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Regardless of whether one thinks the Flynn Effect is caused by nutrition, schooling, video games, or all of the above, a common belief is that it disproportionately impacts the left half of the bell curve.  A blog reader known as “Greying Wanderer” writes:

One aspect of the nutrition idea is nutrition might not have improved for everyone to the same degree i.e. nutrition among the upper and upper middle class may already have been fine and stayed the same while the average nutrition in the classes below went up.

Say for the sake of argument with a population average of 100 IQ those to the right of the median have an average of 115 and those to the left have an average of 85 then I guess you could have a situation where only the left side of a population like that was malnourished giving them say a depressed average of 70 IQ thus giving the combined population a depressed average of 93 (70+115). If so then improving the nutrition of the left side might lead to them reaching their natural average of 85 and the whole population increasing their average to 100 (85+115). So an increase in 7 points of the population’s average IQ even though the people on the right side of the Bell curve haven’t changed at all.

Just a thought.

That would fit an increase in average IQ without an increase in innovation.

Greying Wanderer’s argument is plausible in his example of only a 7 point Flynn Effect, but on the Raven IQ test, the Flynn Effect has been over 30 points.  In order to get a 30 point gain driven only from the left or the curve, you would need to assume that before 20th century nutrition, IQ 85 people had non-verbal IQ’s of only 25!  The notion that the left part of the curve was that severely disabled seems unlikely.

But we don’t have to speculate because we have actual data. The average British born in 1967 got 55 items on the Raven test right.  Only 5% got 39 or less right (see Figure 2 and Table 1 of this document).  From these data points we can crudely equate a Raven raw score of 55 with IQ 100 (for this age/cohort) and a Raven raw score of 39 with IQ 75, and assign all other scores very rough IQ’s through linear extrapolation (they don’t recommend this, but what choice do we have?).

Now, for the British born in 1877, the average score (IQ 100 for their cohort) was 24 and the top 10% (IQ 119 for their cohort) scored 39.  But by the standards of the newer cohort, those are IQ’s of 51 and 75 respectively.  Now James Flynn suggests that the IQ’s of the older cohort may have been depressed by up to 10 points by age (they took the test at age 65, while the new cohort took it at 25).  The new cohort also got to take the test home, while the old cohort had to sit supervised in some room, which may have depressed their IQ’s another 5 points.  Correcting for these factors brings them up to IQ 66 and IQ 90 respectively on the new norms.  So summing up, an IQ of 66 on new norms equals and IQ of 100 on old norms (a difference of 34 points) and an IQ of 90 on new norms equals an IQ of 119 on old norms (a difference of 29 points).  So maybe the Flynn Effect is 15% smaller for the brightest 10%, but it’s still roughly 2 standard deviations over 90 years.

But haven’t height gains been stronger at the low end?

Because psychologist Richard Lynn hypothesized that nutrition is responsible for the 20th century rise in both height and IQ test performance, height is a useful analogy for the Flynn Effect.  According to Arthur Jensen (see page 354 of his book The g Factor), a sample of 8,585 British adult males was measured without shoes in 1883 and had a mean height of 67.46 inches. Looking at the actual frequency table (see pg 25 of this document), I was able to infer the tallest and shortest 5% and compare these figures with a 21st century sample of non-Hispanic white men from a Western country (see table 12 of this document):

                                  Year 1883               Year 2003-2006         Difference

Shortest 5%               63 inches              65.9 inches                 2.9 inches

Average                      67.46 inches          70.4 inches                2.94 inches

Tallest 5%                  72 inches                74.9 inches                2.9 inches

So it seems height gains have affected virtually the entire height distribution roughly equally, rather than disproportionately impacting the low end.  However I can’t be too sure of that because I don’t know how representative the 1883 sample was because another source claims British men in the 19th century were 166 cm tall (which equates to 65.35 inches) so maybe there were huge masses of very short people that were missed in some studies.  However James Flynn claims that at least in Norway, height gains have been greater for tall people than for short people.

There also seems to be a myth that only the poor lower classes showed height gains since the 19th century, and that elites have always been tall.  But British scientist Francis Galton collected height data on various occupations and found that males aged 26+ of the Professional occupation (which was very elite in those days) averaged 67.91 inches tall (see table 10 in HBD Chick’s blog post) which is well below the average of young white men today and probably even more below the average if compared to today’s elite young white men.  This is strong evidence that all social classes were strongly affected by nutrition.