Sex Differences in Intelligence The Developmental Theory

 Introduction  A number of books and many thousands of papers have been published during the last century and a half on sex differences in intelligence as defined by Johnson, Carothers and Deary (2009) “to mean the ability to use combinations of pre-existing knowledge and abstract reasoning to solve any of a variety of problems designed to assess the extent to which individuals can benefit from instruction or the amount of instruction that will be necessary to attain a given level of competence” and measured as the IQ derived as the average of cognitive abilities obtained in tests like the Wechsler, the Stanford-Binet, the Cattell Culture Fair and numerous others. From the early twentieth century, virtually all authorities have contended that males and females have the same average intelligence. This book disputes this position and presents the developmental theory which states that in infants aged up to 4 years, girls have a higher average intelligence than boys, from the age of 6 to 15 there is little or no sex differences in intelligence, while from the age of 16 males begin to have higher average intelligence than females reaching an advantage of 4 to 5 IQ points in adults aged 21 and older.

The Nineteenth Century  In the nineteenth century, several authorities contended that men have a larger average brain size than women, that brain size is a determinant of intelligence and consequently that men have a higher average intelligence than women. This contention was advanced by Paul Broca (1861), the French physician and professor of medicine at the University of Paris, who is best known for his discovery that language is normally located toward the front of the brain’s left hemisphere, a region that is known as Broca’s area. A decade later, the same contention was asserted by Charles Darwin, who wrote in The Descent of Man: “The chief distinction in the intellectual powers of the two sexes is shown by man attaining to a higher eminence, in whatever he takes up, than woman can attain — whether requiring deep thought, or imagination, or merely the use of the senses and hands” (Darwin, 1871). In the next decade, George Romanes, professor of medicine at the University of Oxford, took the same view: “Seeing that the average brain weight of women is about five ounces less than that of men, on merely anatomical grounds we should expect a marked inferiority of intellectual power in the former… In actual fact, we find the inferiority displays itself most conspicuously in a comparative absence of originality, and this more especially in the higher levels of intellectual work” (Romanes, 1887).

The Twentieth Century  With the invention of the intelligence test in the early twentieth century in France by Binet & Simon (1905a, 1905b), studies began to be published on sex differences in intelligence in which most authorities asserted that there is no difference between males and females. One of the first to advance this conclusion was Thorndike (1910, p. 35), who wrote of “the trivial difference between the central tendency of men and women which is the common finding of psychological tests”. This contention was also advanced in early studies in the United States by Terman (1916), who wrote that in the American standardisation sample of the Stanford-Binet test on 4–16 year-olds “the superiority of girls over boys is so slight … that for practical purposes it would seem negligible”, and in England by Burt & Moore (1912).

In 1932, this contention was confirmed in the survey of the intelligence of all 11-year-olds in Scotland, in which boys (N=44,210) obtained a score of 34.506 and girls (N=43,288) obtained an almost identical score of 34.411 (Scottish Council, 1933). This study also found that the range of IQs was significantly greater in boys than in girls, given as IQ standard deviations of 14.9 for boys and 14.1 for girls by Deary (2020, p. 41). The effect of this is that 58.6 per cent with IQs below 60 were boys and 57.7 per cent with IQs above 130 were boys. Many later studies have confirmed that males have greater variance of intelligence than that of females, e.g., Lohman & Lakin (2009). 

