Welcome to Your Brain (31 page)

without any meaningful integration in their communities. Second, parents and doctors know more

about autism and are more likely to consider the possibility when evaluating a child with

developmental problems. Third, better treatment options are available, which increases parents’

motivation to identify autism in their children. Many parents are interested in obtaining behavioral

therapy, which, although not curative, may lead to some improvement in symptoms. Of course, by the

same arguments, no one can say for sure that the prevalence of autism has not increased. In fact, some

scientists believe that autism is underdiagnosed even now. What we can say is that the data from past

decades doesn’t provide clear evidence for an increase in the prevalence of autism.

Myth: Vaccines cause autism

The proposed link between vaccines and autism has received a lot of attention over the

past few years. Robert Kennedy Jr. wrote a book about it. Indiana Republican Dan Burton,

who has an autistic grandchild, has held multiple congressional hearings on the topic.

Scientists have spent hundreds of hours and reviewed thousands of patient records to

investigate this connection, but have found no trace of a causal relationship—still, the

speculations continue.

All this excitement began with a 1998 study from a British gastroenterologist. The

paper reported on twelve patients, who were selected based on gastrointestinal symptoms.

Nine met the criteria for autism diagnosis. The parents of eight of the children reported that

the symptoms had begun around the time that the children were vaccinated against measles,

mumps, and rubella (known as the MMR vaccine). The paper noted that the behavioral and

intestinal symptoms may have occurred together by chance, “reflecting a selection bias in a

self-referred group.”

The paper’s interpretation was later retracted by ten of the gastroenterologist’s twelve

coauthors, who stated, “We wish to make it clear that in this paper no causal link was

established between MMR vaccine and autism as the data were insufficient.” Indeed, the

study did not even have a control group, which was essential considering that the outcome

measure, intestinal inflammation, was so vague and common. Others were not able to

reproduce the gastroenterologist’s findings. It also came to light that, before the paper’s

publication, the lead author had been consulting for a group of lawyers who were intending

to file suit against vaccine manufacturers.

Parents may associate vaccination with the onset of autism by coincidence because both

events occur around the same time. Vaccines are given between twelve and fifteen months,

and symptoms of autism typically begin to appear between twelve and twenty-four months.

In one study, starting in 1979, all the cases of autism or autism spectrum disorders in a

London district were identified. Autistic children were no more likely to have been

vaccinated than typical children. The diagnosis of autism was no more likely to occur

immediately after vaccination than at any other time. A study in Sweden also found that

introduction of the MMR vaccine did not correlate with an increase in autism diagnosis.

Indeed, in several independent reviews by the U.S. Institute of Medicine, the U.K. Medical

Research Council, and the Cochrane Library (an international consortium of scientists

formed to evaluate the medical literature), no credible link between vaccines and autism

has been found. The Cochrane group notes that most studies on the subject are flawed by

unreliable outcome evaluation and other sources of investigator bias.

The hypothesis favored by Kennedy is that autism is caused by ethyl mercury in

thimerosal, a preservative that was used in some vaccines (though not the MMR vaccine)

until 2001 in the U.S. The main evidence for this idea is that autism diagnosis has been

increasing over the last few decades, though it is unclear whether this reflects a real

increase in the number of affected people, as discussed below. Even if we accept that there

is an autism epidemic, though, it does not correlate with the presence of thimerosal in

vaccines. In the London study, no jump in autism diagnosis occurred when vaccines

containing thimerosal were introduced in 1988. Thimerosal was present in U.S. vaccines

between 1991 and 2001, but the increase in autism diagnosis began earlier and has not

declined since the preservative was removed. Canada and Denmark removed thimerosal

from their vaccines in 1995 and have since had no decrease in the rate of autism diagnosis.

Sadly, the continued debate about this false trail diverts needed resources from productive

lines of research into the true causes of autism.

Whatever the relative importance of genes or the environment in causing autism, both act by

affecting brain development. The brains of most autistic people do not appear dramatically different

from normal brains, though some autistics have unusually large brains and, for unknown reasons,

unusually small cerebellums. These differences in brain size are not present at birth but instead

develop over the first two years of life, suggesting a problem with the “pruning” of brain connections

that normally occurs during this time period, as we discussed in
Chapter 10
. Most autistic people

have a variety of subtle but widespread problems in the cortex and other areas, including changes in

neuron density or number, and the disordering of the normal arrangement of neurons into functional

groups.

Only a few specific genes have been consistently linked to autism. If multiple mutations are

required to cause the disorder, then geneticists may never be able to identify all the complicated

interactions that are involved. Even partial answers can be useful, however, in suggesting brain

mechanisms of the disorder. For example, autism is linked to mutations in two families of related

genes, called neurexins and neuroligins. These genes encode proteins that control the positioning of

neurotransmitter receptors during the formation of both excitatory and inhibitory synapses in early

development.

This is interesting because about 30 percent of autistic people also have epilepsy, compared with

only 1 percent of the general population. Epilepsy is a disease of brain excitability that occurs when

the balance between excitation and inhibition is disrupted, leading to uncontrolled excitation that

causes seizures in the body. It is easy to imagine how damage to the neurexin or neuroligin genes

could lead to defects in this synaptic balance that cause seizures. It is not much more difficult to

picture such changes causing more subtle functional defects in brain regions that control language or

social behavior, though no one is certain exactly how this happens.

