For a long time I had argued, based on cranium data, that brain size has been increasing in the developed world by about 1.3 standard deviations a century.  But then there was a paper published, which actually looked at secular increases in brain weight in various countries, and found very tiny increases in brain size.  I quibbled with aspects of this study (especially the use of non-age controlled standard deviations to measured standardized changes over time), but ultimately agreed that brain size gains have been much smaller than I had thought.  Still, it didn’t make any sense to me, given the much larger secular increase in head size, which is closely linked to brain size.

Well, now it seems I was right the first time.  Brain size gains really have been about 1.3 standard deviations per century, just as I originally thought.  This is evidenced by an extensive 19th century data-set by Broca which found male brains averaged 1325 g (1259 cm3  in vivo)and female brains averaged 1144 g (1087 cm3  in vivo).  I suspect these brains were taken from people who died in their late 40s (typical of the time period).

To put that in perspective,  a study by Ho et al (1980) found that White American men and women (who presumably died circa 1980) had autopsied brain weights of about 1392 g and 1252 g at age 60 and 1570 g and 1339 g at age 25 respectively.  Assuming age differences in brain weight are linear, we can deduce that 48.5 year white American men and women dead circa 1980 had autopsied brain weights of 1450 g (1378 cm3 in vivo) and  1281 g  (1218 cm3  in vivo) respectively.  So summing up:

men women
brain size of French 48ish year olds dead circa 1877 1259 cm3 1087 cm3
brain size of white american 48.5 year olds dead in 1980 1378 cm3 1218 cm3
absolute difference in 103 years 119 cm3 131 cm3
standard deviation (sd) 91 cm3 90 cm3
difference in sd units per century 1.27 1.42

Averaging men and women together, brain size has increased by 1.35 SD per century.

Now a study of identical twins born with different nutrition levels suggests that malnutrition has virtually the same effect on later brain size that it has on later performance IQ (when both are measured in SD units), but has virtually no effect on verbal IQ.  Thus if 20th century nutrition caused brain size to increase by 1.35 SD, it probably also caused Performance IQ to increase by 1.35 SD, so nutrition likely caused a 20 point per century rise in Performance IQ, a roughly 0 point per century rise in Verbal IQ, and about a 9 point rise in Global IQ.

In other words, nutrition explains about a third of the Flynn effect, which is 30 points per century.  In other words, a third of the Flynn effect is a gain in real intelligence: biological intelligence.  Another third of the Flynn effect can be explained by the rise in schooling, and the final third of the Flynn effect can be explained by people being raised in more educated affluent homes (similar to the well documented 10 point adoption effect)

Some would argue that socialization can not explain secular increases in adult IQ, because the effects of parenting vanish by adulthood, however the Dickens-Flynn models shows that to only be true within generations, not between them.

A good analogy for the Flynn effect would be if average height had increased by 2 standard deviations per century. and then it was discovered only about 0.66 SD of this increase were real.  Another 0.66 SD was because people were wearing higher heeled shoes, and yet another 0.66 SD was because parents were raising their kids to stop slouching.