Is the planet getting stupider?

It’s possible.

Women are more responsible for children’s IQ than men are. Smarter women are less likely to have children and when they do, have fewer offspring.

Via The Intelligence Paradox: Why the Intelligent Choice Isn’t Always the Smart One:

Genes determine about 80% of the variance in adult intelligence. On average, more intelligent parents beget more intelligent children. And the genes that influence general intelligence are thought to be located on the X chromosomes… It means that boys inherit their general intelligence from their mothers only, while girls inherit their general intelligence from both their mothers and their fathers. Their fathers in turn inherit their general intelligence from their mothers (the girls’ paternal grandmothers) only. So women influence the general intelligence of future generations very strongly, through their sons and through their paternal granddaughters. If more intelligent women have fewer children and are more likely to remain childless, then one potential consequence is that the average level of general intelligence in society may decline over time.

Some might respond that over the course of the 20th century people actually got smarter. It’s called the Flynn Effect. But that may be ending:

If improved health and nutrition are chiefly responsible for the secular increase in general intelligence throughout the 20th century, and if these factors no longer contribute to the increase, then the negative effect of the dysgenic fertility should lead to a declining level of average intelligence in advanced industrial nations. This is in fact happening already. There is strong evidence to suggest that the Lynn-Flynn Effect was only a 20th-century phenomenon. It appears to have ended at the end of the 20th century in the most advanced industrial nations (with simultaneously the highest rates of obesity and diabetes). Studies suggest that the average level of intelligence has begun to decline at the beginning of the 21st century in such advanced industrial nations as Australia, Denmark, Norway, and the United Kingdom.

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