I.Q and Human Intelligence

Summary

WAIS

Raven’s Progressive Matrices

Brain size

Evolution of Sex Differences in Intelligence 

 

 

Summary

-IQ of an adult male 4 to 5 points higher
-Faster development for women, but peaks earlier (parallel to physical growth)
-Cerebral volume peaks at age 10.5 in females and 14.5 in males
-IQ of men becomes superior at about 16 years
-Adult men brain bigger by 110 to 150 cubic cm (δ = 0.78 SD)
-19.3 billion neurons in women, 22.8 billion neurons in men
-Highest cortical thickness in men (δ = 0.28SD)
-Men have quicker Reaction Time (Deary, 2006 Ritchie, 2018) a consequence of the positive correlation between IQ and reaction time; Men also have a faster nerve conduction velocity (De Bellis, Keshavan, Beers et al. (2001)), consequence of the positive correlation between IQ and NCV.

 

WAIS (Wechsler Adult Intelligence Scale)

All 42 studies show a male advantage, median +0.24 sd or 4 IQ points

Table 5.5. Sex differences on the WAIS Full Scale IQ and Performance subtests; (ds; positive signs denote males score higher)

Country	Test: N	Full Scale IQ d	Reference
Brazil	WAIS-III: 3494	.07	Victora et al., 2015
Canada	WAIS-III: 1104	.11	Longman et al., 2007
Chile	WAIS-IV: 887	.20	Diaz & Lynn, 2016
China	WAIS-R:1406	.24	Dai et al., 1991
China	WAIS-R:1979	.33	Lynn & Dai, 1993
China	WAIS-R:120	.43	Yao et al., 2004
China	WAIS-111:888	.29	Chen & Lynn , 2020c
China	WAIS IV:311	.62	Gao et al., 2015
Canada	WAIS 111:1104	.11	Longman et al., 2007
Denmark	WAIS:62	.21	Nyborg, 2005
Germany	WAIS-IV:1425	.21	Daseking et al., 2017
Italy	WAIS-R:1168	.45	Tommasi et al., 2015
Japan	WAIS-R:1402	.22	Hattori & Lynn, 1997
Netherlands	WAIS:2100	.27	Stinissen, 1977
Netherlands	WAIS 111:522	.24	Van der Sluis et al, 2006
Romania	WAIS:100	.44	Dumitrascu, 1999
Romania: Roma	WAIS:100	.44	Dumitrascu, 1999
Russia	WAIS:296	.13	Grigoriev et al, 2016
Russia	WAIS:1800	.22	Grigoriev et al, 2016
Scotland	WAIS-R:200	.39	Lynn, 1998
South Korea	WAIS-IV:1228	.31	Lynn  & Hur, 2016
Spain	WAIS 111:1369	.24	Colom et al., 2002
Sudan	WAIS-R:330	.31	Sulman et al, 2018
Sudan	WAIS-R:319	.21	Sulman et al, 2018
Taiwan	WAIS 111:888	.29	Chen & Lynn, 2021a
Taiwan	WAIS 1V:1105	.35	Chen & Lynn, 2018
United States	W-Bell:235	.59	Strange & Palmer, 1953
United States	W-Bell:153	.20	Norman, 1953
United States	W-Bell:392	.29	Goolishian  & Foster, 1954
United States	WAIS:1700	.10	Matarazzo, 1972
United States	WAIS:279	.40	Boor, 1975
United States	WAIS:588	.17	Horn et al., 1979
United States	WAIS:521	.13	Turner & Willerman, 1977
United States	WAIS:649	.09	Doppelt & Wallace, 1955
United States	WAIS:649	.09	Doppelt & Wallace, 1955
United States	WAIS:100	.33	Shaw, 1965
United States	WAIS-R:230	.27	Arceneaux et al., 1996
United States	WAIS-R:206	.28	Ilai & Willerman, 1989
United States	WAIS-R:1880	.15	Matarazzo et al., 1986
United States	WAIS-III: 2450	.18	Irwing, 2912
United States	WAIS IV:2200	.15	Piffer, 2016
United States	WAIS 111: 850	.04	Du Pont et al., 2020
Median		.24

Raven’s Progressive Matrices

32 of the 33 studies found a male advantage, median 0.3 SD or 4.5 IQ points

Table 4.3. Sex differences (ds) for the Standard and Advanced Progressive Matrices in general population samples aged 20-80 years. Minus signs denote higher means obtained by females.

