Is the left hemisphere more developed in human females than males ?

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There is no argument in the scientific literature against the view that girls and women are superior on some verbal skills and that boys and men are better on certain visuospatial skills.   The first logical explanation of this that ought to come to mind would be that females have a larger, more developed left hemisphere and males have a larger, more developed right hemisphere.  After all,  nothing is more solidly established in neuropsychology than the fact that verbal functions are left hemisphere-specialized and visuospatial functions are right hemisphere-specialized.  In a certain sense,  one famous author actually made that argument and managed to convince a lot of people that his argument was valid.  Norman Geschwind,  and a few of his colleagues proposed that prenatal testosterone actually slows down the development of the left hemisphere  -but only at a certain period of its development.  This roundabout theory allowed him to eschew evidence against a basic sex-specific anatomical difference between whole hemispheres.   Is there any evidence of whole-hemisphere sex differences at all then ?  One very suggestive and exciting sex difference has been observed, as far as I know, only in rats.   Male and female rats have opposite asymmetries in cortical thickness on the left and right sides of their brains.  However,  the female rat,  if ovariectomized before birth, manifests the male-typical asymmetry as an adult !   

An anatomist named Crichton-Brown had observed in 1880 that the two brain hemispheres of women are of similar weight,  whereas,  in men the right hemisphere is heavier.  One research team reported that the right hemisphere of the human male fetus is larger than the left,  and that this is not the case of the female human fetus.  Do these anatomical brain differences have a significant impact on cognitive abilities and behavior of normal men and women ?   It is still too early to say.  However,  as usual,  answers to these types of questions are obtained more revealingly from pathology than from research on normal subjects.   There are two data bases which provide information about this issue,  IQ subscale profiles and aphasia,  following unilateral hemispheric lesions.

A very large meta-analysis was published in 1982 by  researchers named Inglis and Lawson.  They found that right hemisphere lesions affected performance (non-verbal) IQ more than verbal IQ,  in both sexes.  However,  left hemisphere lesions affected verbal IQ more than performance IQ only in men. There have been several other studies of the effects of unilateral brain lesions on IQs of men and women.   It has consistently been found that the results suggest more lateralisation of function in men than in women.   Verbal IQ is more selectively affected by left hemisphere lesions in men than in women,  and Performance (less verbal,  more visuospatial)  IQ is more selectively affected by right hemisphere lesions in men than in women.

Women who suffer cerebrovascular accidents (obstructions or ruptures of arteries feeding the brain) in the left hemisphere become less frequently aphasic (unable to understand or produce speech)  than do men.  Two explanations of the aphasia and IQ effects of hemispheric lesions as a function of gender are currently being debated.   Language functions could be represented in a more distributed fashion within the left hemisphere in women,  or language functions could be distributed to a greater extent in the two hemispheres in women,  whereas in men it could be more constrained to the left hemisphere.   One argument for the former point of view is research on the effect of barbiturization of a whole hemisphere.   An anesthetic called sodium amytal is sometimes injected into a catheter located in one of the carotid arteries,  the main artery irrigating the brain hemisphere,  in epileptic patients.   This procedure serves to determine to what extent the non-anesthetized hemisphere can support language and/or memory functions.  It is used for epileptics who do not respond well to anticonvulsant drugs.   If it turns out that language or most memory functions are localized on the side of the brain that the surgeon wanted to resect (remove),  the operation is canceled.   If the patient is lucky enough to have enough language and memory functions in the hemisphere not requiring resection,  he or she is likely to be relieved of his or her epileptic seizures after the resection of the part of the brain containing the epileptic focus.    This diagnostic technique is also known as the Wada technique, after its inventor.   Now the Wada procedure is extremely useful for the study of hemispheric specialization.     Large cohorts of men and women epileptics have now been tested for language functions after anesthesia of the entire left or right hemisphere.   It turns out that men and women have very similar profiles:  they make very few verbal errors and understand well what is communicated to them when it is their right hemisphere that is anesthetized,  but they make a lot of mistakes and understand communication poorly when it is their left hemisphere that is anesthetized.   

Finally,  a third position on the issue of sex differences in hemispheric asymmetry has been formulated by Kimura.  She reports that women manifest aphasia more frequently than men following left frontal lobe lesions,  whereas men do so more frequently following lesions in the posterior part of the left hemisphere.  Kimura believes the left hemisphere is specialized for several important aspects of motor programming,  thus the high prevalence of right handedness and left hemisphere based speech as well,  in both sexes.  She found that, as was the case for aphasia,   manual apraxia (loss of the understanding or expression of learned hand gestures)  results more frequently in women from left frontal lesions,  and in men from left posterior lesions.  She also makes the point that in those aspects of speech in which women excel over men,  it is in speed and precision of speech production and fluency.    Kimura thus speculates that women’s cognitive strengths may boil down to a sort of evolutionary specialization for fine motor coordination implanted in her brain by natural selection of a home based ecology (child care, tool making,  etc.)

My own position with regard to the literature on this particular topic is that the whole hemisphere is not necessarily the proper level of interpretation of human sex differences in the brain,  except when the data being investigated clearly pertain to the whole hemisphere.   Hemispheric barbiturization (anesthesia of a whole side of the brain),  hemispherectomy (removal of the entire cortex of one hemisphere), electroencephalographic  (EEG) topography (using electrodes placed over the entire hemisphere),  and experiments involving electrical stimulation of a wide diversity of human cortical surface areas (a diagnostic procedure in epilepsy), are examples of techniques yielding data appropriate for inferences about interhemispheric sex differences in terms of whole hemispheres.   Localized brain lesions,  tachistoscopic and dichotic listening experiments (see chapter 3),  and anatomical analysis of individual brain systems such as nuclei (closely knit neuronal networks) or fasciculi (large pathways containing many neuronal axons) are techniques which may reveal sex differences either between or within the hemispheres,  thus requiring more prudence in neuropsychological interpretation.