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Previously I blogged about research showing that Victorians had faster simple reaction times than modern people.  Since simple reaction time partly reflects the basic physiological speed of the brain, some folks think Victorians were (genetically) smarter than people today.

In a paper documenting the 20th century decline in reaction speed, scholar Irwin W. Silverman considers the confounding role of height.  Height has increased by 1.5 standard deviations over the last 150 years, and this may be producing spuriously slow reaction times because nerve impulses have further to travel in a taller body.  However Silverman seems to dismiss this possibility, citing research showing taller people have faster reaction times.

However within generations, taller people tend to be genetically smarter than shorter people.  This is thought to be because both height and intelligence (or at least its correlates: money, status) are socially valued, so people who have an above average amount of both, or either, tend to reproduce with one another, causing the genes for both to become associated.  In addition, some of the same genes that influence height, may also influence intelligence.  A related point is that short stature and low intelligence may both reflect genetic mutation load, or inbreeding depression.

So the fact that the nerve impulse has further to travel in tall people may be completely negated by the fact that tall people have genetically faster brains.  In other words, tall people may be so mentally quick, that they still perform well on reaction time tasks despite the test being physically biased against them.

However this genetic relationship between height and intelligence probably only holds within generations.  Between generations, heights differ for nutritional reasons, probably not genetic reasons, so tall modern people do not have a genetic advantage over short Victorians with which to negate the fact that the reaction time tests are physically biased against the tall. 

The confounding role of height may also explain why studies investigating the relationship between intelligence and nerve conduction velocity have yielded extremely inconsistent results.  Speaking of which, has anyone investigated long-term changes in nerve conduction velocity?  Measures of human NCV have been collected since the 19th century, though old studies may be crude.  

Now to further confuse the issue, even though the Victorian sample from which scientist Francis Galton collected his reaction time data was short by modern standards, they were actually tall by 19th century standards.  This is significant because attempts have been made to correct for the elite nature of Galton’s samples by adjusting the sample for occupational status.  However even when you look at the subsets of Galton’s sample who were not elite (i.e. unskilled men, aged 26+) you find they were 66.47 inches tall (see table 10 in this HBD Chick blog post), even though the average 19th century man was, according to one major study, 166 cm (65.35 inches).  

So why were even the non-elite men in Galton’s sample about 0.43 SD taller than the British average?  Perhaps, because as HBD Chick explained, Galton’s sample was not just elite, they were extremely self-selected, and that may have biased the sample independently of occupational status.  They had to come to Galton (he didn’t come to them).  These were people who were intellectually curious and literate enough to even read about Galton’s research, and motivated enough to travel (perhaps in some cases from great distances) to the museum, find Galton’s test and pay good money to take it.  And why would one want to take the test so badly unless deep down, one had reason to believe in one’s own biological superiority?  Is it really surprising that people who wanted so badly to demonstrate that they’re superior, actually were, and that they would be high, not just on intelligence, but on its weak genetic correlates: height and reaction time.  So, if even adjusting for occupation, Galton’s sample was 0.43 SD taller than other Victorians, then perhaps they were also 0.43 SD mentally faster than other Victorians of the same occupation, skewing Galton’s data.  

But above I argued that Galton’s sample did better than modern people because they’re shorter than us, but now I’m arguing they did better than other Victorians because they were taller than them.  Sounds like ad hoc gibberish, and maybe it is.  But remember, within generations, good genes make people both taller and mentally quicker so tall people have faster reaction times than short people.  But between generations, nutrition improves height but does not appear to improve reaction time, so shorter generations should have an unfair advantage on reaction time tests because the nerve impulse has less distance to travel.