Height, Flynn Effect, and shared environment

Some interesting examples of shared environmental effects on height.

Atlantic: ... a database of 2,236 British soldiers who served in World War I, and then they looked up their birth records. The soldiers were relatively representative of the male population as a whole—about two-thirds of the 1890 British male birth cohort enlisted. It turns out that subtle differences in their heights hinted at their origins:

Those from white-collar backgrounds were taller: This follows the theory that wealth buys better food and living conditions, and thus greater height in adulthood. The men who hailed from the top two social classes stood a half-inch taller, on average.

The more kids there were in a household, the shorter they were: Not only because there was less food to go around, but also because it made it more likely that there were more people in each bedroom. “Crowding can help spread respiratory and gastrointestinal infections,” Hatton said. “People sneezing on each other, that sort of thing.” Each additional sibling cost the men an eighth of an inch, and having more than one person per bedroom shaved off a quarter-inch.

Children of literate mothers were taller: When mothers couldn’t read, they were less likely to know about the importance of a balanced diet or clean cutlery. The researchers measured the percentage of women by region who were only able to sign their marriage certificates with an X, rather than their name. People from areas with a high percentage of illiterate mothers were a quarter-inch shorter.

People from industrial districts were shorter than those from agricultural areas: Regardless of income, the Dickensian living conditions of 19th century British cities suppressed height by about nine-tenths of an inch.

Average European male height increased 11 centimeters between 1870 and 1970, about a centimeter per decade, or 1.5 SD in a century. Seems suspiciously similar to the Flynn Effect! In addition, over the same period of time average years of schooling went up significantly: e.g., UK 1870 4 years to 1930 7 years (see Appendix A of this paper). Most individuals born 100 years ago experienced significant deprivation by modern standards.

My own view is that there is nothing particularly mysterious about the Flynn Effect: living conditions, nutrition, and availability of education have all improved drastically in the last 150 years. So g scores should have as well. The Flynn Effect can be consistent with high heritability for non-deprived individuals in modern environments.

See also Swedish height in the 20th century and Flynn on the Flynn Effect.

The north-south gradient in average height found in Europe (see figure) may be a consequence of differential selection pressures that vary by region.