Why the story about sex differences and the hemispheres is not straightforward

In response to reader requests ....

I have noticed that a number of people in the past few months, reasonably enough, have asked why it is that I say that the popular equation – LH=male, RH=female – does not hold water. There are a number of observations that suggest this formula is mistaken. A few of the more ‘headline’ points include that, while, as you may remember, the right hemisphere is slightly larger and heavier than the left, this effect is mainly due to males, and diluted by females, in whom there is a much smaller difference, if any. In keeping with this, it is testosterone that causes the right hemisphere to expand during the second trimester of pregnancy. Then there is the fact that the two most highly validated neuropsychological differences between men and women are that men have a greater superiority in three-dimensional visuospatial manipulation, a skill highly dependent on the right hemisphere, and women have a similar superiority in verbal fluency, a skill highly dependent on the left hemisphere. And men tend to take more global (RH) views, and women more local (LH) views.

But there is much more to be said, and as ever the picture is not, of course, without its nuances. For this reason, I have copied and pasted here from pages 162-7 of The Matter with Things where I include what I call an aside on differences between sexes. Clearly I was so careful to make it no more than an aside that it would appear to have escaped detection altogether.

As someone who feels that one ought to be able to speak honestly, openly, and in good faith about pretty much anything, I really do hope that the consequences of putting up this post, in response to readers’ requests, will not be that people with hobby horses exchange them for high horses, leaving me with the task of cleansing the Augean stables after they have finished … I'm parking a broom by the stable door in the hopes that if necessary people will clean up after themselves.

In any case, there follow here some thoughts, at least, based on the science. Incidentally, it's worth pointing out that the majority of the researchers in this area are, for perhaps obvious reasons, women, and the lead authors on most of the papers I cite are women.

Here goes:

What prompted my aside at this point in The Matter with Things was some findings on the lateralisation of judgment formation.

Thus:

Most PET imaging studies have shown predominantly right‐sided orbitofrontal activation while involved in decision-making, such as assessing conflicting possibilities – eg, frequent small rewards versus occasional large rewards – on a risk task.^[1] Lesion studies suggest that evaluating decisions depends, in general, on the right ventromedial prefrontal cortex (VMPFC cortex).^[2] In a comparison between normal subjects and addicts in a functional MRI task, healthy controls were found to have significant right-sided activation (in the lateral orbitofrontal cortex, superior frontal gyrus, dorsolateral prefrontal cortex, inferior temporal gyrus, inferior parietal gyrus, lingual gyrus, occipital cortex and cerebellum): but the addicts, whose decision-making was chaotic and irrational, showed a reverse pattern:

During decision-making, control participants showed relatively greater activation in the right dorsolateral prefrontal cortex, whereas participants engaged in current or previous drug use showed relatively greater activation in the left orbitofrontal cortex.^[3]

There is, however, evidence from three studies that I am aware of that this may apply only to men, and that in women, it is the left, not the right, VMPFC that is more important in decision-making.^[4] If this turns out to be more widely replicated, it joins a similar anomaly regarding sex-specific lateralisation in the amygdala, where activity in relation to emotionally charged memories is once again right-lateralised in men, left-lateralised in women.^[5] To an extent the finding that the VMPFC and the amygdala vary together makes sense, in that the VMPFC is involved, amongst other things, in dealing with affect-laden memories, as is the amygdala.

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^[1] Rogers, Everitt, Baldacchino et al 1999; Rogers, Owen, Middleton et al 1999; Rubinsztein, Fletcher, Rogers et al 2001.

^[2] Tranel, Bechara & Denburg 2002; Manes, Sahakian, Clark et al 2002.

^[3] Ersche, Fletcher, Lewis et al 2005.

^[4] Sutterer, Koscik & Tranel 2015; Reber & Tranel 2017; Tranel, Damasio, Denburg et al 2005.

^[5] Cahill, Uncapher, Kilpatrick et al 2004; Schneider, Peters, Bromberg et al 2011; Canli, Desmond, Zhao et al 2002.

