Download Why are people`s brains different sizes?

Why are people's brains
different sizes?
There are many tactics people use to end
arguments and confrontations, most of them
having to do with size. Parents, for example,
seem to favor "Because I said so" as a way to put
a stop to a disagreement with their children, who,
after all, are much smaller than they are for at
least a little while. School bullies have the
reputation of being big kids, their stature all the
more advantageous for stuffing the wimpy geeks
down the toilet. And let's say that a married
couple is arguing about the best way to get to
their destination; conceivably, the man could end
the argument by saying, "Of course, I'm right!
Males have bigger brains."
Men do in fact have bigger brains than women.
The average human brain weighs in at 2.7
pounds, or 1,200 grams, which is about 2 percent
of our body weight [source: Bryner]. Males,
though, have about a 100 g advantage after
accounting for differences in total body weight
[source: Schoenemann].
Which brings us to the age-old question: Does
size really matter? That is, does the male in this
scenario have any sort of valid point when he
claims to be smarter with his bigger brain? On
first glance, one might think so, because the way
that we humans differentiate ourselves from our
earlier primate ancestors is by our bigger brains.
But if bigger is better, does that mean we are
only slightly smarter than a walrus, which has a
brain weighing 2.4 pounds (1.1 kilograms), and
much dumber than a sperm whale, which has a
whopping 17-pound (7.7-kilogram) brain
[source: Bryner]?
In this article, we'll investigate the issue of
whether it's how much you have or how you use
it. First though, head to the next page and we'll
take a look at why hats come in all different
sizes, or how we end up with different-size
brains to begin with.
Brain size is determined to some extent by
genetics. In studies of identical twins, who share
the same genes, and fraternal twins, who share
about half the same genes, there is greater
correlation in brain size between the identical
twins [sources: Pennington et al., Wade].
Neuroscientists are still unpacking all of the
mysteries in the brain's suitcase, though, so while
we may not know all of the genes that are at
work in the brain, we can shed light on a few.
Researchers at Harvard Medical School isolated
one of the brain-size genes by working with
mice. When the mice were administered an
increase of a gene named beta-catenin, their
brains doubled in size, and they began to show
more activity in the cerebral cortex [source:
Cromie]. The cerebral cortex, which regulates
intelligence and language, is what sets humans
apart from other species. It gives us the ability to
form ideas and express them, giving us an
advantage over species that may have bigger
brains than we do. So while whales have that 17pound brain, they use most of that giant noggin
telling their bodies to move through the water
[source: Wanjek].
Does that mean we need to start injecting betacatenin to make it through high school calculus?
Not if the mice are any example. Some of the
mice in the Harvard experiment died after their
heads got too big.
These researchers have also worked with another
brain-size determining gene, ASPM, which is an
abbreviation for abnormal spindle-like
microcephaly-associated. As the name of this
gene implies, it's linked with microcephaly, a
condition in which a person is born with a small
head and brain, often resulting in mild
retardation. While an increase in beta-catenin
might lead to a similarly increased brain, a
mutation in ASPM seems to stop the formation
of brain cells. When proteins in ASPM are
shorter, brains are smaller.
To return to the similarities of mice and men,
researchers have shown that a gene called Emx2,
present in both rodents and humans, may control
how the brain is actually divided. If certain parts
of the brain are larger, there may be increased
function in the area that part of the brain controls
[source: Salk Institute].
Other genes may be at work as well, but they're
not the only determinant of brain size. While
different brain sizes are evident at birth,
environmental factors also play a role in the
brain's development. In the first five years of life,
the brain quadruples in size, reaching about 95
percent of its adult volume [source: Suplee]. The
neural connections that babies make in their first
year or so are the connections that will serve
them for the rest of their lives, but recent
evidence shows that brain development in certain
areas continues through the teenage years
[source: Suplee].
So let's say genetics aren't on your side, and no
one fires up the Baby Mozart for you. Does it
matter? Find out if a bigger brain makes any
difference at all on the next page.
Brain Size and
Intelligence: Does Size
Matter?