Country	N:M	N:F	d	Reference
Britain	300	240	.29	Heron & Chown, 1967
Hungary	250	250	.17	Szegedi, 1974
Israel	100	100	.31	Guttman, 1974.
Hawaii	939	971	.37	Wilson et al., 1975
Taiwan	225	225	1.33	Adair & Pollitt, 1985
Belgium	850	979	.31	Deltour, 1993
Belgium	101	174	.38	Deltour, 1993
USA	63	80	.16	Sitkei & Michael, 1996
Belgium **	564	802	.21	Dufouil et al., 1997
Brazil	1921	741	.28	Campos, 1999
USA**	92	114	.31	Salthouse, 2001
Scotland	210	217	.11	Deary et al., 2004
Scotland	230	313	.29	Deary et al., 2004
Guatemala	683	786	.52	Martorell et al., 2005
Brazil**	104	265	.49	Rossetti et al., 2009
Pakistan	997	1019	.04	Ahmad et al., 2008
Morocco	92	110	.38	Sellami et al., 2010
Spain	101	157	.15	Diaz et al., 2010.
Sudan	115	125	.12	Khaleefa et al., 2010
N. Zealand	143	187	.22	Fletcher & Hattie, 2011
Argentina	374	390	-.02	Flynn, 2012
Libya	300	300	.37	Al-Shahomee, 2012
Libya	260	260	.36	Al-Shahomee & Lynn, 2012
Brazil**	454	534	.10	Flores-Mendoza et al., 2013
Brazil**	161	386	.65	Braga et al., 2014
Serbia	62	74	.27	Čvorović & Lynn, 2014
Romania*	618	823	.18	Iliescu et al., 2016
Australia**	128	327	.30	Waschl et al, 2016
Brazil**	381	216	.43	Flores-Mendoza et al., 2016
USA***	393	503	.21	Van der Linden et al., 2017
Poland**	218	218	.12	Gignac & Zajenkowski, 2019
USA***	346	399	.05	Du Pont et al., 2020
Portugal	250	272	.34	Queiro-Garcia et al., 2021
Median	–	–	.30	–

 *Progressive Matrices Plus; **Advanced Progressive Matrices; *** Advanced Progressive Matrices Short Form 

Brain Size

Boys and girls have about the same IQ up to 15 years. From the age of 16 years the average IQ of males becomes higher with an advantage increasing to approximately 4 IQ points in adulthood.

The reason for this is that the brain size of males increases relative to that of females from the age of 16 shown by Lenroot, Gogtay, Greenstein, Wells et al. (2007) and Lenroot & Giedd (2010) and confirmed by neurological studies that have shown that white matter in the brain continues to grow more in males than in females from mid-adolescence (Simmonds, Hallquist, Asato & Luna, 2014; Wang, Adamson, Yuan, Altaye, Rajagopal, Byars & Holland, 2012). This thesis has been further supported by studies showing that males have later physical maturation (Hills & Byrne, 2010), behavioural maturation (Greenstein, Blachstein & Vakil, 2010; Keulers, Evers, Stiers & Jollies, 2010; Yurgelun-Todd, 2007), brain maturation (De Bellis, Keshavan, Beers et al., 2001) and brain development shown by neuroimaging by Bramen, Hranilovich, Dahl et al. (2010) and by Tiemeier, Lenroot, Greenstein et al. (2010). 

The effect of the increasing brain size of males compared with that of females from the age of 16 years is that male intelligence increases relative to that of females.

Table 2.1. Sex differences in intelligence at ages 12 through 21 (ds; positive signs denote males score higher). AR: abstract reasoning.

Age	12	13	14	15	16	17	18	19	20	21	Reference
Female per cent  brain size	92.2	92.5	92.6	91.5	91.2	89.2	–	–	–	86.6	Roche & Malina, 1983; Rushton, 1992
Sex diffs brain size cc	110	120	127	133	140	150	160	170	–		Giedd et al., 2012
US: AR	–	–	0	.04	.09	.10	.16	–	–	–	Feingold, 1988
UK: AR	–	–	.06	.08	.08	.19	.25	–	–	–	Lynn, 1992
Spain: AR	.05	-.20.	.14	.31	.32	.38	.36	–	–	–	Colom & Lynn, 2004
US: whites: g	–	–	–	-.03	.26	.29	.17	.23	.32	.41	Meisenberg, 2009
US: blacks: g	–	–	–	-.11	.07	.05	.07	.00	.10	.10	Meisenberg, 2009
UK: g	-.06	–	–	–	.12	–	–	–	–	–	Lynn & Kanazawa, 2011
US:DAT	-.80	–	–	.60	–	-3	–	–	–	–	Keith et al., 2011
US: CogAT	-.03	-.01	–	-.01	–	.03	–	–	–	–	Lakin, 2013
US: whites: g	.08	.10.	.02	.16	.23	.26	–	–	–	–	Nyborg, 2015
US: blacks: g	-.13	-.18	-.04	-.19	-.34	.43	–	–	–	–	Nyborg, 2015
US: Hispanics: g	.02	.11	-.08	.24	-.23	.30	–	–	–	–	Nyborg, 2015
Spain: IQ	.07	.01	.08	.19	.27	.25	.32	–	–	–	Arribas-Agula et al., 2019
Spain: Verbal  ability	-.03	-.07	-.06	.05	.15	.09	.15	–	–	–	Arribas-Agula et al.,  2019

Row 1 gives the cranial capacity of females as a % of that of males. Cranial capacity of females as a percentage of that of males declines from the ages of 15 to 17 (data from Roche & Malina, 1983) and declines further at age 21+.