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Then I turn more generally to sex differences:

As to the significance of such (sexually divergent) findings, there are very few clues at this stage. One possible theory is that while their effect may lead to behavioural differences in some instances, they might just as well be designed to prevent such differences (when, for instance, they would be maladaptive), by compensating for sex differences in other physiological conditions, such as endocrine function.^[1] If, for example, the right frontal cortex is so crucial in females for underwriting the empathic relationship with the infant, it may not be possible for the same area to make risk decisions, which, from an evolutionary psychology point of view, are more the domain of the male. But we would need more research into what other sex-specific aspects of lateralisation exist to have a basis for a useful global conjecture.

What seems to be clear, however, is that striking sex differences in functional asymmetry do exist, and they have been ascribed, at least in part, to the action of sex hormones, especially testosterone.^[2] Many individual studies have found greater functional asymmetry in men than women,^[3] and all five reviews of the literature of which I am aware reach the same conclusion,^[4] though there are occasional contrary findings.^[5] There are a number of corollaries of this. Men are more likely to display verbal deficits after left hemisphere lesions and non-verbal deficits after right hemisphere lesions, whereas the deficits are less predictable for women.^[6] Additionally male brains exhibit greater intrahemispheric, and females greater interhemispheric, structural connectivity.^[7] What these findings together suggest is that the hemispheres are more specialised in the male than the female brain. ^[8]

This could be expected to lead to complementary patterns of strengths and weaknesses. In normal adults, sex differences in functional cerebral asymmetries have been reported in a wide range of areas, including decision-making, as above, but extending to areas such as language, working memory, spatial orientation, spatial attention, face perception, verbal and musical creativity tasks, emotional ‘processing’ and appreciation of beauty.^[9] Except in the case of language, males have generally been found in every one of these areas to be more reliant on the right hemisphere than females. Doreen Kimura proposed that males prefer to employ right hemisphere, non-verbal systems whereas females prefer to employ left hemisphere, verbal systems.^[10] Women tend to use a more verbal strategy than men in non-verbal tasks.^[11] They also employ a different cognitive strategy from men in navigation tasks, relying predominantly on landmark [local or ‘egocentric’] information, ‘whereas men used both landmark and geometric [global or ‘allocentric’] information’.^[12] In general, as we have seen, we transfer more information, and more swiftly, from the right hemisphere to the left, but this asymmetry seems to be less pronounced in women, who show more symmetrical interhemispheric transfer times than men.^[13]

According to Belinda Pletzer, one of the leading researchers in the area, many lines of evidence show that males process more globally, females more locally: there is ‘a stronger focus on the global level (holistic processing) in men and a stronger focus on the local level (decomposed processing) in women’.^[14] The first is more right hemisphere-dependent and the second more left hemisphere-dependent. Men show greater right hemisphere activation in tasks assessing global attention.^[15] In women, global choices are slower than local choices, whereas in men they are faster. Testosterone is positively related, and progesterone negatively related, to the use of global strategies.^[16] Oral contraceptive users show enhanced global advantage compared to women whose menstrual cycle is natural.^[17]

It has repeatedly been demonstrated that men tend to remember the gist of an emotional story more strongly than women, while women tend to remember the details of the same story more strongly than men.^[18] A study by Deborah Waber and Jane Holmes asked girls and boys simply to copy the Rey-Osterrieth figure.

They found that

at the youngest age, females reproduced more parts of the design (ie, scored higher on accuracy) than did males. This difference was accounted for by the presence, in the girls’ designs, of internal details absent in those of the boys. In addition, females drew more discrete parts than males, even when the total number of parts was statistically controlled. For males the external configuration of the design seemed to be more salient …

Copies of the Rey-Osterrieth figure drawn by (A) a 5 year-old boy and (B) a 5 year-old girl. The different line styles are intended to give information about the order in which parts of the figure were drawn: see original for details. (from Waber 1979)

They further found that, at age 11,

males drew their designs in long, sweeping, continuous lines, while females drew theirs part by part. At age 13, the stylistic difference between the sexes had once again disappeared … The male superiority in spatial ability appears on three kinds of tests. The first is spatial visualisation … The second is perceptual disembedding … The third is mazes … These differences do not appear reliably until adolescence. The language abilities at which females show an advantage are clearly associated with left hemisphere function.^[19]

