Re: Is there a correlation between head size and IQ?

There is evidence to support both sides of this question. For example, Reed and Jensen report an equal head size for a high IQ group and a low IQ group (Reed, T.E., & Jensen, A.R. 1993. Cranial capacity: new Caucasian data and comments on Rushton's claimed Mongoloid-Caucasoid brain-size differences. Intelligence, 17, 423-431) while Wickett reported a relationship between head size and IQ (Wickett, J.C., Vernon, P.A., & Lee, D.H. 1994. In vivo brain size, head perimeter, and intelligence in a sample of healthy adult females. Personality and Individual Differences, 16, 831-838). My view is that head size is unrelated to IQ in healthy people.

The idea that head size is related to intelligence has attracted people for years (Galton, F. 1888 Head Growth in Students at the University of Cambridge, Nature, 38; 14-15; Pearl, R. 1906 On the Correlation between Intelligence and the Size of the Head, Jour. Comp. Neurol. and Psychol., 16: 189-199.) People often draw pictures of people with larger heads than is realistic, possibly because the human mind is what makes us ourselves (Gellert E Children's constructions of their self-images. Percept Mot Skills 1975 Feb;40(1):307-24). Another reason people might be so interested in this is that we might like to be able to 'judge a book by its cover', and tell if someone was smart just by looking at them.

There are two real questions at the center of the question of whether head size is related to IQ. One is whether the authors are trying to prove that a certain 'type' of person is smarter than another. J. P. Rushton is an example of this. Rushton straightforwardly states his belief that "evolution is progressive and that some populations are more 'advanced' than others" (Rushton JP Contributions to the history of psychology: XC. Evolutionary biology and heritable traits (with reference to oriental-white-black differences): the 1989 AAAS paper. Psychol Rep 1992 Dec;71(3 Pt 1):811-21). What he does not state is that these beliefs may be influencing the way he does his research, and therefore he may be finding evidence for what he wants to find. This introduction of bias into research is an important problem, and of great ethical importance in research that addresses differences supposedly due to race or sex (Kaufman JS; Cooper RS; McGee DL Socioeconomic status and health in blacks and whites: the problem of residual confounding and the resiliency of race Epidemiology 1997 Nov;8(6):621-8). Rushton says that brain size is related to IQ in normal adults (Rushton, J.P. 1992 Cranial capacity related to sex, rank and race in a stratified random sample of 6325 U.S. military personnel. Intelligence, 16, 401-413; Rushton JP, Ankney CD Brain size matters: a reply to Peters. Can J Exp Psychol. 1995 Dec;49(4):562-9; discussion 570-6). It is my opinion that Rushton uses bad science (http://goinside.com/98/3/postmod.ht ml; http://www.sciam.com/1197issue/1197review1.html) to support racist views.

The second question at the center of the relationship of head size to IQ is whether other factors are affecting the results. Other factors such as nutrition may be involved, and correcting for these factors may eliminate the relationship between head size and IQ (Passingham RE Brain size and intelligence in man. Brain Behav Evol 1979;16(4):253-70). Michael Peter says that

"nutritional and environmental conditions which foster good development of intelligence can be expected to foster good physical brain development as well. . . .(Therefore it is) useful to control for body size when looking at brain/IQ relations (because the) relation between brain size and IQ is confounded by nutritional and environmental conditions. This does not change the basic observations, but it does change arguments about causation."

In people who are not able to grow healthily either before or after birth, head size may be related to mental function (Hack M; Breslau N; Weissman B; Aram D; Klein N; Borawski E. Effect of very low birth weight and subnormal head size on cognitive abilities at school age N Engl J Med 1991 Jul 25;325(4):231-7) These researchers found that babies who weighed less than 1.5 kg (about 3 and a half pounds) at birth, and who did not grow as large as most babies at eight months, also had lower IQ scores. This research suggests that brain development, and therefore IQ, was affected by the same factors that affected overall growth and head size in these children. When pregnant women were exposed to radiation during the tragic bombing of Hiroshima, their babies had both small head size and mental retardation. (Miller RW Effects of prenatal exposure to ionizing radiation. Health Phys 1990 Jul;59(1):57-61) Children who were exposed to rubella before they were born, who were smaller than average and had smaller than average head size, had normal IQs (Macfarlane DW; Boyd RD; Dodrill CB; Tufts E Intrauterine rubella, head size, and intellect. Pediatrics 1975 Jun;55(6):797-801). Even in children with a medical problem in which the pressure of the fluid in their brains is very high, the IQ score is not a good measure of intellectual ability because some of their mental abilities are affected (mostly motor skills) and some are not (Hammock MK; Milhorat TH; Baron IS Normal pressure hydrocephalus in patients with myelomeningocele. Dev Med Child Neurol Suppl 1976;(37):55-68). So even in people with abnormal growth and brain development, IQ and head size may not be related.

Finally, Mark Tramo has the last word in his article "Brain size, head size, and intelligence quotient in monozygotic twins." By MJ Tramo; WC Loftus; TA Stukel; RL Green; JB Weaver; & MS Gazzaniga, published in Neurology, Vol 50, No 5, May 1998, pp. 1246-52. Dr. Tramo uses magnetic resonance imaging (http://128.227.164.224/mrituto r/index.html) to look at the relationship of head size and Intelligence Quotient (IQ) in twins. They found that twins had similar brain size, body weight, head size, and IQ. Brain size and head size were related. But IQ was not related to brain size or head size. These scientists decided that brain size was related to head size, but not to IQ in young, healthy adults.