Why are people's brains different sizes?

Thanks for my giant brain, Mom!

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Brain size is determined to some extent by genetics. In studies of identicaltwins, 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 namedbeta-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 17-pound 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 beta-catenin 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 withmicrocephaly, 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 braincells．当蛋白质ASPM短,brains are smaller.

To return to the similarities of mice and men, researchers have shown that a gene calledEmx2, 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.

Scientists are studying whether a bigger brain can tip the scales of intelligence.

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Researchers have linked sudden and disproportionate brain growth during the first year of life toautism, suggesting that excessively rapid growth prevents the child from making the connections that guide normal behavior [source:英国广播公司]。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 asencephalization quotients．As brain-imaging 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 18-year-olds to a corresponding IQ score and found that males averaged 3.63 IQ points higher than the females [source:Jackson, Rushton]。然而,这项研究,使用约10000females 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 forgenius．

But should gals just throw up their arms, curse their parents and refuse to make sense ofnuclear 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.爱因斯坦, 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:英国广播公司]。

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.