Many subsequent studies repeated the contention that males and females have equal intelligence. Thus Roberts (1945, p. 727): “It is a striking fact that in mean performance on intelligence-test scales there should be no difference between boys and girls”; Cattell (1971, p. 131): “It is now demonstrated by countless and large samples that on the two main general cognitive abilities — fluid and crystallized intelligence — men and women, boys and girls, show no significant differences”; Hutt (1972, p. 88): “There is little evidence that men and women differ in average intelligence”; Maccoby & Jacklin (1974, p. 65): “It is still a reliable generalisation that the sexes do not differ consistently in tests of total (or composite) abilities”; Brody (1992, p. 323): “Gender differences in general intelligence are small and virtually non-existent”; Eysenck (1981, p. 40): “Men and women average pretty much the same IQ”; Herrnstein & Murray (1994, p. 275): “The consistent story has been that men and women have nearly identical IQs”; Jensen & Johnson (1994, p. 330): “It remains a major unresolved puzzle in differential psychology and neuroscience that the large sex difference in head and brain size is not reflected by the mean IQ difference between males and females, which is virtually nil”; Mackintosh (1996): “There is no sex difference in general intelligence worth speaking of”; Geary (1998, p. 310): “The overall pattern suggests there are no sex differences, or only a small and unimportant advantage of boys and men, in average IQ scores”. 

The assertions that males and females have the same average IQ continued to be made in the twenty-first century. Thus: Lubinski (2000): “Most investigators concur on the conclusion that the sexes manifest comparable means on general intelligence”; Colom et al. (2000): “We can conclude that there is no sex difference in general intelligence”; Loehlin (2000, p. 177): “There are no consistent and dependable male-female differences in general intelligence”; Lippa (2002): “There are no meaningful sex differences in general intelligence”; Jorm et al. (2004): “There are negligible differences in general intelligence”; Bartholomew (2004, p. 91): “Men on average have larger brains than women but display no significant advantage in cognitive performance”; Anderson (2004, p. 829): “The evidence that there is no sex difference in general ability is overwhelming”; Haier, Jung, Head & Alkire (2004, p. 1): “Comparisons of general intelligence assessed with standard measures like the WAIS show essentially no differences between men and women”; Camarata & Woodcock (2006, p. 231): “There appears to be general consensus for the view that males and females are not different in terms of general intellectual ability”; Spelke & Grace (2007, p. 65): “Men and women have equal cognitive capacity”; Hines (2007, p. 103): “There appears to be no sex difference in general intelligence; claims that men are more intelligent than women are not supported by existing data”; Haier (2007): “General intelligence does not differ between men and women”; Halpern (2007, p. 123):“There is no difference in intelligence between males and females…overall, the sexes are equally smart”; Pinker (2008, p. 13): “The two sexes are well matched in most areas, including intelligence”; Mackintosh (2011, p. 380): “The two sexes do not differ consistently in average IQ”; Halpern (2012, p. 233): “Females and males score identically on IQ tests”; Lakin (2013, p. 263): “Research indicates that men and women have equal or nearly equal ability in general intelligence”; Dunst, Benedek, Koschutnig, Jauk & Neubauer (2014): “There are no sex differences in general intelligence”; Sternberg (2014, p. 178): “There is no evidence, overall, of sex differences in levels of intelligence”; Ritchie (2015, p. 105): “Women tend to do better than men on verbal measures, and men tend to outperform women on tests of spatial ability; these small differences balance out so that the average general score is the same”; Saini (2017, p. 85): “When it comes to intelligence, it has been convincingly established that there are no differences between the average woman and man”; Halpern & Kanaya (2017): “There are no overall differences in female and male intelligence”; Toivainen, Papageorgiou, Tosto & Kovas (2017, p. 81): “Sex differences in general cognitive ability are overall small, if not negligible”; Ackerman (2018, p. 8):“There are negligible gender differences in omnibus IQ assessments”; Warne (2020, p. 245): “Males and females are equal in average intelligence”; Halpern & Wai (2020, p. 335): “Data showing differences between men and women in intelligence do not support the notion of a smarter sex”; Deary, Cox & Hill (2021): “There are very small or no sex differences in mean intelligence”. 

Despite the contention of all these numerous experts that there is no sex difference in intelligence, there remains the problem noted by Paul Broca, Charles Darwin and George Romanes that males have a larger average brain size than females, that there is a positive association between brain size and intelligence, and that it appears to follow from this that the larger average brain size of men should give them a higher average IQ than that of women. 

