Richard Herrnstein hypothesized that heritability of IQ and/or educational attainment had been increasing over the course of the 20th century due to more ambitious education testing leading to more assortative mating. But is that true?

From Scientific Reports:

Genetic and environmental variation in educational attainment: an individual-based analysis of 28 twin cohorts

By Karri Silventoinen and a cast of thousands

Abstract
We investigated the heritability of educational attainment and how it differed between birth cohorts and cultural–geographic regions. A classical twin design was applied to pooled data from 28 cohorts representing 16 countries and including 193,518 twins with information on educational attainment at 25 years of age or older. Genetic factors explained the major part of individual differences in educational attainment (heritability: a2 = 0.43; 0.41–0.44), but also environmental variation shared by co-twins was substantial (c2 = 0.31; 0.30–0.33). The proportions of educational variation explained by genetic and shared environmental factors did not differ between Europe, North America and Australia, and East Asia. When restricted to twins 30 years or older to confirm finalized education, the heritability was higher in the older cohorts born in 1900–1949 (a2 = 0.44; 0.41–0.46) than in the later cohorts born in 1950–1989 (a2 = 0.38; 0.36–0.40), with a corresponding lower influence of common environmental factors (c2 = 0.31; 0.29–0.33 and c2 = 0.34; 0.32–0.36, respectively). In conclusion, both genetic and environmental factors shared by co-twins have an important influence on individual differences in educational attainment. The effect of genetic factors on educational attainment has decreased from the cohorts born before to those born after the 1950s.

Some of this might be due to more emphasis on getting everybody through high school (and later, college) over the course of the 20th century.

Another possibility is that assortative mating was always pretty high in the West. My impression is that in Victorian times, unmarried young men and women did much of their courting via letters. For example, poets Robert Browning and Elizabeth Barrett exchanged 573 letters in the 20 months before their 1846 marriage. Perhaps letters tend to lead to assortative mating of levels of prose style?

In the 20th century, telephone calls became more important in courting.

In the 21st century, text messages matter. Perhaps people fall in love with how your beloved always bothers to respond to your jokes by putting on caplocks to text “LOL” rather than just “lol.”