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I am wondering if someone could direct me to a longitudinal study examining IQ test scores over time. For example, testing a group of test subjects every 10 years from age 5 to age 65.

The reason I am looking for such a study is I am interested to know what proof there is or is not that IQ is stable over time. It seems to me that it is accepted that IQ should be stable over a lifetime, but I haven't found any study that demonstrates this.

If we assume that IQ is the result of genetics and early childhood environment, then it follows that it should be stable over a lifespan, but I am looking for a study that demonstrates this through repeated IQ measurement over the lifespan.

I'll just give a bit of background about what I understand about IQ below:

  • IQ is obviously not an exact measure. It's reasonable that a person's measured IQ could vary by around +/- 6 IQ points from their true IQ, therefore it is still a stable measurements if all IQ measurements over a lifespan are within a 12 point range.
  • Brain damage such as catastrophic injury or regular alcohol abuse can permanently lower IQ.
  • During adolescence there has been shown to be greater than normal fluctuation in IQ. I would suppose that, because IQ is a measure against age peers, the wide differences in maturation for individuals in the adolescent age range account for the fluctuations in IQ. However I would be interested to know if there were studies that showed children's IQs to have permanently raised or lowered during adolesence.
  • If an individual has difficulty with an IQ test not related to raw intellegence, for example, if they have ADHD and therefore difficulty concentrating, then treating their issue may allow them to score higher on an IQ test than prior to treatment. In these cases it would seem that the condition masked their intelligence, and treatment did not improve their intelligence but allowed it to be measured more accurately. The same could probably be said of an individual for whom cultural biases in an IQ test (eg with verbal knowledge) worked against them.
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  • $\begingroup$ IQ is not going to be stable from 5 to 65. That's too wide of an interval $\endgroup$ – Fizz Mar 9 at 0:28
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There are some subtleties to this question which you may or may not realize. There are (at least) two similarly sounding questions, but they are actually quite different questions:

  • Does an individual's general cognitive abilities change throughout life?

  • Does the rank-order of a population's cognitive abilities change throughout life? In other words, if we test a population at one time and then again at another time, will those who scored highly on the first test also be the ones who score highly on the second test and vice versa?

The answer to the first is trivially yes. An individual's brain is more powerful and has learned more when they are 22, then when they are 4, say. There is also reduction in cognitive abilities when people become elderly.

The second question is more interesting. For this I would refer first to the paper:

Deary et al (2000). The stability of individual differences in mental ability from childhood to old age: Follow-up of the 1932 Scottish Mental Survey. See pdf here.

They conclude that

The correlation between Moray House Test scores at age 11 and age 77 was 0.63, which adjusted to 0.73 when corrected for attenuation of ability range within the re-tested sample. This, the longest follow-up study of psychometric intelligence reported to date, shows that mental ability differences show substantial stability from childhood to late life.

So the rank-order of abilities within a population probably correlates >0.7 with its rank-order six and a half decades later (age 11 to age 77).

Another interesting study, which is new and I don't believe is published yet, is the following:

Grønkjær et al. Associations between education and age-related cognitive changes in a 41-year follow-up study

In this Danish longitudinal study, they have them first tested at mean age 20, then again 41 years later at mean age 61. They report finding:

The mean change in BPP test scores was -2.94 (SD 5.60) and a retest correlation of 0.81 between the baseline and follow-up BPP scores was observed.

These correlations are not quite in the 0.9-1.0 range, and hence we're certainly not seeing perfect stability. On the other hand, I don't think I know any quantitatively measured psychological trait which shows a greater longitudinal stability (in terms of rank-ordering) than general cognitive ability.

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Eff basically has a good answer, but more recent studies, such as one published in Nature 2011 have some additional data with respect to the observed variation over time not being just random measurement error but partially explainable through changes in the volume of some brain areas:

In the absence of neurological insult or degenerative conditions, IQ is usually expected to be stable across lifespan, as evidenced by the fact that IQ measurements taken at different points in an individual’s life tend to correlate well. Nevertheless, strong correlations over time disguise considerable individual variation: for example a correlation coefficient of .7 (which is not unusual with verbal IQ) still leaves over 50% of the variation unexplained. [...]

The changes in brain structure that correlated with changes in IQ allow us to explain some of the variance in terms of brain development. Specifically, 66% of the variance in Time 2 V[erbal]IQ was accounted for by Time 1 VIQ, a further 20% was accounted for by the change in grey matter density in the left motor speech region, with the remaining 14% unaccounted for. Similarly, 35% of the variance in Time 2 P[erformance]IQ was accounted for by Time 1 PIQ, with 13% accounted for by the change in grey matter density in the anterior cerebellum, leaving 52% unaccounted for. [...]

Further studies are required to determine the generalisability of this finding: for example, is the same degree of plasticity present throughout life or are the adolescent years covered by this study special in this regard.

What I find interesting about this study is that they were able to explain a lot more of the change in verbal IQ than performance (and thus full scale) IQ.

And if you're looking at the stability of IQ raw scores (as opposed to rank order), early adulthood is when this happens [with respect to rest of life], so age of 5 is way to soon.

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This is based on a meta-analysis, so larger circles show greater data weight & precision.

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    $\begingroup$ Yes, I should've added the meta-analysis you posted in the end. I actually knew of that one, but forgot about it when answering. However, regarding the neuroscience study, I remain skeptical. Very small sample size (N = 33). This is a huge issue in neuroscience. $\endgroup$ – Eff Mar 9 at 14:09

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