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Explanations of the Correlation

Further information: Impact of health on intelligence and Heritability of IQ

An individual’s height is determined by a complex interplay between genes and environment. While statistical analysis of multiple research studies have produced a correlation between height and intelligence, it is important to acknowledge how various genetic and environmental factors influence height, and why this may be associated with measurements of intelligence.

It has been suggested that increases in average height, in response to improved nutrition, have been accompanied by an increase in brain size, and is one explanation for an overall rise in measured intelligence among all tested populations that has been called the Flynn Effect.

Heritability estimates for height can be generated using comparisons among close relatives, such as between parents and children or siblings. To measure the effect of genetic inheritance, height comparisons are correlated with total shared genetic markers. Common percent values range between 60-80%,[1] with high variation attributed to differences in a population’s genetic history and environment (such as climate, nutrition, and lifestyle).

The most notable contributing environmental influences associated with the correlation of height and IQ include “diet, disease, psychosocial stress and inadequate cognitive stimulation”.[2] Populations who live under stable conditions tend to have higher values of genetic influence, while populations susceptible to events of poor nutrition and illness show greater determination from their environment.[3]

An auxological approach that emphasizes multiple influences from all aspects of life, with special attention paid to prenatal and very early child development, seem to offer the most robust and accurate portrayal of overall growth and development. From these perspectives, we can better understand if the correlation persists, especially within developing countries prone to environmental uncertainties.[4]

Men on average are taller and have a higher IQ than women, likely as a result of the height-intelligence correlation (if height is controlled for, women have a higher average IQ).[5] This may be explained by generalizations of the Trivers-Willard Hypothesis, which predicts that tall and/or big couples are likely to have more sons than the average couple, whereas beautiful couples are likely to have more daughters than the average couple.[5][6]  As those traits are considered desirable in mates of those respective genders, they confer an evolutionary advantage to offspring.[6] Height and intelligence may be genetically correlated as a result of this, because men tend to choose beautiful women as mates, and women tend to choose men who are either tall, intelligent, or both as mates.[5] Over time this might lead to an increased average height and intelligence of the population that is primarily inherited by men.

Height and intelligence may be correlated, but there is not significant proof of it being caused by genetics. There are many genes associated with genetic transmission of intelligence, but current theories of intelligence point to inheritance of intelligence from parents to be due to both genetic and environmental factors, especially as concrete proof of the relationship between certain genes and intelligence is lacking.[7]

The correlation between height and intelligence could also be confounded by how cultural and psychological factors affect the expression of intelligence. It is impossible to create a culture-free test for intelligence,[8] as different societies value and cultivate different skill sets and ways of interpreting the world. For example, some cultures put more emphasis on hard work than being clever, making it difficult to accurately test and compare the intelligence of different groups within a population.[8]

There is evidence that intellectual development is influenced by the expectations placed on an individual.[9] In an experiment involving elementary school children, researchers found that when teachers believe students are above average intellect, these students tend to do better in school and receive higher IQ scores than students who are perceived as average or below average intellect, regardless of the actual  competence of the students. This self-fulfilling prophecy caused by expectations is known as the Pygmalion Effect,[9] and it may have an effect on taller student who are generally viewed as more mature or dominant.[10]

Sexual selection and cross-trait assortative mating might also contribute to the correlation between height and intelligence.[11] Because both height and intelligence positively affect an individual’s overall fitness, individuals generally look for these traits in potential partners. In other words, smart men are more likely to mate with tall women, and smart women are more likely to mate with tall men, as both traits are affected by perceptions of cross-trait attractiveness.[11]

1. "How much of human height is genetic and how much is due to nutrition?". Scientific American. Retrieved 2016-11-15.
2. Jump up^ Gale, Catharine (2005-06-01). "Commentary: Height and intelligence". International Journal of Epidemiology. 34 (3): 678–679. doi:10.1093/ije/dyi064. ISSN 0300-5771. PMID 15831563.
3. Jump up^ Colom, Roberto; Lluis-Font, Josep M; Andrés-Pueyo, Antonio (2005-01-01). "The generational intelligence gains are caused by decreasing variance in the lower half of the distribution: Supporting evidence for the nutrition hypothesis". Intelligence. 33 (1): 83–91. doi:10.1016/j.intell.2004.07.010.
4. Jump up^ Lampl, M.; Mummert, A.; Schoen, M. (2015-10-01). "Auxological perspectives on 'growth' in DOHaD". Journal of Developmental Origins of Health and Disease. 6 (5): 390–398. doi:10.1017/S2040174415001403. ISSN 2040-1744.
5. ^ Jump up to:a b c Kanazawa, Satoshi; Reyniers, Diane J. (2009-01-01). "The role of height in the sex difference in intelligence". The American Journal of Psychology. 122 (4): 527–536. ISSN 0002-9556. PMID 20066931.
6. ^ Jump up to:a b Kanazawa, Satoshi (2005-08-21). "Big and tall parents have more sons: Further generalizations of the Trivers–Willard hypothesis". Journal of Theoretical Biology. 235 (4): 583–590. doi:10.1016/j.jtbi.2005.02.010.
7. Jump up^ McAskie, M.; Clarke, A. M. (1976-05-01). "Parent-offspring resemblances in intelligence: theories and evidence". British Journal of Psychology (London, England: 1953). 67 (2): 243–273. ISSN 0007-1269. PMID 779915.
8. ^ Jump up to:a b Cole, Michael. "The Illusion of Culture-free Intelligence Testing". Co-Laboratory of Comparative Human Cognition. UC San Diego. Retrieved 2016-11-15.
9. ^ Jump up to:a b Rosenthal, Robert; Jacobson, Lenore. "Pygmalion in the classroom". The Urban Review. 3 (1): 16–20. doi:10.1007/BF02322211. ISSN 0042-0972.
10. Jump up^ Schank, Roger (1994). Beliefs, Reasoning, and Decision Making: Psycho-Logic in Honor of Bob Abelson. Psychology Press. p. 319. ISBN 0805814566.

^ Jump up to:a b Keller, Matthew C.; Garver-Apgar, Christine E.; Wright, Margaret J.; Martin, Nicholas G.; Corley, Robin P.; Stallings, Michael C.; Hewitt, John K.; Zietsch, Brendan P. (2013-04-04). "The Genetic Correlation between Height and IQ: Shared Genes or Assortative Mating?". PLOS Genetics. 9 (4): e1003451. doi:10.1371/journal.pgen.1003451. ISSN 1553-7404. PMC 3617178. PMID 23593038.