Row 2 gives the differences in brain size in cubic centimetres between males and females obtained by magnetic imaging : the differences increase over the ages 12 through 19.

Rows 3, 4 and 5 give data for Abstract Reasoning from the Differential Aptitude Test for the United States, the United Kingdom and Spain all showing a male advantage from ages 14 through 18.

Row 6 gives results for 15- to 21-year-old whites for the ASVAB (Armed Services Vocational Aptitude Battery) scored for g, showing a female advantage at age 15 followed by increasing male advantages from age 16 reaching .41d (6.15 IQ points) at age 21.

Row 7 gives results from the same data for Blacks, also showing a female advantage at age 15 followed by male advantages from age 16 but these are very small and not statistically significant (smaller sex differences in intelligence among Africans, see below the evolutionary explanation of sex differences and sex differences).

Row 8 gives the differences in g for a UK longitudinal sample, showing girls at age 12 had a significantly higher score than boys but at age 16 the same boys had a significantly higher score than girls.

Row 9 gives results for the American DAT averaged from Gv (visual-spatial) (verbal), Glr (free recall memory),Gsm (short-term memory) and Gs (processing speed), showing girls scored higher at age 12 (also at ages 5-7 and 8-10) while boys scored higher at ages 14-15 and 16-17.

Row 10 gives results for the American CogAT averaged from verbal, quantitative and non-verbal reasoning ability, showing girls scored higher at age 12, 13 and 15 while boys scored higher at age 17.

Rows 11 through 13 give results for the NLSY 97 (National Longitudinal Study of Youth) scored for g for whites, blacks and Hispanics, showing generally small sex differences from ages 12 through 15 but significant male advantages at age 17.

Row 14 gives results for g (general intelligence) for Spain, showing no significant sex differences at ages 12 through 14, and increasingly higher scores by boys from ages 15 through 18.

Row 15 gives results for verbal ability for Spain, showing girls scored higher at age 12. In a Spanish sample there was a female advantage in 12- through 14-year-olds and male advantages from the age of 15 through 18 years of .05d increasing to .15d.

To calculate the magnitude of the higher adult male IQ that would be predicted from the larger male brain size I took Ankney’s figure of the male-female difference in brain size expressed in standard deviation units of 0.78d and Willerman et al.’s (1991) estimate of the correlation between brain size and intelligence of 0.35. These figures would give adult males a higher average IQ of 4.0 IQ points (0.78 multiplied by 0.35 = .27d = 4.0 IQ).

The Evolution of Sex Differences in Intelligence 

In infants aged between 1 and 4 years, girls have higher average intelligence than boys, between the ages of  6 and 15 years there is virtually no difference in intelligence between males and females, while at the age of 16 years males begin to have higher average intelligence than females increasing an advantage of 4 IQ points in adults.

 The explanation of the higher intelligence of girls than of boys in infancy is that girls mature earlier than boys, as shown by Lenroot, Gogtay, Greenstein et al. (2007) in a longitudinal investigation of sex differences in brain development using magnetic resonance imaging (MRI) of 387 subjects aged 3 to 27 years, showing that cerebral volume peaked at age 10.5 in females and 14.5 in males. The likely evolutionary explanation of earlier maturation of girls is that it is advantageous for them to begin reproducing in early puberty when they are sufficiently mature to have babies and look after them. Between the ages of  6 and 15 years, the greater brain size of boys compensates for their later maturation resulting in virtually no difference in average intelligence between males and females during these ages.   

The likely evolutionary explanation of the higher intelligence of males from the age of 16 years is that in all mammalian group-living species, males compete for territory or high status in dominance hierarchies to secure access to females and reproduction as documented in detail by Wynne-Edwards (1962) and Wilson (1975). During the evolution of the hominids, greater intelligence would have contributed to success in this male competition by enabling males with greater intelligence to form useful alliances, display leadership qualities in hunting and warfare, and to dominate other males with lesser intelligence. The continued maturation and increasing intelligence of males in later adolescence and into adulthood from the age of 16 years would have enabled males to acquire the experience and skills required to work their way up the dominance hierarchies and obtain sufficient status to secure access to females. In contrast, females do not need to acquire these skills. The advantage of  intelligence is present in contemporary societies where it is a significant determinant of rank indexed by socio-economic status as documented in the United States by Jencks (1972), who showed they are correlated at 0.46, and in Britain by Nettle (2003) and Saunders (2012, 2019).  