Some scientists suspect that all these differences between autistic and normal brains result from a

primary defect in connections between brain areas. In particular, many autism symptoms could be

explained by damage to connections that allow the frontal cortex and other so-called association

areas (which coordinate the use of many different types of information) to influence brain regions that

are important for routine behavior and sensation. Without these connections, the brain would be

unable to regulate incoming sensations, which could cause the hypersensitivity to environmental

stimuli seen in many autistic people. The association areas are also important for facilitating flexible

responses to circumstance, including suppressing habitual behaviors when appropriate in a particular

context, which could account for rigid and repetitive behavior. Finally, many of these association

areas are directly involved in social behavior (see
Chapter 16)
.

One question is why the genetic factors that underlie autism would persist in the population. It’s

possible that individually, the genes confer some benefit. For example, autistic people tend to be very

good with details, perhaps because of a lack of higher control from the frontal cortex. A small number

of people in the population with an exceptional ability to focus on tasks could be a good thing for

society. In the words of the famous autistic Temple Grandin, “What would happen if the autism gene

was eliminated from the gene pool? You would have a bunch of people standing around chatting and

socializing and not getting anything done.”

Chapter 25

A Brief Detour to Mars and Venus: Cognitive

Gender Differences

Men and women are exactly the same.

Just kidding. If we had to toe that ideological line, this would be a very short chapter. Now, it is

true that many sex differences are exaggerated—and some are just plain invented. The world is full of

nurturing men and aggressive women, and the sexes are equally smart overall. But as anyone who’s

raised kids probably knows already, boys and girls are born with some different equipment between

their ears.

Of course there are major differences in the brain regions that determine which sex you’d rather

see in tight pants (see
Chapter 20)
. But get your mind out of the gutter for a moment, and let’s consider

why men and women might think differently when they’re not in bed. We know that hormones

influence how the brain works and that sex hormones like testosterone and estrogen are present in

different amounts in males and females. These hormones have an especially strong influence before

and soon after birth, when babies’ brains are being formed, but they also have direct effects on adult

brains. Men’s and women’s brains are shaped differently too, probably as a result of these hormones

—though again, most of the differences are very subtle. Women’s brains have slightly more surface

area and more connections between areas, while men’s brains have slightly more volume, even when

we allow for their larger bodies.

Given these differences, it’s hardly surprising that men and women might tend to behave

differently. But human behavior is determined not only by biology, but also by experience and training

—what we commonly call culture. Most kids want to behave in ways that please their favorite adults.

If girls are punished for getting their clothes dirty, while boys sense that their parents are secretly

happy about such a show of masculinity, then we can’t conclude that girls are naturally inclined to be

fussy about their appearance. Many teenagers believe that men find smart women less attractive

(though thankfully most of us come to know better), and the effectiveness of all-girls’ schools in

promoting academic achievement suggests the possibility that girls may adjust their behavior—and

their apparent abilities—to accommodate this stereotype.

Myth: Women are moodier than men

We can’t deny that women are moody. What most people don’t realize is that men are

moody too. In fact, their moods vary as much from hour to hour as women’s moods. How

do we know this? When psychologists give beepers to men and women and ask them to

write down their mood whenever it goes off, men and women report similar variations.

Curiously, both men and women tend to remember women’s mood swings better, so if

people are asked later to remember how moody they or their partners were in the previous

week, more mood swings are reported for women than men.

It is true that mood disorders, including depression and anxiety, are about twice as

common in women as in men. Some of that disparity may be because women are more

willing to go to the doctor when they feel bad, but even when we account for that cultural

difference, women are still at greater risk. No one is really sure why, though some people

have guessed that women’s life experiences may expose them to more stress, which is

linked to depression and anxiety (see
Chapter 17
). Men and women are equally prone to

manic-depressive disorder, which is strongly linked to genes.

Prior beliefs can also influence how people’s performance is evaluated by others. Beginning in

the 1970s, debate raged in the classical music community over whether women could play as well as

men, since the top orchestras were made up overwhelmingly of men. Then feminists convinced U.S.

orchestra directors to start having musicians audition behind a screen so the judges could hear the

music without seeing the player. Surprise! Twenty years later, half the players in the top five

orchestras in the U.S. are women. In Europe, though, blind auditions are rare, orchestras are largely

male, and many musicians still believe that women can’t play as well as men.

So how do we distinguish between biological and cultural influences on behavior? We can’t

separate the two absolutely, since the environment shapes the way our brains work, but we can make

educated guesses. For instance, behaviors that differ between males and females in other species are

more likely to reflect biological differences. (Rats, for instance, don’t have much culture.) Behaviors

that are reliably masculine across various cultures are also more likely to have a biological basis

(though the biology in question could be men’s greater muscle strength, not necessarily their brains).

With that in mind, let’s look at some of the more convincing biological sex differences that are

documented in people.

The most reliable difference is in spatial reasoning. Not that men don’t like to ask for directions