Why is IQ declining in Norway?

Norwegian conscripts (Wikicommons: Soldatnytt) – in Norway, the conscript register provides invaluable information on IQ. This information can then be linked to other registers of the Norwegian population registry

IQ scores have been gradually increasing since the 1930s at the rate of 3 points per decade (Rindermann 2018, pp. 85-89). This increase, named the Flynn effect, has been much debated. Are we becoming more intelligent because of better nutrition and more stimulating learning environments? Or are we simply becoming more familiar with tests and test-taking?

Whatever the cause, this increase seems to be slowing throughout the West (Flynn 2007, p. 143). In Scandinavia, mean IQ peaked during the late 1990s and has since declined (Teasdale and Owen 2005).Why? There is plenty of speculation. Perhaps the poor are outbreeding the rich. Or perhaps lower IQ immigrants are replacing higher IQ natives. Whatever the cause, it seems to be something that is more advanced in Scandinavia than elsewhere in the West. 

Two Norwegian economists, Bernt Bratsberg and Ole Rogeberg, have tried to pin it down. 

Population intelligence quotients increased throughout the 20th century—a phenomenon known as the Flynn effect—although recent years have seen a slowdown or reversal of this trend in several countries. [...] Using administrative register data and cognitive ability scores from military conscription data covering three decades of Norwegian birth cohorts (1962-1991), we show that the observed Flynn effect, its turning point, and subsequent decline can all be fully recovered from within-family variation. (Bratsberg and Rogeberg 2018)

"Within-family variation" means that IQ is declining even among people with more or less the same genetic background, i.e., siblings. More to the point, this within-family decline seems to account for most, if not all, of the IQ decline among Norwegians. So this is not a matter of the poor outbreeding the rich or immigrants outbreeding natives. In fact, it doesn't seem to reflect any sort of genetic change.

All of this assumes, of course, that the genetic differences between siblings are the same now as in the past. That assumption may appear reasonable, but appearances can deceive.

Siblings in Norway are increasingly half-siblings. Among Norwegian women with only two children, 13.4% have had them by two or more men. This figure rises to 24.9% among those with three children, 36.2% among those with four children, and 41.2% among those with five children (Thomson et al. 2014). This multiple paternity is produced not so much by family size as by the passage of time. The probability of a relationship breaking up will increase over time. Brothers who are born farther apart are more likely to have different fathers.

Stepfamilies typically arise when a woman divorces and has a child by a second husband. Or she may divorce, have a child out of wedlock, and then marry a man who provides for the family without ever fathering any children. In this chain of sexual partners, the second or third man is qualitatively different, on average, from the first man. The stepfather, in particular, is less able to compete in the marriage market for one reason or another. In general, he has been less successful in life, and life, in itself, is an IQ test.

So, within a stepfamily, one would expect to see an IQ decline between older and younger siblings. The methodology of Bratsberg and Rogeberg (2018) is especially sensitive to this stepfather effect because the IQ decline is a function of time, i.e., the decline is most visible between brothers born farther apart. As this birth interval increases, however, so does the probability that the younger brother is a half-brother.

How strong is this stepfather effect? Bratsberg and Rogeberg studied the recent IQ decline in Norway by looking at pairs of brothers. The IQ data come from the military conscript register, and only men are subject to conscription. Now, to produce a pair of brothers, a woman has to have, on average, three children. Among Norwegian women with three children, 36.2% have had them by two or more men. So this is not a trifling matter.

There would be no problem if the word "father" in the registries means "biological father." It is now common practice, however, for a stepfather to adopt his wife's children. At that point, he becomes the "father" for all intents and purposes. This has been the case since 1986, when Norway passed The Adoption Act: "If a spouse or cohabitant has adopted a child of the other spouse or cohabitant, the said child shall have the same legal status in relation to both spouses or cohabitants as if he or she were their joint child." The Adoption Act (1986), Chapter 3, Section 13.

This paternity issue affects different registers to different degrees. The family register is most vulnerable. It is updated annually and thus identifies the current legal father as the “father” (Black et al. 2011, note 4). The birth register is least vulnerable. The mother will usually identify the biological father as the "father." The exceptions are cases of infidelity or cases where a new relationship has formed during the pregnancy and the stepfather wishes to be recognized as the child's father.

Bratsberg and Rogeberg (2018) were most interested in the conscript register. This was where they obtained the IQ data. They then used the family register to locate the conscript's brother or brothers, at which point they returned to the conscript register to find that person's IQ. The family register, however, will most likely identify a stepfather as a "father." One would have to go back to the birth files and double-check, but this doesn't seem to have been done.