      A further probable evolutionary explanation for the higher average intelligence of men than of women lies in sexual selection, the process by which females tend to favour males with high intelligence and accept them as mates because they consider them likely to be good providers for themselves and their children. This theory was first advanced and described as sexual selection by Charles Darwin (1971) to explain why in most species males are bigger and stronger than females. Darwin argued that males have to compete with each other for mates, and females tend to accept those who are bigger and stronger and, in humans, more intelligent, with the result that “man has ultimately become superior to woman”. This thesis has been elaborated by Geoffrey Miller (2000) who writes: “Male nightingales sing more and male peacocks display more impressive visual ornaments. Male humans sing more and talk more in public gatherings, and produce more paintings and architecture… Men write more books. Men give more lectures. Men ask more questions after lectures. Men dominate mixed-sex committee discussions”.  He argues that these are strategies successfully used by intelligent males to attract females.  

         The higher average intelligence of males than of females is greater in Europeans than in sub-Saharan Africans. This was shown by Jensen & Johnson (1994) who reported that among 7-year-old whites boys had a 1.1 higher WISC IQ than girls, but among blacks girls had a 0.6 higher IQ than boys. This difference was confirmed by Meisenberg (2009) who reported that among 20- and 21-year-olds the white male advantage was .356d but the black male advantage was only .10d. The difference was further confirmed by Nyborg (2015) who reported a female advantage of .24d on g in black 16-17 year-olds and a male advantage of .30d in whites. The same difference was reported for mathematical problem-solving ability, given as .23d for blacks and .41d for whites in the meta-analysis by Hyde, Fennema & Lamon (1990). Consistent with these results, Rushton (1992) reported that the male-female difference in brain size is greater in whites than in blacks. He reported that for enlisted military personnel the male-female difference in brain size was 204 cc. in whites and 189 cc. in blacks and for officers it was 210 cc. in whites and 197 cc. in blacks.   

The likely evolutionary explanation for the greater advantage of males over females in intelligence in Europeans than in sub-Saharan Africans is that in the tropical and sub-tropical evolutionary environment of sub-Saharan Africa, black males had only a weak need for greater intelligence than females because plant and insect foods were available throughout the year and females could collect these for themselves and their children without the support of males. The effect of this was that males were only under weak selection pressure to evolve the higher intelligence required to hunt for animal foods. When early humans migrated into Europe, they found that plant and insect foods were not available in the winter and spring and males had to hunt large animals to obtain food for themselves and their females and children. Hunting large animals is more cognitively demanding than gathering plant and insect foods, so European males came under selection pressure to evolve higher intelligence than females. In confirmation of this explanation, it has been shown by Humphreys, Lin & Fleishman (1976) that white males have significantly greater hunting ability than have white females, compared with that of black males and females.   

If this theory is correct, the male intelligence advantage should be greater in North East Asians than in Europeans because North East Asians experienced a harsher and more cognitively demanding environment. This expectation is confirmed by the sex differences in the Wechsler Full IQs in the 45 adult samples given in Tables 5.4 and 5.5. Eight of these samples were North East Asians (four from China, two from Taiwan, one from Japan and one from South Korea) in whom the median male advantage was .31d . The other thirty-seven were Europeans in whom the median  male advantage was .21d.  

References for the whole page

Lynn R. (2021) ‘Sex Differences in Intelligence, The Developpemental Theory’

Lynn R. (2015) ‘Race differences in intelligence. An evolutionary Analysis‘ Chapiter 16

Ian J. Deary (2006) ‘Age and sex differences in reaction time in adulthood, results from the United Kingdom health and lifestyle survey’, Psychology and aging

Van Der Linden D., Curtis S.D, Madison G. (2017) ‘Sex differences in brain size and general intelligence (g)’, Intelligence

Lynn R., Irwing P. (2006) ‘Intelligence: Is there a sex difference in IQ scores ?’ Nature 442, E1, doi:10.1038/nature04966

Arribas D., Aguilaa F. et al. (2019) ‘Testing the developmental theory of sex differences in intelligence using latent modeling: Evidence from the TEA Ability Battery (BAT-7)‘

Lynn R. (2017) ‘Sex Differences in Intelligence. Reply to Comments’ Mankind Quarterly 58:1145-156.

APPENDIX