Genetic and Environmental Influences on Intelligence

What role do genetic and environmental influences play in determining intelligence? This question has been one of the most controversial topics throughout the history of psychology and remains a hot topic of debate to this day.

In addition to disagreements about the basic nature of intelligence, psychologists have spent a great amount of time and energy debating the various influences on individual intelligence. The debate focuses on one of the major questions in psychology: Which is more important—nature or nurture?

Do Genetics or Intelligence Play a Bigger Role in Determining Intelligence?

Today, psychologists recognize that both genetics and the environment play a role in determining intelligence.

It now becomes a matter of determining exactly how much of an influence each factor has.

One important thing to note about the genetics of intelligence is that it is not controlled by a single "intelligence gene." Instead, it is the result of complex interactions between many genes (or polygenic). Next, it is important to note that genetics and the environment interact to determine exactly how inherited genes are expressed, a concept known as epigenetics.

For example, if a person has tall parents, it is likely that the individual will also grow to be tall. However, the exact height the person reaches can be influenced by environmental factors such as nutrition and disease.

A child may be born with genes for exceptional brightness, but if that child grows up in a deprived environment where they are malnourished and lack access to educational opportunities, they may not score well on measures of IQ.

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Evidence of Genetic Influences on Intelligence

All traits are heritable, with intelligence being one of the most researched. If genetics were the only factor to consider when predicting intelligence, we would expect identical twins to have identical IQ scores. The research shows:

• Studies show that IQ scores of identical twins are strongly correlated, and are more similar than those of fraternal twins. Identical twins' test scores also correlate highly when given assessments that measure reading, mathematics, and language skills.
• Siblings who were raised together in the same environment have more similar IQs than those of unrelated adopted children who were brought up in the same household.
  

In addition to inherited characteristics, other biological factors such as maternal age, prenatal exposure to harmful substances, and prenatal malnutrition may also influence intelligence.

Evidence of Environmental Influences on Intelligence

Historically, some theorists believed humans are born with a blank slate, and capable of becoming any type of person with any type of career, influence, and social status. The research shows:

• Identical twins who were raised separately have less similar IQs than those of identical twins who grew up in the same household.
• School attendance has an impact on IQ scores.
• Children who were breastfed for 12 months or longer had a higher IQ (about 3.7 points) at age 30.
• Vitamin and nutrient deficiencies are linked with lower IQ.

In one example, studies have found that first-born children tend to have higher IQs than later-born siblings. Why? Many experts believe that this is because first-born children receive more attention from their parents. Research also suggests that parents expect older children to perform better on a variety of tasks, whereas later-born siblings face less task-focused expectations.

Take Away

Today, scientists agree that both genetics and environment impact the physical development of the brain and the cognitive development of the mind which ultimately determines intelligence. Our genetics may provide us with an intellectual capacity but our environment determines if we reach our potential. That being the case, both would appear to be equally important.