But how would we know? What is the reference population?
“Similarly, rapid expansion of brain size and cognitive abilities in humans has been key to our evolutionary success,” says study senior author David Kingsley, a developmental geneticist at Stanford University. However, at the same time, bipolar disorder and schizophrenia impact more than 3 percent of the world population. Kingsley reasoned this vulnerability to mental disorders might also stem from recent evolutionary changes controlling human brain size and structure. – Charles Choi, “Could Human Evolutionary Changes Be Behind Mental Disorders?” at Discover
Do we know that life forms with far more limited intelligence do not suffer in the same way? If a bear had bipolar disorder or a cat had schizophrenia, how would we know?
The human intelligence level means that, in principle, other human intelligences, capable of analysis, may be trying to tune in to what is wrong. It may or may not mean that the problem is new.
See also: Darwinian conniptions over domestic violence If we “evolved” to be a certain way, researchers would not need to find a remote, pre-industrial people to demonstrate it.
and
Some see consciousness as a form of mental disorder.