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Danish national IQ
Best estimate of Danish-Scandinavian IQ based on recent, large samples is ~103 relative to UK norms.
What is the Danish national IQ? If we look at Lynn and Vanhanen 2012, it is estimated at 97.2. If we look at Becker's re-analysis, it's given at 98.45 (version 1.2). However, this is based on only two studies, one of which is quite questionable (Buj 1981). Rindermann 2007 notes:
Only one international comparison study has been carried out using a uniform intelligence test measured over a short time period under more or less standardised conditions. This is the study with the Cattell Culture Fair Test 3 (CFT3) non-verbal scale (Buj, 1981), probably conducted in the 1970s in 21 European countries and Ghana. The tests were administered in capital cities or in the biggest town in each country. But researchers believe the data from this study are of dubious quality: nobody knows the author; he did not work at a university; the way he collected so much data is unknown; the description of samples and testing procedure is scanty; and only one single two-page-long publication exists.
Both are old: 1981 (collected in 1970s?), and 1968 (Vejleskov). Can it really be that there is no more data for Denmark? The given IQs are suspiciously low from the perspective of both the climatic models and in terms of development, where Denmark is almost always in top 10 in the world. One would expect something like 100-103, depending on how much success you want to attribute to special Scandinavian autism-altruism personality combo.
I know of at least one missing study because I was a co-author:
Dutton, E., & Kirkegaard, E. O. (2014). Fluid g in Scandinavia and Finland: Comparing results from PISA Creative Problem Solving and the WAIS IV matrices subtest. Open Differential Psychology.
The Scandinavian (Sweden, Norway, Denmark) standardization of the WAIS IV on the matrices subtest is presented. The score of Scandinavia on the WAIS IV matrices is higher than Finland (weighted means 105.1 and 103.1, relative to a US norm of 100). However, the difference is not statistically significant. Finland scores higher than Scandinavia on PISA Creative Problem Solving 2012. We meta-analyze the data from both studies and estimate the Scandinavian Matrices IQ at 99.1 and the Finnish at 102.3 or 102.4 (based on US norms) depending on which sample sizes are used. Finally, we discuss theories that attempt to explain this difference.
We can probably assume equivalence of Scandinavian IQs, yielding an estimate of ~103 on the Greenwich IQ scale. US is 2 points below UK in the usual norms. Note though that this test was restricted to Danish speaking population (meaning Danes + second generation foreigners) and filtered out people with mental issues, causing some inflation of the IQ relative to true national average.
I also know of another recent comparison based on large-scale survey data:
Weber, D., Dekhtyar, S., & Herlitz, A. (2017). The Flynn effect in Europe–Effects of sex and region. Intelligence, 60, 39-45.
The Flynn effect describes the observed improvement in cognitive performance over time among individuals of the same age. We examine if the Flynn effect varies across three European regions and whether there are sex differences in the extent of improvement over time. Using SHARE-data, with a study population of 34,300 non-institutionalized individuals, aged between 50 and 84 years, we find that the Flynn effect is larger in regions which experienced the most rapid pace of development over time (Southern Europe), than in regions with relatively higher levels of development but less change over time (Central and Northern Europe). With respect to sex differences in the Flynn effect, we find that women, on the whole in Europe, have a larger Flynn effect than men. In the regional analyses, non-significant trends indicate that women gain more than men in Northern and Central Europe.
Authors note about the tests and standardization:
We investigated two cognitive tasks, assessing episodic and semantic memory, which were administered both in the 1st and the 5th wave. In the episodic memory task, ten words were read out about 5 min prior to asking the respondents to recall as many words as possible. In the category fluency task, assessing semantic memory, participants were asked to name as many animals as possible within one minute. We standardized the scores on both cognitive tasks with mean 100 and SD 15 to enable comparisons between the two measures within our analyses, as well as with broader literature based on IQ scores. Although we refrain from equating our two memory tasks with IQ scores, they could be viewed as assessments of fluid (episodic) and crystallized (semantic) intelligence.
The IQ here seems standardized to the full sample of individuals across waves, hence the approximate mean of 100 across the two waves (4 points of Flynn gains in 8-9 years? Suspicious). In any case, we see that Scandinavian IQ is at the top in both waves, as one would expect. Note that these IQs are not on the Greenwich national scale, but scaled to the rather unhelpful mean and SD of this particular west European mega-population (no Slavs represented). The within country SDs are a bit deflated due to this, but the adjustment would be small. The data then suggests a rather stark gap of ~9 points between Scandinavia and Italy-Spain, whereas other data would indicate perhaps 3-5 points. Perhaps it's best to disregard the southern data as questionable, and stick to the comparison of the central-northern countries, which reveals a gap of ~3.75 (4, 6, 1, 4) points. We can probably assume central IQ is about the same as UK, hence giving a Scandinavian estimate of ~103.75.
What about the standard IQ test standardization samples? Since the Wechsler tests (WISC, WAIS) have been used in Denmark for decades and they require at least a translation, there should be norms for each version. What does Google Scholar find? In this study, I see references to Danish (1974) and Swedish (1977) standardizations of the WISC. This obscure 1972 study mentions some results from the Danish WISC. This study tested a representative sample of 621 Danish kids using the WISC-3 but no information given about the Danish version. Jensen mentions in an old study (1978) that there is no Danish version of the WAIS-1 test, but gives mean IQ of the non-verbal tests (WAIS non-verbal and Raven's) as 106.8 and 98.8 (no Flynn correction) for a tiny sample of twins (n = 12). Probably should not be taken too seriously, as twins used to show decreased overall IQ, and the sample is tiny, but it means there's probably other data for the non-verbal part of the WAIS-1 for Denmark. Actually, this book chapter says the WAIS-1 was translated to Danish in 1958, but apparently norms were never published.
I see a paper in someone digging around for more Scandinavian IQ data.