Graphics copyright © 1983 Richard C Atkinson, Rita L Atkinson & Ernest R Hilgard

Brain Lateralisation & Gender

The brain’s two hemispheres do, in fact, engage in some notable specialisations. Which means, at times, one half will be  ‘cerebrally dominant’. For example, the left hemisphere specialises in language while the right is concerned with visuospatial tasks. How complex this is, though, is reflected in the fact that language is exclusive to the right hemisphere in less than 20% of left-handers and is bilateral - ie: in both hemispheres - in another 20% of left-handers. However, in the majority of left-handers language is found in the left hemisphere.

Generally speaking, male brains are said to be more lateralised - having a preference for the right side. This would help explain why males are better at Mathematics and map-reading.

Females are usually thought to use both hemispheres more equally than males. Females often

have a larger corpus callosum, meaning their two hemispheres are better connected so they can use the two halves together more. This could explain superior female fluency in speech, thought, body language...and, of course, multitasking!

Another sex difference  frequently found in brain structure is that the anterior commissure  (which communicates sensory information) is larger in women and male homosexuals.

Evidence for gender differences in brain lateralisation includes:-

• Some studies - eg: J McGlone (1978) - show that damage to one side of the brain can lead to difficulties with verbal tasks (damage to the left hemisphere) or difficulties in visuospatial tasks (damaged right hemisphere) for men, but not women
• Women with Turner’s Syndrome (X0 genotype) use both sides of the brain even more than normal and display very feminine’ behaviour
• Men who do not have normal exposure to androgens in the womb tend to use both sides of the brain more
• From studies using rats, it is thought that high levels of testosterone slow neuron development in the left hemisphere
• Females are generally acknowledged to do better at language tasks, tasks in verbal fluency, speed of speaking, and tasks involving mathematical calculation. Males usually perform better at spatial tasks such as maze performance and mental rotation tasks
• Males with damage to the left hemisphere generally have more problems with speech than females with equivalent damage

Part 2

A physical reflection of different patterns of brain lateralisation between the sexes may be the findings of Jennifer Kulynych, Katalin Vladar, Douglas Jones & Daniel Weinberger (1992) that, using MRI scans, the left temporal plane was 38% longer than the right temporal plane in men but there was no real difference between the 2 planes in women.

Bennett Shaywtiz, Sally Shaywitz, Ken Pugh, Todd Constable, Pawel Skudlarski, Robert Fulbright, Richard Bronen, Jack Fletcher, Donald Shankweiler, Leonard Katz & John Gore (1995) used functional scanning to record blood flow to different brain areas during cognitive tasks. They found that, for a rhyming task, activity was localised to the left hemisphere in males but the activity in females was symmetrical. Pugh, Shaywitz, Shaywitz, Constable, Skudlarski, Fulbright,Bronen, Shankweiler, Katz, Fletcher & Gore (1996), in a similar piece off research, got similar results. However, J A Frost, J R Binder, J A Springer, T A Hammeke, P S Bellgowan, S M Rao & R W Cox (1999) failed to replicate such findings In a task requiring recognition of psuedowords,Susan Rossell, Edward Bullmore, Steve Williams & Anthony David (2002) found that men were faster if the pseudoword was presented in the right of their visual field whereas women were faster if it was presented in the left of their visual field. Brain scans showed that the task generated activity mainly in the left hemispheres of the male participants whereas activity was more evenly distributed between the two hemispheres for the females. Similarly Jeri Jaeger, Alan Lockwood, Robert Van Valin, David Kemmerer, Brian Murphy & David Wack (1998) found via PET scans that, when male participants in a language task, generated past tenses of verbs, activity was lateralised to the left whereas it was more bilateral in the female participants.

One problem in mapping brain activity to (self-reported) thoughts and (observable) behaviour is that, although blood flow can be measured to indicate neural activity, the activity may occur not specifically due to different brain structure but because different strategies are employed. Also some procedures measure activities in which men and women differ in performance capability while other procedures measure activities in which men and women are recognised as being equally competent.

Another difficulty in attempting to attribute the causes of thought and behaviour is that nurture is often involved as well as nature - see Nature-Nurture - with the differences between male and female brains often being a combination of biology and environment.

More Differences between Male & Female Brains

In their research Ruben Gur, Bruce Teretsky, Mae Matsui, Michelle Yan, Warren Bilker, Paul Hughett & Raquel Gur (1999) identified several underpinning factors to the apparent differences between male and female brains...

• The greater amount of the fatty ‘white matter’ throughout male brains gives them superiority at spatial reasoning.
• The white matter also prevents ‘information spread’ in the cortex, resulting in males tending to pay attention to only one thing at a time - supposedly why men proverbially can’t walk and chew gum at the same time!
• The white matter in females is concentrated in the corpus callosum - which means female brains are more balanced and better organised
• One result of more white matter in the female corpus callosum is that the right side of the brain can join in language tasks – language areas of the brain being primarily in the left hemisphere – associated with female superiority in language skills

Michael De Bellis, Matcheri Keshavan, Sue Beers, Julie Hall, Karin Frustaci, Azadeh Masalehdan, Jessica Noll & Amy Boring (2001) studied 61 males and 57 female children aged from 6.9 to 17 years. They used MRI scans to investigate how volumes of grey and white matter and the corpus callosum change with age. They found that volume of grey matter fell significantly with age more in males than females but that the volumes of white matter and the corpus callosum both increased with age more in males than females. Although the whole cerebral volume did not change significantly with age, it was consistently larger in males than females. De Bellis et all concluded from these results that boys’ brains mature faster. They at least partly attributed these effects to sex hormones - with oestrogen delaying pruning while testosterone promotes myelination.

Daniel Voyer (1996) carried out a meta-analysis, with 396 male-female comparisons from 266 studies and concluded that, overall, males show greater lateralisation than females. However, another recent meta-analysis did not find consistent gender differences - especially in relation to the corpus callosum.

Child abuse and neglect affects the brain

It appears brain abnormalities can be caused by child abuse and neglect.

This is particularly so in the limbic system which is associated with emotions. Abuse can cause disturbances that can lead to seizures and other abnormalities that show in the electrical activity measured by an electroencephalogram (EEG). In abuse cases investigated this way, the abnormality has been in the left hemisphere and has been linked to Depression and memory deficits.

The corpus callosum is smaller in those who have been abused as children. A reduction in size of 24%-42% has been found in boys who have suffered neglect. In girls who have been sexually abused, a reduction of 18%-30% has been found. However, neglect of girls appears to have no effect on the size of the corpus callosum. It si thought that a reduction in size of the corpus callosum means less activity between the two hemispheres and this has been associated with changes in mood.