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 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 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.