He should move to China.
I just have a hard time reconciling the belief that it is difficult to change IQ through environmental factors and the notion IQ points have gone up.
I don't have any desire to study the issue in depth enough to really get an answer but Sam Harris and Jordan Peterson make the argument that IQ is largely inherited. A proposition that is easily observable. All their critiques point to the Flynn effect as a rebuttal. I just have a hard time accepting that your average 19th century man was functionally retarded. That does not feel right. I know this isn't good evidence but have you ever seen civil war letters. Those were regular guys and they would write beautiful letters home. Letters that no one with a 70 iq could write. I just don't understand how they are gauging this.
Bad IQ tests and undernourished people. There were more retarded people back then due to nutritional deficiencies so the average was lower, but once you exclude these the average would be roughly similar. Many IQ tests were heavily culturally loaded back then too and immigrants would be especially bad but also rural people(a much larger percentage at the time).
From wikipedia:
Some studies have found the gains of the Flynn effect to be particularly concentrated at the lower end of the distribution. Teasdale and Owen (1989), for example, found the effect primarily reduced the number of low-end scores, resulting in an increased number of moderately high scores, with no increase in very high scores.
[11] In another study, two large samples of Spanish children were assessed with a 30-year gap. Comparison of the IQ distributions indicated that the mean IQ-scores on the test had increased by 9.7 points (the Flynn effect), the gains were concentrated in the lower half of the distribution and negligible in the top half, and the gains gradually decreased as the IQ of the individuals increased.
[12]
So, let's say the median american has an IQ of 100 as of 2018. The median(the middle point in a distribution) IQ of 1800 was probably the same, but you had a larger number of people with very low IQs dragging the average down.
If there were only 3 people:
IQs in 1800: 60, 100, 110. Median=100. Average=90
IQs in 2018: 80, 100, 110. Median=100. Average=96.7
Not nearly as simple but it's something in that direction.