In a 1968 study of the Pygmalion Effect:

Rosenthal and Jacobson gave an intelligence test to all of the students at an elementary school at the beginning of the school year. Then, they randomly selected 20 percent of the students - without any relation to their test results - and reported to the teachers that these 20% of 'average' students were showing "unusual potential for intellectual growth" and could be expected to "bloom" in their academic performance by the end of the year. Eight months later, at the end of the academic year, they came back and re-tested all the students. Those labeled as "intelligent" children showed significantly greater increase in the new tests than the other children who were not singled out for the teachers' attention. This means that "the change in the teachers' expectations regarding the intellectual performance of these allegedly 'special' children had led to an actual change in the intellectual performance of these randomly selected children"

My understanding is that a person's IQ doesn't change. See Wikipedia

How is this result possible?

  • @Articuno agreed but the experiment was conducted over a year and it saw significant change in IQ (20% if I recall correctly). This is beyond IQ changing slightly over the entire childhood. So I still think there's a contradiction.
    – Celeritas
    Jan 21, 2014 at 3:29
  • I think you've "dumbed down" the findings. The fact that some change occurs in childhood is uncontroversial. More controversial is going to be whether the intervention reported by Rosenthal and Jacobson can be replicated, and how sizeable the effect of such interventions is. Nov 28, 2017 at 18:38

3 Answers 3


IQ changes during childhood development are well known. See IQ Testing 101, page 218, for an example:

Campbell et al. reported IQ gains from age 3 to 21 years.

The gains during elementary and high school [...] were smaller than the short term gains but still substantial (4 points at age 8, 6 points at ages 6.4, 12, and 15).

From Ceci and Williams. Schooling, Intelligence, and Income:

  • staying in school can itself elevate one's IQ

  • children whose schooling was delayed experienced a decrement of five IQ points for every year that their schooling was delayed.

Given that IQ does change throughout childhood, that should remove your incredulity about the study's results.

  • Hmm I guess I was mistaking about how IQ works. However I think IQ is supposed to be culturally independent so I don't see how school would affect this.
    – Celeritas
    Jan 21, 2014 at 4:02
  • I don't think they mean "culture" in that sense.
    – Oddthinking
    Jan 21, 2014 at 5:19
  • @Celeritas the number is supposed to be culturally independent. To accomplish that the testing is made culturally dependent. As a result someone will likely not score the same on IQ tests taken in different countries (or even different years in the same country) even if his mental faculties remain the same.
    – jwenting
    Jan 21, 2014 at 20:33
  • @jwenting interesting, do you have a source for this fact?
    – Celeritas
    Jan 21, 2014 at 22:11
  • this article implies that IQ tests are culturally dependent, I guess there's a difference between the official statement that they aren't and what peoples opinion of them are psychology.jrank.org/pages/161/Culture-Fair-Test.html
    – Celeritas
    Jan 21, 2014 at 22:15

My understanding is that a person's IQ doesn't change.

This is an misunderstanding. When it is corrected, the conflict disappears.

(The other answers agree that this is a misunderstanding, but I don't think they adequately illustrate that how very incorrect this is.)

First, the provided notability link from Wikipedia does not claim that IQ remains the same. It doesn't support this misunderstanding.

IQ can change to some degree over the course of childhood. However, in one longitudinal study, the mean IQ scores of tests at ages 17 and 18 were correlated at r=.86 with the mean scores of tests at ages five, six, and seven and at r=.96 with the mean scores of tests at ages 11, 12, and 13.

So there is a strong correlation - on average they don't change much, but they do change over childhood.

This idea that IQ changes over childhood is supported by other studies. HEre is a selection.

However, IQ doesn't just change over childhood. More importantly, it doesn't just change over long time periods. It may change over the source of a single day. Here is a selection of studies supporting that idea:

Assuming that intelligence is a constant over the lifetime of a person over-estimates the ability of IQ tests to pull out some fundamental property of a person, rather than measuring their performance on the day.

  • 2
    So, it's more like measuring somebody's 100m time?
    – user5582
    Jan 22, 2014 at 15:19
  • -1 since the study the OP discussed was over 1 year, NOT over 10 or even 5 years. I don't see how the information above - even if correct - applies. Your end of answer (showing changes over single day) also have nothing to do with the study being discussed.
    – user5341
    Feb 13, 2014 at 16:09
  • To be fair, this is mostly the fault of the OP that asked a useless trivia question that has nothing to do with the real study. It should have been "can the IQ changes be statistically significantly affected in random group of people"
    – user5341
    Feb 13, 2014 at 16:13
  • @DVK: I haven't quite understood the nature of your concern. The OP's objection was based on a fundamental misunderstanding, hopefully now corrected. I tried to show how big a misunderstanding it was with the day-to-day IQ fluctuations. But I also showed some examples of longer-term changes in IQ. True, my answer doesn't address whether the particular cited study was valid; merely that it wasn't invalid due to the objections the OP gave, which addressed the question they seemed to be asking.
    – Oddthinking
    Feb 13, 2014 at 22:17

Summary: there seems to be strong evidence that IQ is both highly hereditary (in simple terms, "nature"), but is also extremely negatively affected by bad socioeconomic status ("nurture"). If we allow the assumption that the latter effect takes place over a relatively long period of time (as opposed to say, a special few weeks of neglect early on), then we have to admit that IQ can indeed change, depending on the level of "nurture" that takes place over time.

The heritability of IQ article on wikipedia gives a good summary, with decent references. Specifically, in their article: The heritability of IQ, Devlin et al. found the heritability to be around 0.5, while in Variability and stability in cognitive abilities are largely genetic later in life, Plomin et al. found it be even higher, at 0.8. This means that the IQ of closely related individuals is highly correlated.

On the other hand, in Socioeconomic Status Modifies Heritability of IQ in Young Children Turkheimer et al. find that the correlation goes down to almost 0 (even for "identical" twins), for extremely impoverished families. From the article's abstract:

The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.

So, here we see strong evidence that the environment greatly affects IQ. Assuming that this effect occurs over most of childhood (unfortunately I'm unable to find any articles along these lines at the moment), there is no reason to believe that IQ cannot change. Indeed, an improvement in the children's condition (special treatment, extra tuition, or simply encouragement / placebo effect), as we see in the study you cite, should lead to some kind of improvement.

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