Yes, the decline is genetic

Average polygenic score for educational attainment, by year of birth in Iceland. The blue line is a quadratic fit for the full range of birth years. The red line is a linear fit for people born in or after 1940. (Kong et al. 2017)

For most of the 20th century mean IQ went up at the rate of 3 points per decade throughout the Western world. This is the Flynn effect (Rindermann 2018, pp. 85-89). Much of the increase seems to have involved a change in mental priorities rather than a rise in intelligence: a culture of doing as we're told has given way to a culture of having several possible responses and picking the right one. But some of the increase seems real, being perhaps due to better nutrition and a more stimulating learning environment.

The Flynn effect is now running out of steam (Flynn 2007, p. 143). In Scandinavia, mean IQ peaked during the late 1990s and has since declined (Teasdale and Owen 2005). Using the Norwegian population registry, two economists, Bernt Bratsberg and Ole Rogeberg, attribute this decline largely, if not entirely, to "within-family variation." In other words, IQ has been declining even among people of similar genetic background, i.e., siblings. So this decline is not due to the poor outbreeding the rich or immigrants outbreeding natives.

All the same, a genetic cause cannot be excluded. In Norway, siblings are less and less genetically similar; they are increasingly half-siblings. This factor can especially affect the methodology of Bratsberg and Rogeberg (2018) because the IQ decline is most measurable between siblings who are born farther apart. As this birth interval increases, so does the probability that the younger siblings is a half-sibling.

This is not a minor factor. Bratsberg and Rogeberg were looking at pairs of brothers. (The IQ data come from the military conscript register, and only men are subject to conscription). To produce a pair of brothers, a woman has to have three children on average. Among Norwegian women with three children, 36.2% have had them by two or more men (Thomson et al. 2014). Furthermore, because those children tend to be born farther apart than children born to the same father, they contribute more to within-family variation.

If the genetic basis of intelligence has been declining within Norwegian families, specifically between older and younger half-siblings, two things must be happening:

1. The average divorced mother has her second child by a man who belongs to a lower-IQ segment of the Norwegian population.

2. Such men have been contributing more than other men to succeeding generations of Norwegians, at least during the last forty years. This point is important. Even if we look only at first-born sons, mean IQ has steadily declined among Norwegians born since c. 1975 (Bratsberg and Rogeberg 2018).

The first point has been proven by Lappegård et al. 2011) in their study of fatherhood and fertility in Norway. Multi-partner fatherhood is most common among men with the lowest level of education (10 years of schooling, "i.e., compulsory education"). Second place goes to men with college or university education (14 to 17 years of schooling, "tertiary degree"), and third place goes to men with upper secondary (11 to 13 years of schooling). 

At age 45, about 15 percent of all men in the 1960-62 cohort with a compulsory education had had children with more than one woman, compared to about 5 percent among men with a tertiary degree. If looking at fathers only (Figure 6), the pattern becomes even more pronounced. At the lowest educational level, 19.3 percent of those who had become fathers, had children with more than one woman, compared to 6.1 percent of those at the highest educational level. (Lappegård et al. 2011)

As for the second point, Lappegård et al. (2011) found that reproductive success is more variable among men with the lowest level of education. Such men have the highest rate of childlessness of all three groups, while having the highest level of multi-partner fertility. Moreover, multi-partner fertility has increased over time among these men, while childlessness has remained constant. Their overall reproductive success has thus gone up:

Like childlessness, multi-partner fertility has increased across cohorts, but unlike childlessness it has increased more among men with lower education than among those with higher education. From the 1940-44 cohort to the 1960-62 cohort the proportion of fathers who had children with more than one woman more than doubled (from 8.9% to 19.3%) in the compulsory schooling group, while it only rose by about 30% in the highest tertiary group, from 4.7 to 6.1 percent. (Lappegård et al. 2011)

Thomson et al. (2014) made the same observation:

In all countries [Australia, United States, Norway, Sweden], however, education is negatively associated with childbearing across partnerships, and the differentials increased from the 1970s to the 2000s.

Moreover, official statistics do not fully capture multi-partner fatherhood. It can be difficult to identify the paternity of children whose biological father is little more than a sperm donor. Lappegård et al. (2011) allude to this difficulty: "some of these men have never been in a stable relationship with the mother." This is less of a problem in Norway, where "only about 1-1.5 percent of the total number of children has no registered father."  The registered "father" may nonetheless be a cuckolded husband or a boyfriend who has agreed to assume paternity of the unborn child. The second situation is not uncommon if the woman is still young and attractive.

