Several studies have confirmed a slight reversal of the famous Flynn-Effect. In the 1980s James R. Flynn discovered that throughout the 20th century IQ scores had been increasing by about 3 points per decade. This effect might have been due to increasingly stimulating environments for toddlers and little children.
However, more and more studies tend to find a reversal of the Flynn-Effect (e.g. here). There are many interesting hypotheses about its causes and I would like to add my own. IQ is correlated with the personality trait “openness”. Markus Jokela (2012) has found in a study that
Higher levels of openness to experience in both sexes and higher levels of conscientiousness in women were associated with lower fertility [...]
This has been true before the 60s, but the effect has become stronger since the availability of the contraception pill. High openness correlates with being on the fast end of the life-history spectrum (openness to action) as well as the slow end (openness to ideas). It is the latter that should be highest correlated to IQ. I have argued that the slow end of the life history spectrum is populated with “hunter-gatherer” types, who (historically) had fewer children than “farmer” or “pastoralist” types.
The reversal of the Flynn-Effect could, therefore, be explained by lower fertility among high IQ hunter-gatherer types. As hunter-gatherers are collective breeders, hunter-gatherer types might find it harder to raise children in a world with little support for child-rearing than farmer types. Birth control allows them to make voluntary decisions about having children. Being more conscientious would reinforce the choice to have few/no offspring as this would entail an additional burden in child-rearing. If my hypothesis is correct it would follow that it is, in particular, hunter-gatherer type women who suffer from post-partum depression due to lack of social support.
Apart from reduced fertility, there may be additional factors that reduce trait openness in the gene-pool. One of them is a higher incidence of autism among hunter-gatherer types. Hunter-gatherer types have highly reactive amygdalas and are therefore less resilient than other types. They were not born for highly competitive and socially stressful environments. Schools and stressful modern work environments might, therefore, be additional factors that might lead to high IQ hunter-gatherers not succeed in life due to increased mental problems such as social anxiety and depression.
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