Gender and Brain Imaging

by Douglas Eby

There seems to be an endless fascination with looking for differences in behaviors, attitudes, abilities and brain function between the sexes, but as a scientific pursuit it is a search with both technical and political/philosophical dimensions, and findings that are often inconsistent or open to variable interpretation.

As writer Diane Halpern notes, "Cognitive abilities are heterogeneous, and whether we find gender differences depends on what, who, and when (in the life span) we test."[1]  Dr. Carol Tavris comments that scientists in the past "tried to argue that women had smaller frontal lobes and larger parietal lobes than men did - until researchers reported that parietal lobes might be associated with intellect. A miracle followed : anatomists suddenly 'found' that women's parietal lobes were smaller than they had believed."[2]

Although most gender studies have ignored it, sexual orientation may be as significant as sexual identity: one study at least has found that in terms of brain hemisphere functioning and certain cognitive abilities, homosexual males resemble heterosexual females rather than heterosexual males.[3] 

Both the popular and scientific press love to report studies showing differences, while tending to ignore the usually more common research showing no real gender distinctions. And the differences that do show up are in terms of small separations of group mean values. Graphically, you can picture this as two bell-shaped curves, one for male and one for female scores, that don't quite overlap completely. It is all too easy to forget that men and women are far more alike than different, and that people change over time, both individually and societally.

The major cognitive gender differences have been declining significantly over the past three decades, including the "traditional" ones of superior female verbal, and male math, abilities.

One of the more recent explanations for this is proposed by the theory that children are no longer so segregated as far as "male" and "female" activities, and that behavior has a profoundly interactive relationship with hormones and the development of cognitive skills and brain structures - for example, rough-and-tumble play, previously more common in boys, might better develop spatial abilities.[4]

Researchers have lately been using very sophisticated imaging technologies to look at a wide range of both healthy and disordered brain processes, and finding some sex differences. These imaging tools include CT (and CAT) scanning - computed or computerized (axial) tomography : three dimensional X-ray; PET - positron emission tomography : evaluation of amount and localization of metabolism using a radioactively tagged substance like sugar or oxygen; MRI - magnetic resonance imaging : an extremely high powered magnetic field to show the structure of the brain; EEG - electroencephalogram : detection of the brain's electrical activity.

The research and clinical application of these tools is still a relatively new science, and there are sometimes difficulties with the comparisons and evaluations of findings.

One of the latest theories to explain cognitive variations is "brain lateralization" : the idea that the two brain hemispheres have different types or levels of abilities such as spatial orientation and language, and that women are less differentiated or dependent on one side compared with men.

Some current brain research tends to support this notion, such as a study showing sex differences in the ratio of EEG activity on each side [5], but a larger number of findings recently have failed to support the idea, including evaluations of the size and shape of the corpus callosum (the nerve bundle joining the two sides of the brain) using MRI [6,7] and greater, rather than decreased, localization of emotionally induced EEG (brain wave) arousal in women. [8]

Another favorite topic - higher verbal ability in women - has been evaluated by psychologists Marcia Linn and Janet Shibley Hyde in 165 studies representing almost 1.5 million subjects, and they conclude there are no gender differences in verbal ability.[9] 

As confirmation, a study using CT (X-ray) of brain lesions that caused speech problems showed no sex differences.[10]  But there still are notable gender distinctions in rates of certain speech problems: males outnumber females about 3 to 4 times for stuttering and 10 times for severe dyslexia, and both of these disorders seem to have at least some biological as well as psychological factors.

For many years, males have tended to score higher on math skills tests. In another major survey, Dr. Janet Hyde analysed 100 studies of math performance, representing over three million subjects, and concluded that there were no gender differences in problem solving in elementary or middle school ages, but small distinctions did show up in later age groups.[11] 

In the past, women have consistently scored lower on the SAT-M (math) section, but that discrepancy virtually disappears when the time limits on the testing are removed [12], which may indicate that women simply tend to use a different cognitive process in math problem solving.

According to Dr. Richard Haier of the UC Irvine Brain Imaging Center, this kind of question can be studied through PET scan research, but a priori hypotheses for specific brain areas are necessary before comparing males and females.

The incidence of female depression is usually found to be twice as high as the rate for men, and is estimated to affect about 7% of American women. Some of the new brain imaging technologies are beginning to look for possible anatomical or chemical factors to help explain this, although a significant part of the variation may be that women are more socially pressured toward the disorder, are more susceptible, or simply more willing to seek formal therapeutic help.