It seems, then, that modern Norwegian culture is facilitating the reproductive success of low IQ men. One such man was Anders Breivik's stepfather:

My stepfather Tore, one of my best friends Marius and my more distant friends Kristoffer, Sturla and Ronny are all living manifestations of the complete breakdown of sexual moral. All five have had more than 300 sexual partners (two of them more than 700) and I know for a fact that three of them have one or more STDs (probably all of them).

[...] My mother was infected by genital herpes by her boyfriend (my stepfather), Tore, when she was 48. Tore, who was a captain in the Norwegian Army, had more than 500 sexual partners and my mother knew this but suffered from lack of good judgement and moral due to several factors (media - glorification of certain stereotypes being one).

[...] Tore, my stepfather, worked as a major in the Norwegian military and is now retired. I still have contact with him although now he spends most his time (retirement) with prostitutes in Thailand. He is a very primitive sexual beast, but at the same time a very likable and good guy. (Breivik 2011)

The evidence from Iceland

Iceland isn't Norway but it is culturally similar. According to a recent study, the genetic basis of intelligence has been declining in that country since the cohort born in 1910. The authors used a "polygenic score," based on alleles associated with high educational attainment, to measure the genetic potential for academic achievement from generation to generation:

Here, we investigate the effect of this genetic component on the reproductive history of 109,120 Icelanders and the consequent impact on the gene pool over time. We show that an educational attainment polygenic score, POLYEDU, constructed from results of a recent study is associated with delayed reproduction (P < 10-100) and fewer children overall. The effect is stronger for women and remains highly significant after adjusting for educational attainment. Based on 129,808 Icelanders born between 1910 and 1990, we find that the average POLYEDU has been declining at a rate of ~0.010 standard units per decade, which is substantial on an evolutionary timescale.

This is the same polygenic score I've discussed in previous posts, such as Frost (2018). Certain genetic variants are associated with high educational attainment and others with low educational attainment. Do these variants determine our capacity for intelligence? For the most part, yes, but I suspect that many of them have a stronger bearing on time preference or willingness to sit still in a classroom. The authors concede this point:

We postulate that, in addition to being correlated with cognitive ability (32, 33), POLYEDU is capturing a portion of the propensity to long-term planning and delayed gratification. (Kong et al. 2017)

These other traits still matter. Together, they form a mental/behavioral package that coevolved with the rising middle class over the last millennium, eventually spreading through all social strata (Clark 2007; Clark 2009a; Clark 2009b). That evolution is now unravelling. Reproductive success is shifting toward individuals with "fast life-history": lower cognitive ability, weaker orientation toward the future, and, for men, a larger number of sexual partners with less investment in the resulting offspring (Frost 2012; but see also JayMan 2012). This shift began seventy years before the decline in IQ scores.

It seems, then, that the Flynn effect has masked a longer-term decline in the genetic basis of intelligence and other mental/behavioral traits. This is in line with other recent findings. Woodley et al. (2013) argue that mean reaction time has increased in Great Britain by 13 points since Victorian times, although this finding may be an artefact of better sampling of the general population over time (hbd* chick, 2013). Another study, however, using Swedish subjects, has confirmed this lengthening of reaction time, particularly in cohorts born since the 1970s (Madison 2014; Madison et al. 2016).

Conclusion

The recent reversal of the Flynn effect seems to result from two trends:

1. a positive trend based on increasing familiarity with tests and test-taking, as well as improvements in nutrition and a more stimulating learning environment;

2. a negative trend due to dysgenic factors.

For most of the 20th century the positive trend overwhelmed the negative trend. In Norway, the negative trend has had the upper hand in post-1975 cohorts, partly because the positive trend has exhausted all room for improvement and partly because the current culture is facilitating the reproductive success of sexy, low-IQ men.

I've long believed that human evolution didn't stop in the Pleistocene. Nor did it slow down. In fact, we've changed much more over the past 10,000 years than over the previous 100,000, and I'm talking here not only about our outward appearance but also about our inward qualities of mind and behavior. But we can quickly lose what we so quickly gained. This reverse evolution is now taking place, and it’s visible even in the relatively closed system of Iceland's gene pool.

