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The article People Getting Dumber? Human Intelligence Has Declined Since Victorian Era, Research Suggests says,

As for Dr. te Nijenhuis and colleagues, they analyzed the results of 14 intelligence studies conducted between 1884 to 2004, including one by Sir Francis Galton, an English anthropologist and a cousin of Charles Darwin. Each study gauged participants' so-called visual reaction times -- how long it took them to press a button in response to seeing a stimulus. Reaction time reflects a person's mental processing speed, and so is considered an indication of general intelligence.

What is the correlation between this type of "reaction time" test and "general intelligence"?

If there is such a correlation, should that imply that elite sports-players (e.g. of sports which require good reaction time, such as cricket, squash, baseball, boxing, etc.) tend to be abnormally able to succeed in endeavours such as academia, which may be presumed to correlate with "intelligence"?

  • I don't understand the last paragraph. Is that just a general question about what could be predicted if the quoted claim is true? – user5582 Nov 23 '13 at 20:08
  • @Articuno Yes it is: it's not part of the original notable claim, instead it's more my own naive understanding of what the claim seems to me to predict. Consider it a 'bonus' question if you will. You can answer only the first question and still be on topic, but perhaps I would only/more fully understand the answer if you can address the second question too. – ChrisW Nov 23 '13 at 20:18
  • Cool, that make sense. – user5582 Nov 23 '13 at 20:37
  • I think somehow a factor in all this is that reaction time can be decreased with learning for any specific task (on a short term scale) – Andrey Nov 25 '13 at 15:03
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    If reaction time is an indicator of intelligence, that means most house flies are more intelligent than I am... – Flimzy Nov 26 '13 at 16:58
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Short answer: YES, it is surprising, but it is true.


Based on existing answers and comments, there seems to be some confusion, so let's start by clarifying:

  • This question is not about whether we were smarter/faster in Victorian times. It is not about 120 year old data.
  • This question is not about how quickly people answered questions. It is how quickly they respond to a stimulus. (But, how quickly people make choices between options is a closely-related area of research.)
  • This question is not about causality, it is about correlation.
  • This question is not about what you expect might be the answer, based on your conjectures. It, like every question on this site, is what the literature shows based on empirical data.

So what does the literature say?

Wikipedia gives us a quick overview:

Researchers have reported medium-sized correlations between reaction time and measures of intelligence: There is thus a tendency for individuals with higher IQ to be faster on reaction time tests.

Research into this link between mental speed and general intelligence (perhaps first proposed by Charles Spearman) was re-popularised by Arthur Jensen, and the "Choice reaction Apparatus" associated with his name became a common standard tool in reaction time-IQ research.

The strength of the RT-IQ association is a subject of research. Several studies have reported association between simple reaction time and intelligence of around (r=-.31), with a tendency for larger associations between choice reaction time and intelligence (r=-.49).

Seems astonishing to you? Jensen (who was credited with repopularising the idea by Wikipedia, above) agrees:

It seems almost incredible that individual differences in reaction time (RT) in simple tasks that involve no intellectual content and are so easy as to be performed by most persons in less than 1 s should be correlated with scores on nonspeeded, complex tests of reasoning ability, vocabulary, and general knowledge - the kinds of content that compose IQ tests. Nevertheless, in recent years, the correlation between Rt and IQ has become an empirically well established fact, based on thousands of subjects in scores of studies conducted in many laboratories around the world.

In this this next article, he discusses how the idea was counter-intuitive and initially rejected:

Jensen has done some experiments:

The abstract is a bit light on (and I haven't read the paywalled article), but claims:

[Reaction Time] and [Movement Time] show reliable individual differences which are significantly correlated with intelligence as measured by Raven's Standard Progressive Matrices.

Moving away from Jensen, do we have anyone else making the claim? Here's the reference was used by Wikipedia.

The association between reaction times and psychometric intelligence test scores is a major plank of the information-processing approach to mental ability differences.

They measured the correlation amongst 900 56-year-old Scottish people between reaction time and AH4, a type of intelligence ("general mental ability") test.

Note: Quick stats lesson recap: Correlation coefficients range from 1 (total positive correlation) to 0 (no correlation) to -1 (total negative correlation). If there is a correlation between higher mental ability and shorter reaction time, we should expect the r-values to be negative.

AH4 Part I total scores correlated -.31 with simple reaction time, -.49 with four-choice reaction time, and -.26 with intraindividual variability in both reaction time procedures. [...] Separate analyses were conducted after partitioning the total group according to sex, educational level, social class grouping, and number of errors on the four-choice reaction time task. None of these factors significantly altered the effect sizes.

Two of the same co-authors from the previous paper, builds on the correlation between psychometric intelligence and reaction times, and the unexplained correlation between intelligence and longer life, and suggests, with (presumably) the same sample of people, it is actually the faster reaction times that are the important factor.


As an aside: Another related concept is Inspection Time:

the exposure duration required for a human subject to reliably identify a simple stimulus

A meta-analysis found that it also correlated with IQ.


