JayMan left a comment in the last post which arguably reflects a common line of thought on this issue:
But then, there is the link between IQ and mental illness (as seen from the now closed Secular Blood), which presumably includes depression. Are the lives of the low-IQ that much more miserable in general to make up for the apparently higher rates of depressed people among the high-IQ?
This is, I believe, one of those ideas which have been concocted by the less accomplished or lower status people to console themselves for the relatively worse deal they’ve received in life. Just as we often hear the refrain that “money isn’t everything” said by those who don’t have any (of course it isn’t, but being rich is better than being poor), or in such well-known phrases as “the meek shall inherit the earth: (when all experience argues against it), we sometimes hear the expression of the sentiment that being intelligent isn’t all that great. (I believe that Nietzsche had a great deal to say on this matter and what it it means for moral systems.)
Just a quick look at the literature will quash any notion that a positive correlation between mental illness and IQ exists. One of the co-authors of the following is a leading IQ scientist, Ian Deary.
Intelligence in childhood and risk of psychological distress in adulthood: The 1958 National Child Development Survey and the 1970 British Cohort Study
a r t i c l e i n f o a b s t r a c t
Lower cognitive ability is a risk factor for some forms of severe psychiatric disorder, but it is unclear whether it inﬂuences risk of psychological distress due to anxiety or the milder forms of depression. The participants in the present study were members of two British birth national birth cohorts, the 1958 National Child Development Survey (n=6369) and the 1970 British Cohort Study (n=6074). We examined the association between general cognitive ability (intelligence) measured at age 10 (1970 cohort) and 11 years (1958 cohort) and high levels of psychological distress at age 30 (1970 cohort) or 33 years (1958 cohort), deﬁned as a score of 7 or more on the Malaise Inventory. In both cohorts, participants with higher intelligence in childhood had a reduced risk of psychological distress. In sex-adjusted analyses, a standard deviation (15 points) increase in IQ score was associated with a 39% reduction in psychological distress in the 1958 cohort and a 23% reduction in the 1970 cohort [odds ratios (95% conﬁdence intervals) were 0.61 (0.56, 0.68) and 0.77 (0.72, 0.83), respectively]. These associations were only slightly attenuated by further adjustment for potential confounding factors in childhood, including birth weight, parental social class, material circumstances, parental death, separation or divorce, and behaviour problems, and for potential mediating factors in adulthood, educational attainment and current social class. Intelligence in childhood is a risk factor for psychological distress due to anxiety and the milder forms of depression in young adults. Understanding the mechanisms underlying this association may help inform methods of prevention. (Link.)
Another article, also co-authored by Deary, found that
Each standard deviation decrease in IQ results in a 12% increase in the risk of contact [with “specialised psychiatric services”], independent of gender and childhood residence.
Now, it could be that there is a specific association betwen depression – and not other mental illnesses – and intelligence. Here, Audacious Epigone found no link between depression and intelligence using the GSS as a source.
It also depends on what exactly we’re talking about. In the other post, the group was divided into only two, the less intelligent and the more intelligent, i.e. the two halves of the bell curve. Maybe the highly intelligent (top 1% or better) are more prone to depression than those less so, but on its face it seems doubtful, as those are normally among the more accomplished members of society.
The link between lower intelligence and mental illness could work in at least a couple of ways. One is that the less intelligent appear much more likely to make poor life choices, of the kind that cause one to end up in jail, with illegitimate children, or with a drug habit, for instance. That these conduce to unhappiness seems patent. Another consideration is that when the less intelligent act impulsively, they manifest a pre-existing mental illness. Finally, those who are more intelligent may be less constitutively mentally ill, they act less impulsively (higher future time orientation), or they may be able to cope with life stresses better.