Subsequent work has confirmed these associations. The positive association between brain size and cognitive ability was first shown quantitatively by Galton (1888) in a study of students at Cambridge University that reported a correlation of .11 between head size and examination results. Although this correlation is positive, it is quite low because examination results are only a moderately good measure of intelligence and the restriction of range of the sample. Subsequent studies have reported higher correlations. A review of studies of head circumference and IQ giving a correlation of .30 was reported by Van Valen (1974). Brain size is measured more accurately by MRI (Magnetic Resonance Imaging) and the first study of the association of this with intelligence was carried out by Willerman, Shultz, Rutledge & Bigler (1991) and reported the correlation as .35. This association has been confirmed in subsequent studies, e.g. at r = .43 (Raz, Torres, Spencer et al., 1993), at r =.40 for college students in Turkey (Tan, Tan et al., 1999), at r = .43 (Gignac, Vernon & Wickett, 2003) and at r =.33 in a meta-analysis of 37 studies (McDaniel, 2005). In a more recent meta-analysis, a lower correlation of r = .24 was reported by Pietschnig, Penke, Wicherts, Zeiler & Vorocek (2015) but Gignac & Bates (2017) showed that this result needs to be corrected for restriction of range and data quality to r = .31. These results have been further confirmed in a study by Van der Linden, Dunkel & Madison (2017) that reported partial correlations (controlling for body height) of brain size with the g factor of .245 for men and .265 for women. It has also been shown that cortical thickness is positively correlated with intelligence (Karama, Ad-Dab’bagh, Haier & Deary, 2009; Karama, Colom, Johnson & Deary, 2011). 

The larger average brain size of males than of females is also well-established. It is present in the fetus (Wheelock, Hect, Hassan et al., 2019) and at birth, when the average male brain is approximately 12 percent heavier than that of females (Swaab & Hoffman, 1984). The larger average brain size of adult males was shown when controlled for body size by Ankney (1992) and Rushton (1992). It was confirmed in a meta-analysis of 77 studies of sex differences in human brain structure by Ruigrok, Salimi-Khorshidib, Laia, Baron-Cohen et al. (2014), who concluded that men have a 12 percent greater average brain volume than women. These results have been further confirmed in a study by Ritchie, Cox, Shen et al. (2018) that compared 2,750 females (mean age = 61.12 years, SD = 7.42, range = 44.64–77.12) and 2,466 males (mean age = 62.39 years, SD = 7.56, range = 44.23–76.99) and reported that average male brains were larger with a 1.4d effect size, meaning that 92% of men have a brain size above the mean for women. 

Thus, the problem remains that brain size is positively associated with intelligence, and that males have a larger average brain size than females. It appears to follow that males should have a higher average IQ than females, yet numerous experts have asserted that males and females have the same average intelligence. There has been some acknowledgement of this problem. Butterworth (1999, p. 293) noted the problem, writing that “[w]omen’s brains are 10% smaller than men’s, but their IQ is on average the same” but he did not offer a solution. Halpern (2012, p. 233) also noted that women have a smaller average brain size than men but asserted that “[t]here is no evidence that larger brains are, in any way, better than smaller brains”. This is a remarkable contention in view of the results of the numerous studies noted above, including the meta-analysis of 37 studies by McDaniel (2005) concluding that brain size is positively correlated with intelligence at .33. 

An attempt to resolve the problem was made by Jensen (1998), who suggested that females have the same number of neurons in the brain as males but these are smaller and more closely packed. This is an improbable hypothesis and has been shown to be incorrect by Pakkenberg & Gundersen (1997), who reported that men have an average of four billion more neurons in the brain than have women, a difference of 16 percent. Further data showing that men have more neurons than have women have been given by Pelvig et al. (2008) and by Escorial, Roman, Martinez et al. (2015). Deary, Cox & Hill (2021) have also noted the problem, writing that “[t]he fact that there are substantial sex differences in brain size but very small or no sex differences in mean intelligence is likely to be because multiple aspects of the brain’s structure, function, and connectivity are compensatory for any apparent brain size difference”. They do not elaborate this suggestion.

Richard Lynn