In another study in children, 40 girls and 39 boys between the ages of four and 12 years undertook a perceptual judgment task. ‘Boys were significantly more global in their perceptual judgments than girls at all ages. Younger children of both sexes were less global than older children. Results were consistent with developmental models that suggest an early left hemisphere advantage for girls and a right hemisphere advantage for boys’.^[20] And in a study of adult male and female composers, the compositions that received the highest ratings were by subjects that had smaller left hemisphere advantages in verbal processing, suggesting a greater reliance on the right hemisphere. However, though male composers across the board showed this smaller left hemisphere advantage compared with non-musicians, the opposite was the case with female composers taken as a whole, who showed greater left hemisphere advantage. This is in keeping with the fact that in male adults musical talent appears to be associated with high scores on spatial tests, whereas adult women show no such association.^[21]

I am sometimes asked whether one can say that the left hemisphere is in some sense ‘male’ and the right hemisphere ‘female’. I know why the question is asked, but there can naturally be no such simple equation; and, as I pointed out at the beginning of The Master and his Emissary, if anything the evidence might be taken to point the other way. Quite apart from what I have just related, one needs to take into account a number of other facts that ought to give pause for thought. For example, though the right hemisphere is larger than the left, this effect seems to be more marked in males.^[22] The surface area of the right hemisphere is also greater than that of the left, but the effect is greater in males.^[23] It is testosterone that causes the right hemisphere to grow faster in utero in males than the left, actually inhibiting left hemisphere growth in order to produce resulting compensatory growth of the homologue regions in the right hemisphere.^[24] Higher levels of foetal testosterone cause rightward enlargement of a posterior part of the corpus callosum, the isthmus, which projects mainly to parietal and superior temporal areas.^[25] The larger grey matter volumes found in the right temporoparietal junction and posterior superior temporal sulcus in males correlate positively with foetal testosterone levels.^[26] Testosterone causes relative cortical thinning in the left hemisphere in males, and causes initially relative cortical thickening in the right hemisphere in females, though this effect in females is reversed later in development.^[27] Left hemisphere language-associated cortical regions are proportionally larger in the female brain.^[28] Prenatal testosterone causes left hemisphere lateralisation of language in girls and right hemisphere lateralisation of emotion recognition in boys.^[29] A left hemisphere focus for speech functions matures earlier in females than males.^[30] While there are 20–30% more neurones in the male neocortex than the female neocortex overall, the difference is greater in the right hemisphere.^[31] Women have significantly larger neuronal somata in the left hemisphere than the right, which, it has been posited, relate to higher rates of right-handedness in females than males, and the female language advantage.^[32] And the two most reliable sex differences, neuropsychologically speaking, are that females in general have greater verbal facility and males better visuo-spatial skills, and that this is related to testosterone.^[33] Females do better academically where material is familiar and prepared.^[34] Finally, in rats, too, it has been found that the right cortex is thicker than the left in males, but the left thicker than the right in females.^[35]

The situation is undoubtedly complex.^[36] There are differences between the effects of prenatal and adult levels of testosterone, metabolites of testosterone have differing actions, the same levels of testosterone in males and females have differing actions, and there is in general no simple dose-response curve.^[37] Meanwhile, the lesson we should learn is that all studies of brain structure and function should report results for males and females separately, and indeed, since sex hormones affect lateralisation of brain functioning, arguably differentiate women according to phase of the menstrual cycle and contraceptive use: ‘studies comparing men and women may fail to detect significant differences without controlling for hormonal contraceptive use’.^[38] We do not know how many important brain functional asymmetries have been missed by aggregated data cancelling out an otherwise significant effect.^[39] Where it is lateralised to different hemispheres in males and females, the most clearly lateralised function might potentially show no lateralisation whatever if equal numbers of men and women in a study are, as is often the case at present, grouped together.

