Sex and lateralisation Where the gender debate first arose, was from claims about differences between men and women in the way they use the two halves of the cortex.
The original hypothesis was that men used their logical left side while women used the emotional irrational right side. However, the argument soon arose that, if language was a function of the left side, how was it that women were better at expressing themselves verbally?
This is rather a simplistic view of the controversy, however, the theory was modified to suggest men have greater lateralisation, that their abilities are more compartmentalised, while, in women utilisation of the two halves is more diffuse.
From the sixties onward, Landsell was working with people who had damage to one side of the cortex or the other. The knowledge of the time indicated that damage to the left hemisphere should lead to deficits in verbal tasks, while right-side damage should produce deficits in visuospatial tasks. This proved particularly true for men, but the prediction was not borne out well for women. It led him to speculate that the abilities of the two hemispheres overlapped to an extent.
Electroencephalogram measurements have also shown a difference. When given abstract problems to work out, men showed a great deal of activity in the right side of their brain, while for women the activity was more generalised to both sides. Similar studies with teenage boys and girls gave similar results.
With women who had Turner's syndrome, which comes about because they have only one X chromosome, XO, and are considered to behave in a very feminine manner, this diffusion of organisation was particularly marked. The phenomenon has also been found in men whose exposure to androgens in the womb was reduced.
Workers following hormonal hypotheses have found that in rats given testosterone at birth, the females developed a larger corpus callosum. Others have found that male rats showed a thicker right hemisphere, except when they were very old. One developmental theory is that high levels of prenatal testosterone slow neuron growth in left hemisphere.
However, Shute(4) analysed blood samples from groups of males and females whose hormones were within the normal range. For spatial tests, females with high androgen levels performed better than their lower androgen counterparts. However, low testosterone men performed better than high testosterone men, leading the researchers to conclude that high androgens may inhibit the acquisition of spatial skills, and that there may a low optimum level.
Other tests have claimed that females are superior in language, verbal fluency, speed of articulation and grammar, also arithmetic calculation. Their perceptual speed, for instance in matching items is better, and so is their manual precision. Males are reckoned to be better at tasks that are spatial in nature, such as maze performance and mental rotation tasks. Also mechanical skills, mathematical reasoning and finding their way through a route. Certainly, among brain injury patients, after damage to the left hemisphere, long term speech difficulties occur three times more often in males.
Some critics asked why, after a hundred years of research, these findings have only just appeared. One reason may be that most of the subjects studied originally were male war veterans. But, in any case, nobody had looked for sex differences. What we are discussing are average differences which are statistically significant but their effect is very small within a very wide range of individual variation. The investigator must be specifically looking for them, using a large number of subjects.
