Has average intelligence started to decline?

After years on the increase, average intelligence test performance could be in decline. That’s according to Thomas Teasdale and David Owen who took advantage of the Danish tradition of testing the intelligence of all 18-year-old men being considered for conscription into military service.

Consistent with the observed world-wide increase in average intelligence – the Flynn Effect – the 25,000 young men assessed for military service in Denmark in 1999 performed significantly better, by about 2 IQ points, than the 33,000 tested in 1988. However, the 23,000 men tested in 2003/2004 performed significantly worse than the 1998 group, at a level almost equivalent to the 1988 cohort. This apparent decline in average intelligence matches a similar observation made in Norway among their conscripts.

So what’s causing this reversal in braininess? Teasdale and Owen rule out any effect of diet – after all, there’s been no change in average height, which would be expected to suffer if diet quality had deteriorated.

Prior research found that test performance was higher among those men with a negative attitude towards military service. On that basis, Teasdale and Owen also reject the suggestion that the decline could be due to malingering – that is, deliberate poor performance on the intelligence test to avoid military service.

Instead, the researchers surmise that the performance decline is due to “some qualitative change in the emphasis on abstract reasoning and problem-solving within the Danish educational system or a decreased emphasis on speed”. They also cite a rising proportion of immigrants in the young population as another possible contributing factor.

Finally, Teasdale and Owen noted that with average intelligence test scores beginning to rise in developing countries, the decline observed here, if representative of a larger pattern, could mark the beginning of the end for any observed differences in average IQ test scores between nations.

TEASDALE, T., OWEN, D. (2008). Secular declines in cognitive test scores: A reversal of the Flynn Effect. Intelligence, 36(2), 121-126. DOI: 10.1016/j.intell.2007.01.007

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

10 thoughts on “Has average intelligence started to decline?”

  1. It’s all 18-year-olds in Denmark for a given year. Conscription is compulsory so they all have to go along to be assessed (unless they have medical exception). The numbers have declined over the years because of a falling birth-rate and more men being disqualified for medical reasons.


  2. “Prior research found that test performance was higher among those men with a negative attitude towards military service. On that basis, Teasdale and Owen also reject the suggestion that the decline could be due to malingering”at the same time it would be good to know the percentage of 18-year olds getting exemption on medical ground. It might be that brainier ones are better at getting exempted…The paper was a good read though and it’ll be interesting to see where things will go in the future.


  3. The massive use of computer technologies doesn’t allow young people’s brain muscles to develop. Any media, according to McLuhan, is at the same time an extension and a castration. Google for instance is an extension and a castration concerning our research and answer-finding capabilities.


  4. This all presupposes, of course, that not only is there something called ‘IQ’, and that ‘IQ’ can be measured; but also that an ‘IQ test’ actually measures accurately that thing called ‘IQ’. Most likely, all that is being demonstrated here is a difference in manner/mode/type of education over time that emphasises or trains or favours slightly differing aspects of human functioning (‘intelligence’); leading to differing results when these tests are taken by different groups over time.


  5. The average intelligence of Americans has been in decline for years – just look at news story comments and craigslist ads.


  6. It has been shown in a great many different studies that people of lower intelligence tend to have on average significantly more children than those of higher intelligence. Given than environmental challenges no longer cull less intelligent people from the populace it is a simple matter of survival of the fittest. Horny stupid people simply out breed smarter individuals that naturally reproduce less successfully. Expect to see a world populated by very attractive, horny and dim people in the centuries to come.


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