|Original research study is here|
James Thompson (read his entire blog post)
These results are from a study of experts in intelligence in general, and on the Flynn Effect in particular. The expert consensus is that the Flynn effect (rising IQ scores throughout the 20th century) was the result of improved general health and mass education, but is now petering out or reversing in the West. The experts expect East Asia, India, and African to see continued gains over the next several decades. The USA, however, will see a significant downward shift, due mostly to 1) migration from lower IQ countries, and 2) lower IQ people having more children than higher IQ people. It is of course interesting that the experts also cited "decline in educational values" and "worse education and school-systems."
With regard to the increase, recognize that "improved general health" means that fewer people were exposed to hunger, lead exposure, etc. Mass education doesn't necessarily boost intelligence, but it does make one more familiar with abstractions, e.g., "How are a tiger and a horse alike?" It is important that the Flynn Effect was always observed to be stronger for Fluid, rather than Crystallized Intelligence (i.e., the sort of knowledge that one might, ideally, obtain at school). Schooling also makes one less reluctant to guess on IQ tests (which is not penalized). Schooling also makes IQ testing less of a novel experience, which it was originally intended to be.
US Census data supports the lower fertility of the more educated. In 2014, these were the Childlessness Rates for Women aged 40 to 50, by educational attainment:
No High School Diploma 9.9%
High School Diploma 15%
Some College 13.9%
Bachelor's Degree 18.2%
Graduate/Prof'l Degree 22.5%
Thought experiment: Imagine the United States fifty years from now (2066) with these numbers, and then imagine it with the numbers reversed (i.e., 22.5% childlessness for non-high school grads and 9.9% childlessness for graduate degree holders).