists and is affecting us right now.
If the NIMH data on mice is true, we should be seeing decreased childbirths, increased rates of homosexuality and bisexuality, increased importance placed on personal appearance and influence, and decreased importance placed on family units and childcare. This is what the mouse utopia experiment predicted overpopulation humans would cause naturally. I think we are seeing this today in both my generation Generation Y and Generation Z.
Though I use the term overpopulation here only as a biological term, not a literal one as I believe we are far from the actual population we could potentially sustain on our planet.