Human intelligence is not only difficult to measure; it's also difficult to define. Most researchers, these days, will tell you that our intelligence is a combination of what we know (our knowledge), our skills and our ability to understand and reason -- and that our cognitive abilities continue to grow throughout our lives, rather than being set at birth. The basis of intelligence is likely a combination of several factors. For example, are you biologically destined to be only as smart as your parents? Biology, though important, is only part of the intelligence package. Other factors, including everything from what we eat to where we live, can also affect our intelligence. But first, let's look at what's in our genes.
How much does our intelligence depend on our genes? For more than a century, researchers have been studying how much our genetic legacy influences our intelligence. After all that time, they've determined that our genes do influence intelligence and IQ. However, the percentage of that influence may range anywhere from 40 to 80 percent [source: Norrgard].
Additionally, our brain structure and functionality -- both biological factors -- contribute to our level of intelligence. Using brain imaging, neuroscientists have identified differences in brain structure, specifically differences in our parieto-frontal pathways, that seem to affect our intelligence positively (or negatively, depending on the brain). Well-functioning pathways correlate to better brain functioning, brain efficiency and information processing, which all point to better IQ scores.
4: Early Nutrition
As it turns out, you really are what you eat. And what your mom ate during her pregnancy. Prenatal and early nutrition are linked to brain structure, behavior and, yes, intelligence.
The greater nutrition in the foods we eat, especially for males in the weeks just after birth, the greater the size of the caudate -- that's the part of our brain that specializes in learning and memory -- and the greater our verbal IQ scores. And the effects also seem to apply to babies whose prenatal diets were rich in long-chain polyunsaturated fatty acids, such as DHA (docosahexaenoic acid). When pregnant and lactating women have diets rich in these fatty acids, their offspring are more likely to score higher on intelligence and achievement tests at ages 4 and 7.
How much nature versus nurture affects human intelligence is a long-studied and long-debated topic. The term "nature" refers to how genetics and heritability influence our intelligence, and "nurture" describes how certain environmental factors affect our intelligence. These factors include everything from our family's parenting style and home environment to how we're educated and the experiences we have throughout our lives.
Researchers often study twins who've been separated at birth to understand further the roles nature and nurture play in human intelligence. They theorize that if intelligence is purely biological, identical twins separated at birth should still have equal IQs. But that's not always the case, they find. While you may be genetically predisposed to an average intelligence level, a quality education and life experiences may enable you to turn an average IQ into a great one over a lifetime.
2: Birth Order
People have been studying whether or not birth order affects human intelligence for more than a century, yet we're still just not sure. For years, it was believed that firstborns and older children in families were more intelligent than their younger siblings. And there are more firstborn children who've become space-bound astronauts, U.S. presidents and Nobel Prize winners than kids with older siblings. Why? Families with just one child may have more time and financial resources to put toward educating that child. Recent studies, however, observe that it may not be intelligence -- our potential for learning, understanding and reasoning -- that's affected by birth order, but rather our IQ (intelligence quotient). IQ tests measure our intelligence aptitude and compare us to our peers. Firstborn children, on average, score three points higher on IQ tests than their closest, next-born siblings.
We may be genetically predisposed to a certain brain volume, structure and pathways -- a certain level of intelligence set by our biology -- but how much we achieve isn't based in biology alone. The type of life we lead also affects intelligence. Environmental factors, such as the diet we eat, the toxins we're exposed to both in the womb and as we age, and even the neighborhood we choose to live in -- be it dangerous or safe -- all influence how genes are expressed in our lives.
Let's use cigarette smoke as an example. Exposure to the toxins in cigarette smoke -- while in the womb and throughout a lifetime -- is known to lower our intelligence quotient, decreasing IQ scores by more than seven points when compared to individuals who aren't exposed to cigarette smoke [source: American Friends of Tel Aviv University]. Such environmental factors are thought to influence our intelligence levels throughout our lives.
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