In a search to find something, anything, socially pathological that correlates inversely with IQ at a statistically significant level, suicide rates seemed a plausible place to look. In the words of Lisa Simpson:
As intelligence goes up, happiness goes down. In fact, I made a graph... I make a lot of graphs.
Using WHO and IQ and the Wealth of Nations data, I found an even stronger gender-combined correlation of .54. But unlike the IQ testing data, suicide rates are generally obtained from the governments of the countries themselves, so their reliability is always in question. For example, Egypt is apparently free of suicides, while most African countries do not report on rates at all.
Among nations of primarily European descent (Canada, the US, New Zealand, Australia, in addition to the actual continent), suicide rates and IQ actually correlate inversely, albeit weakly and without statistical significance, at .08. Among this same European club, purchasing power parity and suicide rates inversely correlate at .34. In the West, while IQ doesn't appreciably influence suicide rates, wealth mildly reduces them. Thus, it seems that there are biological and/or cultural differences other than intelligence that play into suicidal tendencies.
The findings of a 2005 study out of Sweden contradict Lisa's assertion:
In one of the largest studies on suicide ever conducted, researchers found that men with especially low scores on intelligence tests are two to three times more likely than others to kill themselves.
The study was carried out in Sweden, which, along with other Scandinavian countries, has one of the highest suicide rates in the world. Epidemiologist Finn Rasmussen at the Karolinska Institute in Stockholm and two colleagues monitored nearly 1 million Swedish men, measuring their IQ when they entered national service at 18 and following them until age 44. In that time, 2,811 men committed suicide, with the highest rate found among those who scored lowest on the logical component of the IQ test.
This makes sense to me. Intelligent people tend to be more aware of what is attainable (very broadly speaking, in terms of knowledge, wealth, influence, entertainment) and also more able to attain it, than the average joe. The less endowed, in contrast, are generally less aware of what is out there and less able to attain it than others are. The smart want much, but also have the means to attain much. The modest can't obtain as much, but don't want as much, either. The want-have gap is roughly the same among those in these two broad groups.
Humans are social animals, of course. The lack of meaningful interaction puts those already predisposed to pessimism and depression at greater risk to themselves. Solitude brings with it time to reflect, instead of being constantly distracting by the needs and opportunities other people create. In the words of Denys de Catanzaro (via GNXP's God Fearing Atheist):
"It may take an intelligent animal [with time for self-reflection] to know when the situation is hopeless, to realize that purpose for life is removed in those circumstances, and that death can be self-induced."
Catanzaro's theorizes that suicide offers an advantage for gene propagation. Those who are old, estranged from their families, unable to provide for themselves, or unsuccessful with the opposite sex are more likely to commit suicide than others are. Since all of these 'conditions' put burdens on family members or offer little chance at reproduction, removing the burden is beneficial for gene propagation in that other family members will have a better chance of successfully raising offspring with the deadweight off their backs, and through evolution the poor louse knows as much. So he 'heroically' takes himself out.
The relationship between suicide rates and total fertility rates at the national level lends credence to that idea (although at the state level, there is no statistically significant relationship). Using the same cautionable numbers mentioned above, the correlation is an inverse .60 (as fertility decreases, suicides go up). Having children to care for is the strongest evolutionary reason to keep on chugging. Having kids to live for makes one value his own life that much more.
The West is already failing to replenish itself. It doesn't need a gradual uptick in the number of its people killing themselves to go along with that trend.