I used to be unconcerned about dysgenics. Any negative trends would surely take hundreds of years to produce serious consequences. So we would have plenty of time to get all of the relevant facts, discuss everything thoroughly with everyone, and reach a consensus. Well, I was wrong. Our dystopic future is close at hand.

References

Bratsberg, B., and O. Rogeberg. (2018). Flynn effect and its reversal are both environmentally caused. Proceedings of the National Academy of Sciences Jun 2018, DOI: 10.1073/pnas.1718793115

https://sci-hub.tw/https://doi.org/10.1073/pnas.1718793115  

Breivik, A. (2011). A European declaration of independence.

https://publicintelligence.net/anders-behring-breiviks-complete-manifesto-2083-a-european-declaration-of-independence/

Clark, G. (2007). A Farewell to Alms. A Brief Economic History of the World. Princeton University Press: Princeton and Oxford.

Clark, G. (2009a) The indicted and the wealthy: surnames, reproductive success, genetic selection and social class in pre-industrial England.

http://www.econ.ucdavis.edu/faculty/gclark/Farewell%20to%20Alms/Clark%20-Surnames.pdf

Clark, G. (2009b). The domestication of man: The social implications of Darwin. ArtefaCTos 2: 64-80.

https://www.researchgate.net/publication/277275046_The_Domestication_of_Man_The_Social_Implications_of_Darwin

Flynn, J.R. (2007). What is Intelligence? Beyond the Flynn Effect. Cambridge University Press.

https://books.google.ca/books?id=qvBipuypYUkC&printsec=frontcover&hl=fr&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

Frost, P. (2018). A new yardstick. Evo and Proud, May 14

http://evoandproud.blogspot.com/2018/05/a-new-yardstick.html

Frost, P. (2012). Are the cads outbreeding the dads? Evo and Proud, November 3

http://evoandproud.blogspot.com/2012/11/are-cads-outbreeding-dads.html

Hbd* chick (2013). a response to a response to two critical commentaries on woodley, te nijenhuis & murphy (2013), hbd* chick, May 27

http://hbdchick.wordpress.com/2013/05/27/a-response-to-a-response-to-two-critical-commentaries-on-woodley-te-nijenhuis-murphy-2013/

JayMan (2012). It's not the cads, it's the tramps. JayMan's Blog, December 28

https://jaymans.wordpress.com/2012/12/28/its-not-the-cads-its-the-tramps/

Kong, A., M.L. Frigge, G. Thorleifsson, H. Stefansson, A.I. Young, F. Zink, G.A. Jonsdottir, A. Okbay, P. Sulem, G. Masson, D.F. Gudbjartsson, A. Helgason, G. Bjornsdottir, U. Thorsteinsdottir, and K. Stefansson. (2017). Selection against variants in the genome associated with educational attainment. Proceedings of the National Academy of Sciences 114(5): E727-E732.

http://www.pnas.org/content/early/2017/01/10/1612113114.full

Lappegård, T., Rønsen, M., & Skrede, K. (2011). Fatherhood and fertility. Fathering 9: 103-120.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.839.2752&rep=rep1&type=pdf

Madison, G. (2014). Increasing simple reaction times demonstrate decreasing genetic intelligence in Scotland and Sweden, London Conference on Intelligence, Psychological comments, April 25

#LCI14 Conference proceedings

http://www.unz.com/jthompson/lci14-questions-on-intelligence/

Madison, G., M.A. Woodley of Menie, and J. Sänger. (2016). Secular Slowing of Auditory Simple Reaction Time in Sweden (1959-1985). Frontiers in Human Neuroscience, August 18

https://www.frontiersin.org/articles/10.3389/fnhum.2016.00407/full

Teasdale, T.W., and D.R. Owen. (2005). A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse. Personality and Individual Differences 39(4): 837-843.

https://doi.org/10.1016/j.paid.2005.01.029

Thomson, E., T. Lappegård, M. Carlson, A. Evans, and E. Gray (2014). Childbearing across partnerships in Australia, the United States, Norway, and Sweden. Demography 51(2): 485-508

https://link.springer.com/article/10.1007/s13524-013-0273-6

Woodley, M.A., J. Nijenhuis, and R. Murphy. (2013). Were the Victorians cleverer than us? The decline in general intelligence estimated from a meta-analysis of the slowing of simple reaction time. Intelligence 41: 843-850.

https://pdfs.semanticscholar.org/e8cc/634169c7c5d3e4738fe08091c86177be1380.pdf