As to the elite sports-player conjecture, it seems to contain a number of assumptions that are not necessarily true:

  • Most importantly, the correlation is moderate, and not total. Plenty of intelligent people may have slower than normal reaction times. Plenty of unintelligent people may have faster than normal reaction times.
  • Many sports-players do succeed in academic endeavours.
  • It isn't safe to assume that elite sports-players would want to enter academia, even if their abilities allowed it.
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    Now, If you practice to have better Rt, will it improve your overall IQ ? – jsedano Dec 4 '13 at 1:15
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    Like my question about correlation between prenatal tobacco and intelligence, I think this can suffer from confounders bias: people reacting faster are more likely to have a general better health, better growth environment, etc.. – caub May 9 '14 at 21:24
  • @jsedano hehe, that's hard to tell, I've seen (from a machine-learning course) than neurons are highly generic (auditive neurons could be trained for vision on rats), but they specialize, and become more uncorrelated, and only a few type of them renew, so I'd tend to think no to your question – caub May 9 '14 at 21:33
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Neuroscience has evolved a lot since those papers were published. Also, thousands of research have focused on intelligence... why consider only 14 of them in almost 130 years? Is it because of the use of reaction times? I'm not sure it's possible to compare data obtained 130 years ago with those of modern computerized testing. But I digress...

The reaction times studied in these papers seem to me like they only focused on visual reflexes which, as Kip cited, isn't really indicative of superior intelligence otherwise most baseball players of old would've chosen a different career.

There might not be a correlation between visual reaction times and intelligence, but there are reaction time studies that show there is indeed a link between learning and reaction times. One that comes to mind is that of Shtulman and Valcarcel (2012). It's behind a paywall, but I can summarize it for you guys.

In this research, a test consisting of 200 true/false questions was constructed. Questions were on 10 different topics of science and mathematics known for being rich in misconceptions. Some of the questions were labeled "consistent", where the actual answer was the same as the naïve answer. The others were labeled "inconsistent", meaning that the answer was different from the naïve answer. After the experiment, it was revealed that questions that need to be answered "false" always take more time to process than answers to which the answer is true, which would indicate that your brain has issues dealing with answering with a negative. But the real kicker is that questions that were labeled "inconsistent" took more time to answer on average than questions that were labeled "consistent". The authors interpret these results as meaning that to answer correctly on questions that refer to concepts that are inconsistent with naïve conceptions, one has to inhibit the prior conception, a process that takes some time and effort. In this specific study, a slower reaction time is an indicator that proves someone has realized a conceptual change and is inhibiting a prior conception to manifest a better, more scientific one.

This seems to be concordant with data obtained by Kevin Dunbar's team where inhibition has been linked with conceptual change in different fMRI studies. In one, it was shown that conceptions that are inconsistent with prior conceptions are harder to learn because your brain is actively inhibiting them, interpreting the conflicting observations as errors (Fugelsang & Dunbar, 2005). In another, it was shown that physics students still instinctively cling to the concept of impetus, despite many classes showing otherwise, and that it's only because the concept of impetus is inhibited that a proper mastery of newtonian mechanics can be demonstrated (Dunbar, K. N. & Stein, C. (2007). Do naïve theories ever go away? In Lovett (Ed.), Thinking with data (pp.193-205), New York : Lawrence Erlbaum Associates.). This study was not peer reviewed, but it was replicated by Steve Masson (2011) (in french) for his PhD, using concepts tied to electricity instead of newtonian physics, and the results were mostly identical as those claimed by Dunbar.


TL;DR
When it comes to answering tests, slower reaction times means that conceptual change was achieved and that naïve conceptions needed to be inhibited for the proper conceptions to manifest. Slower reaction time is in fact an indicator of learning taking place. Thus, faster reaction times != more intelligence.

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    This shows that difficult questions take more time to answer correctly - whether one is highly intelligent or not. Of course, someone taking a few extra milliseconds to answer a difficult question CORRECTLY, is a better sign of intelligence that someone who is faster but answers it NAIVELY. But in the case of two people who give the same correct answer, is speed important? But notice the original claim is NOT about answering a question, but responding to a stimulus (i.e. press a button when a light turns on) Finally, the fact that the data is old doesn't mean it is wrong. – Oddthinking Dec 3 '13 at 3:04
  • Shtulman & Valcarcel haven't answered yet (though their data should be able to) as to wether or not reaction times vary widely between individuals that answer correctly. One could assume that for people that have realized deeper conceptual change, answering would be faster, but that doesn't mean they are more intelligent, rather that they have mastered the concept more completely (which could be the result of time spent studying rather than because they are more intelligent). As for old data, I merely said it couldn't be compared to modern data collected using different instruments. – Dungarth Dec 3 '13 at 3:15
  • My reading is that the claim is: with modern or old equipment, higher IQ people (i.e. that answer questions correctly) tend to do better on reaction tests (i.e. that don't measure correctness of answers but speed at responding to a signal.) – Oddthinking Dec 3 '13 at 14:41

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