A PET study by Dr. Buchsbaum of UC Irvine showed relatively higher metabolism in the frontal (brain) cortex of unipolar (depressed) patients.[13]  Panic disorder and agoraphobia are somewhat more common in women, and PET scans show higher temporal lobe activity in these conditions [14], and also unusual architecture as shown by MRI.[15]

Some intriguing gender variations have shown up in studies of schizophrenia, including a finding of overall larger ventricles (brain cavities/spaces) in males [16], but larger third ventricles in women [17], and a significant difference in sizes of specific parts of the corpus callosum among women patients compared with men [18].

Males are diagnosed with autism at a rate of 3 to 4 times higher than females, and a recent MRI study shows differences in brain frontal lobe shape in people with autism compared to those with mental retardation, or with control subjects.[19]

Bulimia was recently estimated to affect 4 to 5% of college women, but less than half of 1% of men. Using techniques such as MRI and PET, researchers are finding differences in such eating disorders in areas including levels of anxiety-related hormones [20], dimensions of forebrain structures [21], and changes in brain metabolism which may distinguish bulimia from depression and anorexia [22], and anorexia from treated or normal status in the same person. [23]

Although most of these findings are preliminary and inconclusive, they do hold promise for helping to better understand the three pound universe in our heads - the biology of our behaviors. The research into gender differences needs to be approached with caution, though.

As psychologist Roy Baumeister notes, "By seeking, reporting, and discussing sex differences, psychologists lend scientific prestige to the distinction between men and women. It endorses a way of looking at the world in which men and women are fundamentally different" (in ref. 2).

And, as Dr. Haier observes, "You can compare men and women on a couple of hundred brain areas and you're bound to find differences, but without some theory in advance what to look at and what you might find...it's kind of a big fishing expedition."

But that also helps keep the search endlessly fascinating.

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   references

1 Halpern DF. The disappearance of cognitive gender differences: what you see depends on where you look. American Psychologist Aug., 1989

2 Tavris C. The gender gap. Vogue, April,1989

3 Sanders G , Ross-Field L. Neuropsychological development of cognitive abilities. Int J Neuroscience 36:1-16, 1987

4 Geary DC. A model for representing gender differences in the pattern of cognitive abilities. American Psychologist,Aug.,1989

5 Corsi-Cabrera M, Herrera P, Malvido M. Correlation between EEG and cognitive abilities: sex differences. Int J Neuroscience 45:133-141, 1989

6 Byne W, Bleier R, Houston L. Variations in human corpus callosum do not predict gender. Behavioral Neuroscience 102(2):222-7, 1988

7 Clarke S, Kraftsik R et al. Forms & measures of adult & developing human corpus callosum: is there sexual dimorphism? J Comp Neurology 280(2):213-30, 1989

8 Smith BD, et al. Hemispheric asymmetry and emotion: lateralized parietal processing of affect and cognition. Bio. Psychology 25:247-60, 1987

9 Hyde JS, Linn MC,eds. The Psychology of Gender: Advances Through Meta-analysis. 1986

10 Kertesz A. Sex equality in intrahemispheric language organization. Brain and Language 37:401-8, 1989

11 Hyde JS, Fennema E, Lamon SJ. Gender differences in mathematics performance: a meta-analysis. Psych Bulletin 107(2):139-155, 1990

12 Gallagher S. Predictors of SAT mathematics scores of gifted male and gifted female adolescents. Psych Women Quarterly June, 1989

13 Buchsbaum MS. Brain imaging in the search for biological markers in affective disorder. J Clinical Psychiatry 47 Suppl:7-12, 1986

14 Heh CW. Anxiety and anxiety disorders. Unpublished paper,UC Irvine, 1990

15 Fontaine R, et al. Temporal lobe abnormalities in panic disorder: an MRI study. Biological Psychiatry 27(3):304-10, 1990

16 Andreasen NC, et al. Magnetic resonance imaging of the brain in schizo- phrenia. Arch General Psychiatry 47(1):35-44, 1990

17 Shelton RC, et al. Cerebral structural pathology in schizophrenia: evidence for a selective prefrontal cort. defect. Am J Psychiatry 145:154-63,1988

18 Nasrallah HA, et al. A controlled magnetic resonance imaging study of corpus callosum thickness in schizophrenia.Biol Psychiatry 21:274, 1986

19 Hashimoto T, et al. Magnetic resonance imaging in autism: preliminary report. Neuropediatrics 20(3):142-6, 1989

20 Brewerton T, et al. Eating disorders, anxiety, and 5-HT. Biol Psychiatry 27:41A, 1990

21 Black K, et al. Brain morphology in eating disorders. Biol Psych 27:62A, 1990

22 Wu J, et al. Greater left cerebral hemispheric metabolism in bulimia assessed by positron emission tomography. Am J Psych 147:309-12,1990

23 Herholz K, et al. Regional cerebral glucose metabolism in anorexia nervosa measured by positron emission tomography. Biol Psych 22:43-51, 1987

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