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IQ tests have been around since Binet invented them in the early 20th century roughly 100 years ago. The point of the test was to measure your mental against your chronological age (hence the quotient) and then multiplied by 100, so that the average person's IQ was supposedly 100.

I have seen several claims (including this one) that IQs have risen steadily at about 3 points per decade since the early 1900s. So roughly 10 decades at 3 points each makes for what today ought to be a 130 point IQ.

Most tests today though, still score you on percentiles based on an average score of 100 points. How can these facts be consistent?

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    $\begingroup$ Binet's (and Simon's) test gave the mental age. It was William Stern (1914 (1912)) who suggested the quotient of mental age and chronological age multiplied with 100 and coined the term ›Intelligence quotient‹ and the abbreviation ›IQ‹. (— Stern, William (1914) [1912 (Leipzig: J. A. Barth, original German edition)]. Die psychologischen Methoden der Intelligenzprüfung: und deren Anwendung an Schulkindern [The Psychological Methods of Testing Intelligence]. Educational psychology monographs, no. 13. Guy Montrose Whipple (English translation). Baltimore: Warwick & York.) $\endgroup$ – huh May 28 '15 at 13:24
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Intelligence Quotient is a relative score, comparing you to the current average. It is not a fixed, raw intelligence score.

When current IQ tests are developed, the median raw score of the norming sample is defined as IQ 100 and scores each standard deviation (SD) up or down are defined as 15 IQ points greater or less,[2] although this was not always so historically. By this definition, approximately two-thirds of the population scores between IQ 85 and IQ 115, and about 5 percent of the population scores above 125.[3][4]

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Raw scores on IQ tests for many populations have been rising at an average rate that scales to three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect.

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Since the early 20th century, raw scores on IQ tests have increased in most parts of the world.[45][46][47] When a new version of an IQ test is normed, the standard scoring is set so performance at the population median results in a score of IQ 100. The phenomenon of rising raw score performance means if test-takers are scored by a constant standard scoring rule, IQ test scores have been rising at an average rate of around three IQ points per decade. This phenomenon was named the Flynn effect in the book The Bell Curve after James R. Flynn, the author who did the most to bring this phenomenon to the attention of psychologists.[48][49]

http://en.wikipedia.org/wiki/Intelligence_quotient

In addition to various IQ tests giving varying scores, each of them is also standardized to the current average intelligence score. So every test can give you a different score. Put in other words: An old IQ-test will compare you to what was average IQ at the time the test was standardized - which may be lower than todays average.

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In the US or the UK, the tests are regularly re-normed to keep the average score 100. But if you use the UK/US average as a baseline to look at IQ scores around the world, then the "average IQ" globally is closer to 88, and dropping, despite the Flynn effect (whose effect has already maxed out in many developed countries, where IQ levels are sometimes now in decline). Generally, people in lower IQ countries tend to have more children, who then grow up to be low-IQ adults (again, generally speaking, and obviously not in every case), which leads to a global decrease in the average IQ. This chart/animation shows what the estimated average is in 2015, what it was in 1950, and estimates what the global average will be in 2050 (the animation takes a few seconds to load).

This 2014 article in the Daily Mail offers a good summary of the research being done into the global drop in IQ scores. Yes, I know, it's the Daily Mail (actually Mail Online), but it does provide concise and easily Google-able references to the main researchers in this field, for example Jan te Nijenhuis, Michael Woodley, and most (in)famously Richard Lynn and Tatu Vanhanen, who pretty much pioneered the idea of how nation-level IQs have major effects on a nation's outcomes (which research has made the duo pretty unpopular, as the implications of their work aren't pretty).

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