Researchers have linked sudden and
disproportionate brain growth during the first
year of life to autism, suggesting that excessively
rapid growth prevents the child from making the
connections that guide normal behavior [source:
BBC]. Another study indicated that children and
adolescents with attention deficit hyperactivity
disorder (ADHD) possess brains that are 3 to 4
percent smaller on average than those without
ADHD [source: Goode]. Scientists have also
revealed that brains shrink with age, though
cognitive functions may remain unaltered
[source: Britt].
But the question everyone wants to know is,
what link exists between a big brain and a big
IQ? Is bigger better? Since we're talking about
the brain, then surely an enhanced version must
lead to more smarts and more talent, right?
Well, it depends which scientist you ask.
Scientists have been divided about what they're
measuring and how they're measuring it.
Anthropologists have long used a skull's interior
volume and compared it against body size for a
rough estimate of intelligence, measurements
known as encephalization quotients. As brainimaging techniques have improved, though,
scientists have measured actual brains with
greater precision. But is it size or is it neurons
that we need to measure? Is it weight or
circumference? Should encephalization quotients
use total body weight or lean body mass? Should
we correct for body size at all? How do you
measure intelligence?
With so many brains tackling these questions, it's
hard to reach a consensus on what might be the
most meaningful measure. That hasn't stopped
researchers from drawing conclusions, though. In
2005, psychologist Michael McDaniel evaluated
studies that used brain-imaging and standard
intelligence tests and found that unequivocally,
bigger brains correlated with smarter people
[source: McDaniel].
Since males have the bigger brains, they must
have the smarts, right? In one study, scientists
converted the SAT scores of 100,000 17- and 18year-olds to a corresponding IQ score and found
that males averaged 3.63 IQ points higher than
the females [source: Jackson, Rushton]. The
study, did, however, use about 10,000 more
females than males, which may have affected the
average, but the study's authors believe that the
greater the brain tissue, the greater the ability for
cognitive processing [source: Bryner].
Remember those studies with twins on the last
page? In one study, after the scientists drew
conclusions about the role of genetics in brain
matter, they gave the twins intelligence tests.
They found a link between intelligence and the
amount of gray matter in the frontal lobes. Since
frontal lobes appeared to be controlled by
genetics, the results indicate that parents pass
along the potential for genius.
But should gals just throw up their arms, curse
their parents and refuse to make sense of nuclear
physics? Nope. You've got to go out and shake
what your momma gave you. These areas may
just lay the groundwork for intelligence down
the line or indicate the potential for genius if a
person works hard. Albert Einstein may be a
perfect example that it may not be overall size
that matters, but size of certain sections beyond
just the frontal lobe. Einstein, for example, had a
perfectly normal-size brain, but certain parts of it
were larger than normal, including the inferior
parietal region, which affects mathematical
thought [source: Wanjek].
It's also worth noting that the strangest things
seem to increase brain size. Scientists have found
that the brains of London's cab drivers enlarge
and change as they learn complicated routes. Cab
drivers who have been navigating the streets for
years had significant structural changes, as they
exhibited a larger posterior hippocampus and a
slightly smaller front hippocampus [source:
BBC].
So until we know more about all the exact
mechanisms of brain growth, you may as well
check out the stories on the next page. They just
may make you brainier.
1) List two things that contribute to the brain growth.
2) The author MOST LIKELY included paragraph 14 to show...
a. Brains come in different sizes.
b. Bigger brains are better.
c. Scientists disagree about how to measure brains.
d. Brain size does not matter.
3) The author MOST LIKELY included paragraph 16 to show...
a. Men are smarter than women.
b. Studies about men’s and women’s intelligence and brain size are inconclusive.
c. Men score higher on the SAT.
d. Women have a different kind of intelligence than men.
4) The main idea of this passage is...
a. There may be a link between brain size and intelligence.
b. Women have smaller brains than men.
c. IQ tests are used to measure a person’s brainpower.
d. Mice that are injected with certain genes become smarter.
5) What would another title for this passage be and why?
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