IQ: Environmental and Genetic Influences

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  • 0:05 Nature vs. Nurture
  • 1:03 Genetic Influences
  • 2:44 Environmental Influences
  • 4:38 Lesson Summary
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Lesson Transcript
Instructor: Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

Do genetics play a role in intelligence? What about the environment a person grows up in? Explore some of the evidence that exists for each of these factors, as well as how researchers attempt to answer these questions in the following lesson.

Nature vs. Nurture

There are two major areas of disagreement in the study of intelligence. One area involves a debate about the basic nature of intelligence. What is the other?

The second debate in intelligence research focuses on one of the most prominent issues in every field of the social sciences. I am referring to the age old question of whether nature or nurture is the most important. So what plays a bigger role in intelligence? Are genetics the most important aspect or is it environment?

Most researchers recognize that both nature and nurture influence intelligence. Both genetics and the environment interact with one another in this process. However, it is less clear how much influence each one has. It can be difficult to separate the influences of heredity and environment, but there is a significant amount of information that can help us. Let's look at the evidence supporting each side of this debate.

Genetic Influences

Some of the most convincing evidence for a genetic component to intelligence comes from twin studies. There have been many studies that compare the IQ of identical twins and the IQ of fraternal twins. Identical twins share 100% of the same DNA because they come from the same fertilized egg. Fraternal twins come from two separate fertilized eggs and they share only 50% of the same DNA. Since both identical twins and fraternal twins share the same prenatal and home environments, any similarities or differences in the IQ of identical twins compared to fraternal twins can reasonably be attributed to DNA.

It was found that identical twins have IQ scores that are more similar than the IQ scores of fraternal twins. Even identical twins who were raised in separate households were found to have IQ's that were more similar than that of fraternal twins raised together. This means we can assume that genetic influences account for the similar IQ scores of identical twins.

Another way researchers have gathered evidence for the influence of heredity on IQ is through adoption studies. Adopted children share similar genetics to their biological parents, while their environment is more closely matched to that of their adoptive parents. The IQ scores of these adopted children have been found to be more closely related to their biological parents. This provides evidence for a genetic component to intelligence as well.

It is important to note that twin studies and adoption studies cannot completely separate genetic and environmental influences. These studies do, however, provide support for the idea that genetics play a role in the development of intelligence.

Environmental Influences

So, there's evidence that genetics plays a role in IQ, but what about environment? There's evidence from a number of different sources that indicates it has a significant role as well.

Remember the twin studies that we discussed earlier? Even though the IQ scores of all identical twins were more similar than those of fraternal twins, there was a difference in this similarity between identical twins who were raised together and those who were raised apart. Identical twins who were raised apart had IQ scores that were less similar than the IQ scores of identical twins who were raised together. The only reasonable way to account for this difference is environmental influence.

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