Concluding remarks

I think there are a few broad comments worth making. Firstly it seems clear that the same brain area may be used to quite different ends in males and females - this is the point I made right at the beginning in relation to the right ventromedial prefrontal cortex. I think it is fair to say that from an evolutionary psychology point of view women are more important for the continuation of the species, and nature seems to have favoured a more robust arrangement of brain functions in women, with greater overlap, and taken more risks with men, who are from this point of view more expendable. Women are more important, as an extension of this, from the point of view of the cohesion of society than men, and it is interesting to me that one kenning for a woman in Anglo-Saxon, freothuwebbe, means the peace-weaver. Though there may be many contrary individual cases, in general women are more socially orientated, while men tend to be more loners or explorers, less concerned with being agreeable. This is all familiar territory in evolutionary psychology research.

However it would be odd if men were really the left hemisphere-dominant caricatures that pop psychology once suggested, since writing poetry, composing music, painting works of art and conceiving works of architecture, not to mention making new insights in mathematics, and indeed in science more generally, all absolutely require high levels of right hemisphere functioning. But perhaps the payoff is that men are less destined to be empathic – I don't know the research on this, although I believe the difference may be more one of the expression of empathy than its experience. (For very good reasons I have not wished to become an expert on an area that would embroil me in sociopolitical controversy. ) There is certainly greater specialisation of the hemispheres in men, and this, as I've pointed out, brings advantages and disadvantages, giving rise to so-called greater male variability, with greater representation at both extremes of the normal distribution. Let me emphasise these are obviously generalisations and, though I should not have to stress this, generalisations are just that – having little if anything to say about individual cases.

I hope this has been, if nothing more, at least a clarification of why the clichés about the right and left hemisphere and the sexes need to be retired in favour of a more complex picture.

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^[1] De Vries 2004

^[2] Schuepbach, Skotchko, Duschek et al 2012; Wisniewski 1998. For a fascinating review of sex differences in the brain, see Cahill 2006.

^[3] See, eg, Tian, Wang, Yan et al 2011; McGlone 1980; Inglis & Lawson 1981; Corballis & Sidey 1993; Juárez & Corsi-Cabrera 1995; Cory-Slechta, Weston, Liu et al 2013; Shaywitz, Shaywitz, Pugh et al 1995; Voyer 1996; Hausmann & Güntürkün 1999; Meinschaefer, Hausmann & Güntürkün 1999; Rasmjou, Hausmann & Güntürkün 1999; Hausmann, Ergun, Yazgan et al 2002; Hausmann, Waldie & Corballis 2003; and Liu, Stufflebeam, Sepulcre et al 2009.

^[4] Voyer 1996; Hiscock, Perachio & Inch 2001; Voyer 2011; Hausmann 2017; and Cahill 2006.

^[5] See, eg, Ashton & McFarland 1991; Sommer, Aleman, Bouma et al 2004; Schlosser, Hutchinson, Joseffer et al 1998.

^[6] Lansdell 1961; McGlone 1977; McGlone 1978; McGlone & Kertesz 1973.

^[7] Ingalhalikar, Smith, Parker et al 2014.

^[8] See, eg, Russo, Persegani, Papeschi et al 2000; Koles, Lind & Flor-Henry 2010; Vikingstad, George, Johnson et al 2000; Miller, Jayadev, Dodrill et al 2005.

^[9] LANGUAGE: see, eg, Bryden 1979; Witelson, Beresh & Kigar 2006; Franzon & Hugdahl 1986; Shaywitz, Shaywitz, Pugh et al 1995; WORKING MEMORY: Speck, Ernst, Braun et al 2000; Bell, Willson, Wilman et al 2006; Ramos-Loyo & Sanchez-Loyo 2011; SPATIAL ORIENTATION: see, eg, Johnson, McKenzie & Hamm 2002; Rilea, Roskos-Ewoldsen & Boles 2004; Witelson 1976; Chiarello, McMahon & Schaefer 1989; Corballis & Sidey 1993; Li 2014; Dabbs, Chang, Strong et al 1998; Galea & Kimura 1993; Lawton, Charleston & Zieles 1996; Barkley & Gabriel 2007; Voyer & Bryden 1990; SPATIAL ATTENTION: Hausmann, Ergun, Yazgan et al 2002; Müller-Oehring, Schulte, Raassi et al 2007; Razumnikova & Vol’f, 2011; Hatta, Ohnishi & Ogura 1982; FACE PERCEPTION: Rizzolatti & Buchtel 1977; Borod & Koff 1983; Killgore & Yurgelun-Todd 2001; Tiedt, Weber, Pauls et al 2013; Proverbio, Riva, Martin 2010; Proverbio, Brignone, Matarazzo 2006; Bourne 2005; Nikolaenko 2005; CREATIVITY TASKS: (verbal): Fink & Neubauer 2006; (musical): Hassler & Nieschlag 1989; EMOTION: Grimshaw, Bryden & Finegan 1995; Cahill, Uncapher, Kilpatrick et al 2004; Schneider, Peters, Bromberg et al 2011; Lee, Liu, Hoosain et al 2002; Wager, Phan, Liberzon et al 2003; Tranel, Damasio, Denburg et al 2005; APPRECIATION OF BEAUTY: Cela-Conde, Ayala, Munar et al 2009.