When I discussed this issue with one of the co-authors, Ole Rogeberg, he replied that very few of the brothers could be half-brothers because the correlation between brothers for IQ was 0.47, and this figure is similar to a previously published estimate of 0.49 for inter-sibling correlation (Paul 1980). That estimate, however, is largely based on American whites, who are a more heterogeneous population than native Norwegians. The inter-sibling correlation should be higher in Norway, and higher figures have in fact been estimated from British samples. Record et al. (1969) found a correlation of 0.55, based on 5,054 pairs of English siblings. 

As for the correlation between paternal IQ and child IQ, this is because most of the fathers are, in fact, biological fathers. Probably about two-thirds of them. That is enough to produce a father/son correlation, but it is not enough to prevent a within-family decline in IQ between older and younger brothers.

Conclusion

The Norwegian population registry has long been popular with population geneticists. It is actually a collection of different registers with information on different life events, but if you know a Norwegian's unique personal identifier you can easily navigate from one register to another to collect information on that person and on all related individuals. Black et al. (2005) have described these registers at length:

Our primary data source is the birth records for all Norwegian births over the period 1967 to 1997 obtained from the Medical Birth Registry of Norway. All births, including those born outside of a hospital, are included as long as the gestation period was at least 16 weeks. The birth records contain information on year and month of birth, birth weight, gestational length, age of mother, and a range of variables describing infant health at birth including APGAR scores, malformations at birth, and infant mortality (defined as those who die within the first year).

[...] Using unique personal identifiers, we match these birth files to the Norwegian Registry Data, a linked administrative dataset that covers the entire population of Norwegians aged 16-74 in the 1986-2002 period, and is a collection of different administrative registers such as the education register, family register, and the tax and earnings register.  These data are maintained by Statistics Norway and provide information about educational attainment, labor market status, earnings, and a set of demographic variables (age, gender) as well as information on families.

[...] Another source of data is the Norwegian military records from 1984 to 2005 which contains information on height, weight, and IQ.  In Norway, military service is compulsory for every able young man. Before entering the service, their medical and psychological suitability is assessed; this occurs for the great majority between their 18th and 20th birthday.  For the cohorts of men born from 1967 up to 1987, we have information on height, weight, and Body Mass Index (BMI), all of which were measured as part of the medical examination. We also have a composite score from three speeded IQ tests -- arithmetic, word similarities, and figures.

At first sight, this collection of registers seems to be a gold mine of information. Unfortunately, the quality of the information has suffered from a social trend that has been stronger in Scandinavia than elsewhere in the West, i.e., the redefinition of the family. As a result, the word “father” no longer has a consistent meaning. In some cases, such as birth records, it usually means the biological father—the man who provided half of the child’s genetic makeup. In other cases, such as the family register, it means the man who provides the family with at least some economic support. Increasingly, the two roles are no longer played by the same person.

This is the nub of the problem. The family unit is no longer defined as a vehicle for procreation. Its members no longer have to share a biological commonality. Increasingly, it is an administrative entity, and as such it can be dissolved and reformed in many ways.

This redefinition of the family calls into question the value of this population registry. Some of its components, particularly the birth records, still provide reliable information on biological relationships, but the same can no longer be said about information that is collected at later times in a person's life and which is continually updated. As a result, we know less and less about the people who are providing the genetic material of the next generation.

References

Black, S.E., P.J. Devereux, and K.J. Salvanes. (2005). From the Cradle to the Labor Market? The Effect of Birth Weight on Adult Outcomes. IZA Discussion Paper No. 1864

http://anon-ftp.iza.org/dp1864.pdf

Black, S.E., P.J. Devereux, and K.J. Salvanes. (2011). Older and Wiser? Birth Order and IQ of Young Men. CESifo Economic Studies 57(1): 103-120.

ftp://193.146.129.230/pdf/papers/pew/PaulDevereux.pdf

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

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

Government of Norway. (1986). The Adoption Act. Act of 28 February 1986 No. 8 relating to adoption,

https://www.regjeringen.no/en/dokumenter/ACT-OF-28-FEBRUARY-1986-NO-8-RELATING-TO/id443477/

Paul, S.M. (1990). Sibling resemblance in mental ability: a review. Behavior Genetics 10(3): 277-290.

https://pdfs.semanticscholar.org/eda5/6485400cdddc1b974b9db6edf74acb28fb23.pdf

Record, R. G., McKeown, T., and Edwards, J. H. (1969). The relationship of measured intelligence to birth order and maternal age. Annals of. Human Genetics 33: 61-69.

Rindermann, H. (2018). Cognitive Capitalism. Human Capital and the Wellbeing of Nations. Cambridge University Press.

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