Male and female brains differ in
structure and function, but we don’t
know how these differences affect
behavior.
Chapter 16 of my book, 50 Human
Brain Ideas You Really Need to
Know. Here’s the introduction to
the book, the chapter
about topographic mapping, the
chapter about mirror neurons, and
my post about the most important
idea in neuroscience.
Summary : Subtle observable
differences exist between male and
female brains, but how exactly these
relate to differences in behaviour is
unknown. Such gender variations in
the brain
are often exaggerated and
misappropriated, not only by the
mass media but also by scientists, to
reinforce stereotypes and
perpetuate myths.
The science of sex differences has
always been – and still is – fraught
with controversy. Some believe that
behavioural differences between
men and women are mostly due to
cultural influences, while others
argue that sex differences are
largely determined by biology. In
reality, the situation is far more
complex. It lies somewhere in the
middle, and involves two related but
independent factors, which are
often confused or conflated.
One of these factors is biological
sex, which is determined by
chromosomes. Most people have
either two X chromosomes, which
makes them female, or one X and
one Y chromosome, which makes
them male. The other is gender,
which is influenced largely by the
socialization process. As we grow up,
we learn society’s norms about how
males and females look and act; for
most people, sex and gender are
matched, and so they inadvertently
conform to these norms.
Men and women’s brains differ in
subtle ways, and these differences
are probably established in the
womb, due to the effects of sex
hormones, which masculinize or
feminize the organ as it develops.
However, we still do not understand
the effects of sex hormones on the
developing brain, or how the subtle
differences observed between men
and women’s brains are related to
differences in their behaviour.
Battle of the sexes?
The most obvious difference
between the brains of men and
women is overall size – men’s brains
are, on average, between 10 and 15
per cent larger than women’s. In
one recent study, neuroscientists
compared the brains of 42 men and
58 women postmortem, and found
that men’s weighed an average of
1,378g (3lb), compared with 1,248g
(2.75lb) for women. These size
differences have been found
repeatedly, but they emerge only
when comparing large numbers of
people, so some women’s brains are
larger than the average whereas
some men’s are smaller. These
differences partly reflect the fact
that men are generally bigger and
taller than women, but they are not
related to differences in
intelligence.
Men and women’s brains also differ
in overall composition. Male brains
tend to have a slightly higher
proportion of white matter, whereas
those of females have a higher
proportion of grey matter in most
parts of the cerebral cortex.
Consequently, the cortex is slightly
thicker in women’s brains than in
men’s and, according to several
studies, is slightly more convoluted
as well. There are also sex
differences in the size of individual
brain structures. The hippocampus,
a structure involved in memory
formation, is on average larger in
men than in women, as is
theamygdala, which is also involved
in memory, as well as emotions.
Another sexual variation is found in
a structure called the third
interstitial nucleus of the anterior
hypothalamus. The function of this
tiny structure is unknown, but
research from four different
laboratories has repeatedly found
that it is almost twice as large in
males than in females. It has also
been linked to sexual orientation
and gender identity: one study
showed that it is more than twice as
large in heterosexual males than in
homosexual males, where it more
closely resembles that of women;
another found that it is smaller in
male-to-female transsexuals, and
larger in female-to-male
transsexuals. These studies have
been criticized for their small
sample sizes, and the findings have
not been confirmed.
Stereotypes and
myths
Numerous studies show subtle
differences in male and female
behaviour and in cognitive functions,
too. Men tend to be more
aggressive and outperform women
on mental tasks involving spatial
skills such as mental rotation,
whereas women tend to be more
empathetic and perform better on
verbal memory and language tasks.
Findings like these are often
exaggerated to reinforce the
stereotypes that women are bad at
reverse parking and that they love
to chat!
In some cases, individual studies
purporting to show sex differences
in certain tasks are misappropriated.
For example, according to a
tinypostmortem study published in
1982, the corpus callosum, the
massive bundle of nerve fibres
connecting the two brain
hemispheres, is proportionally larger
in women than in men. This was
widely reported to mean that
women are better at multitasking,
even though subsequent work has
failed to replicate the results. A
more recent study showed
thatwomen are marginally better
than men at paying attention to
sounds presented to both ears
simultaneously – this was
interpreted by some as evidence
that ‘men don’t listen’.
Many of these claims are
accompanied by the assertion that
the observed differences between
men and women’s brains are ‘hard-
wired’ and, therefore, irreversible.
We now know, however, that brain
structure and function change in
response to experience, so any
observed differences between the
brains of men and women could also
be due to differences in upbringing
and socialization. To date, though,
very little research has been done
to investigate how different
nurturing styles might influence
brain development.
Box: The extreme
male brain
hypothesis
People with autism tend to perform
poorly on tests of empathizing, or
the ability to put oneself in
somebody else’s shoes, but do well
on tests of systematizing, or finding
repeating patterns. Similarly, women
tend to score higher on the
empathy scale, and men on the
systematizing scale. These
observations led one researcher to
propose the highly controversial
‘extreme male brain’ hypothesis of
autism. The hypothesis states that
autism is an extreme form of the
normal male cognitive profile, which
occurs as a result of high
testosterone levels in the womb.
Accordingly, people with autism can
be considered as ‘hyper-
systematizers’ who focus more on
patterns and fine details than on
other people’s thoughts and actions.
The extreme male brain hypothesis
has been used as an explanation for
why autism is four times more
prevalent in males than in females,
and why people with autism can
excel in disciplines such as maths
and engineering.
No comments:
Post a Comment