To further test Richard Lynn’s theory that the Flynn effect is caused by nutrition and health increasing the size and functioning of the brain, I compared the U.S. army’s anthropometric data from 1966 with data from 2012, looking  for evidence of brain growth since the Vietnam war.  My source for the 1966 and 2012 data are THE BODY SIZE OF SOLDIERS _ U.S. Army Anthropometry-1966 and 2012 ANTHROPOMETRIC SURVEY OF U.S. ARMY PERSONNEL: METHODS AND SUMMARY STATISTICS respectively.

Adult male head circumference 1966


Adult male head circumference 2012



Adult male head length in 1966


Adult male head length in 2012


Adult male head breadth in 1966



Adult male head breadth in 2012


Adult male head height in 1966



Adult male head height in 2012



By 1966 standards,adult male U.S. head circumference, head length, and head breadth increased by 0.83, 0.66, and 0.27, standard deviations (SDs) respectively, though head height seems to have decreased by 0.12 SD (sampling error?).  On average adult male head size measures have increased by 0.41 SD, however a study by JC Wickett et al found that head circumference was the single best predictor of brain size, and head height may even be negatively correlated with IQ, though given the small sample size, it’s important not to over-interpret this.


According to James Flynn, performance on U.S. IQ tests has been increasing by 3 point per decade since the earliest days of testing.  This would suggest an IQ increase of 13.8 points from 1966 to 2012 or about 0.92 SD.  This is only slightly more than the 0.83 SD increase in head circumference over the same period.  And army data may slightly underestimate the head size increases because in 1966, even sons of the elite were often forced to join the army because of the draft, while in 2012, the poor and non-white have been forced to do more than their fair share of army service.  Adjusting for demographic changes in the U.S. army, the head circumference increase might perfectly match the IQ increase.