Still No Convincing Evidence of a Relation between Brain Size and Intelligence in Humans

Lynn (1993) takes exception to my conclusion (Peters, 1991) that for individuals in the normal range with respect to nutrition and intelligence, there is no unequivocal eviden ce for a relation between brain size and psychometrically measured intelligence. In order to make the case, he lists a number of studies that show a positive correlation between intelligence and cranial capacity. In addition, he specifically refers to a very recent study that, in hi s mind, provides proof positive for such a positive correlation (Willerman, Schultz, Rutledge, & Bigher, 1991). In view of such a lengthy list of studies with positive findings, and there are many more in the literature, why would anybody remain sceptical? The answer is simple. The nature of the available evidence is not strong enough to accept the unqualified proposition th at intelligence and cranial capacity are positively correlated. Because it is not possible to addres s each single study in the literature, my concerns about methodological issues are best illustrated with reference to a few selected studies. I will begin with the methodologically strongest study o n Lynn's list (Passingham, 1979). Passingham reported a correlation of r = . 14 between brain w eight and intelligence in his study. However, Passingham also reported that when height wa s partialled out, the correlation coefficient became r = . 03 (ns). In discussing his findings, Passingham commented that other studies had not part ialled out factors like height, weight and age when relating IQ and brain size. In addition , socio - economic class, which enters the equation both in terms of nutrition and environ mental influences, had not been taken into consideration. Like others, Passingham was a lso concerned with the problem of being able to measure either IQ or brain weight directly, bu t not both in the same sample. For instance, in his living sample, IQ was measured directly and br ain weight was estimated through external cranial measurements. In Passingham's sample of decea sed persons, brain weight was measured directly, and intelligence was estimated on the basis of the socio - economic status of the individuals. Lynn appears to think that the estimate of b rain size by means of external cranial measures does not pose a problem, citing Brandt's (198 6) finding that head size has a correlation of r = . 80 with brain size. Lynn does not mention th at Brandt's work is exclusively concerned with infant development. Brandt (1986) is careful to st ate that "There is a close relationship between head circumference growth and brain development in the normally growing infant up to 2 years of age" (p. 452). In adults, the relation between external skull measurements and brain measurements is much less reliable, as the extensiv e anatomical studies of Hoadley and Pearson (1929) show. That there is a problem with regard to indirect brain size measures and or indirect intelligence measures is indicated by the fa ct that for Passingham's sample of deceased individuals (these were drawn from the 1964 Pakk enberg and Voigt study), the brain weight for persons from different socio - economic class es did not differ significantly, while significant differences were found for the living sample. I t comes of little surprise that Passingham concludes "But that they (brain size and intelligence) are in fact related can be proved only by a better study than the present one" (p. 268). It may be n oted in this context that Binet's initial attempt to find a psychometrically reliable measure that would correlate well with school performance began with an effort to correlate head ci rcumference with school performance, an effort which he soon abandoned (Gould, 1981, pp. 146 - 14 7). Binet also commented on the danger of bias in taking external head measurements, a com ment that remains topical for contemporary researchers.Any expectation that the most recent work on the brain size/IQ issue avoids meth odological pitfalls will be disappointed. …