The previous post, “Dysgenic Fertility Trends in Generation-X” reported on the negative correlation between white fertility and educational-attainment (a proxy for intelligence) among women ending their reproductive lives in the early 2010s (i.e., those born in the late 1960s).
I ended the post with the words “Admittedly, this is all somewhat difficult to quantify”. The above, which I compiled from a 2004 paper from Dr. Richard Lynn, represents the best effort at quantifying dysgenics I have yet found.
As it turns out, fertility among White-Americans in the mid-20th century was slightly dysgenic. [See table above]. Among white women born in the first half of the 20th century, the less-intelligent half had most of the babies. From this, we expect to see a (genotypic) IQ decline of ~0.3 points per decade (0.75 points per generation) among US-whites, according to Dr. Lynn.
This data comes from a social survey conducted over the past several decades, which has given participants 10-word vocabulary tests as a proxy for general-intelligence. These 10-word vocabulary tests have since been found to correlate very well with full IQ tests. Lots of other information was also collected, including total number of lifetime babies.
The new factor in the equation, emerging the past generation, is the impact of outbreeding. (This is discussed at greater length (here).
The USA has seen a rise in the share of births of mixed-race babies, such that ~10% of the babies borne to white-American women in the 2000s were fathered by non-white men. Considering that (1) mixed-race children are lost to the white genepool for cultural reasons, and (2) white mothers of mixed-race children are shown to be of lower-intelligence, overall (See table here), it may be that the native-dysgenics delineated in the table above are now offset.
On the other hand, among late Baby-Boomers, Generation-Xers, and Milennials, it could be that the white native-dysgenics are increasing. The impact of dysgenic fertility in more recent times is still something of an open question.