Are we born with the intelligence that we end up with or is intelligence a matter of learning through life? In short, is intelligence a product of nature or nurture?
The Origins of Intelligence
What accounts for the differences in people’s intelligence? It could be biological in the nature of the genetics a person has inherited, or it could be the environment they have around them or even the culture that they live in. Hereditability studies have been conducted to find out the answer to the differences in intelligence. These studies tend to be conducted on twins because monozygotic (identical) twins share 100% of their genes. They are, if you like, nature’s clones.
The studies show that the type of intelligence that produces high scores on IQ (intelligence quotient) tests is highly heritable. In twin studies, the scores of identical twins are always more highly correlated than those of fraternal twins. In fact, scores of identical twins that have been reared apart are more highly correlated than those of fraternal twins that have always been together.
Intelligence then can be considered highly heritable because the higher the parental scores, the higher their offspring scores tend to be. However, sibling’s intelligence can vary widely. This shows that a highly heritable trait can be modified by environmental factors. Good diet and good pre-natal care are beneficial to a baby’s intelligence and conversely, a mother who smokes and drinks or takes drugs during pregnancy may harm the baby’s intelligence. Other factors that have been shown to reduce the child’s intelligence include environmental pollutants and even malnutrition, if the mother doesn’t eat properly all play a part during the nine months of pregnancy. Poor pre-natal care has been shown to hinder IQ in the baby. Each risk factor can reduce a child’s IQ by 4 points and if two or three risk factors occur together then the child’s IQ can be lowered by 20 points. Atmospheric pollutants can produce attention problems, lower IQ scores and poor school achievement.
It is often difficult to separate the relative influences of heredity and environment on human characteristics. People who have similar genetic makeup (e.g., brothers and sisters, parents and their children) typically live in similar environments as well. So when we see similarities in IQ among members of the same family, it is hard to know whether those similarities are due to the genes or to the environments that family members share. Nevertheless, a significant body of research tells us that both heredity and environment affect intelligence.
Twin studies
Many studies have used monozygotic (identical) twins and dizygotic (fraternal) twins to find out how strongly heredity affects IQ. Because monozygotic twins begin as a single fertilized egg which then separates, they are genetically equivalent human beings. In contrast, dizygotic twins are conceived as two separate fertilized eggs. They share about 50 percent of their genetic makeup, with the other 50 percent being unique to each twin. If identical twins have more similar IQ scores than fraternal twins, we can reasonably conclude that heredity influences intelligence.
Most twins are raised together by the same parent(s) and in the same home, and so they share similar environments as well as similar genes. Yet even when twins are raised separately (perhaps because they have been adopted and raised by different parents), they typically have similar IQ scores (Bouchard & McGue, 1981; N. Brody, 1992; Mackintosh, 1998; Plomin & Petrill, 1997). In a review of many twin studies, Bouchard and McGue (1981) found these average (median) correlations:
Correlations of Twins’ IQs:
Identical twins raised in the same home .86
Identical twins raised in different homes .72
Fraternal twins raised in the same home .60