^[10] Kimura 1969.

^[11] Frings, Wagner, Unterrainer et al 2006; McGlone & Kertesz 1973.

^[12] Sandstrom, Kaufman & Huettel 1998; Ramos-Loyo & Sanchez-Loyo 2011; Pletzer, Kronbichler, Nuerk et al 2013. However, in chicks, while the left hemisphere, as in humans, deals with the local, and the right hemisphere with the global, at least one study suggests that male chicks appear to rely more on left hemisphere landmarks, rather than right hemisphere geometrics (Tommasi & Vallortigara 2004).

^[13] Nowicka & Fersten 2001; Moes, Brown & Minnema 2007.

^[14] Pletzer, Kronbichler, Nuerk et al 2013; Pletzer 2014. See also Nikolaenko 2005.

^[15] Lee, Chung, Chang et al 2012.

^[16] Scheuringer & Pletzer 2016; Roalf, Lowery & Turetsky 2006.

^[17] Pletzer, Petasis & Cahill 2014.

^[18] Cahill, Gorski, Belcher et al 2004; Cahill & van Stegeren 2003; Cahill, Uncapher, Kilpatrick et al 2004.

^[19] Waber 1979 (173 & 176).

^[20] Kramer, Ellenberg, Leonard et al 1996.

^[21] Hassler & Nieschlag 1989.

^[22] Carne, Vogrin, Litewka et al (2006) found a significant effect in both sexes. Savic & Lindström (2008) found the effect only in heterosexual males and homosexual women. There is an overall trend to find that this effect is more marked in men, but there are many variables, and the area is not easy to interpret.

^[23] Kong, Mathias, Guadalupe et al 2018; Koelkebeck, Miyata, Kubota et al 2014.

^[24] Geschwind & Galaburda 1985; Stewart & Kolb 1988.

^[25] Chura, Lombardo, Ashwin et al 2010.

^[26] Lombardo, Ashwin, Auyeung et al 2012.

^[27] Nguyen, McCracken, Ducharme et al 2013.

^[28] Harasty, Double, Halliday et al 1997.

^[29] Grimshaw, Bryden & Finegan 1995.

^[30] Kimura 1963b.

^[31] OVERALL: Pelvig, Pakkenberg, Stark et al 2008; Oliveira-Pinto, Andrade-Moraes, Oliveira et al 2016. RIGHT GREATER THAN LEFT: Rabinowicz, Dean, Petetot et al 1999.

^[32] Rabinowicz, Petetot, Gartside et al 2002.

^[33] See, eg, Beking, Geuze, van Faassen et al 2018.

^[34] Halpern, Benbow, Geary et al 2007.

^[35] Diamond, Johnson & Ingham 1975.

^[36] For an overview of the complexities, see Celec, Ostátniková & Hodosy 2015.

^[37] Fascinatingly, Kimura (1992) found that higher levels of testosterone in women masculinised the neuropsychological profile, eg, improved visuo-spatial skills, as might be expected; but that in men lower levels of testosterone were associated with excellence in such skills.

^[38] Pletzer, Petasis & Cahill 2014.

^[39] Tranel, Damasio, Denburg et al 2005.