What Factors Determine Intelligence?

Factors that influence intelligence
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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?

Genetics and Intelligence: Which Plays a Greater Role In Determining Intelligence?

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

It now becomes matter of determining exactly how much of an influence each factor has. Twin studies suggest that between 40 and 80 percent of variance in IQ is linked to genetics, suggesting that genetics may play a larger role than environmental factors in determining individual IQ.

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.

Next, it is important to note that genetics and the environment interact to determine exactly how inherited genes are expressed.

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 brightness, but if that child grows up in a deprived environment where he is malnourished and lacks access to educational opportunities, he may not score well on measures of IQ.

Evidence of Genetic Influences

  • Twin studies suggest that identical twins IQ's are more similar than those of fraternal twins (Plomin & Spinath, 2004).
  • Siblings reared together in the same home have IQ's that are more similar than those of adopted children raised together in the same environment (McGue & others, 1993).

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

  • Identical twins reared apart have IQ's that are less similar than identical twins reared in the same environment (McGue & others, 1993).
  • School attendance has an impact on IQ scores (Ceci, 2001).
  • Children who are breastfed during the first three to five months of life score higher on IQ tests at age 6 than same-age children who were not breastfed (Kramer & others, 2008).

So what are some of the environmental influences that can account for variances in intelligence? Factors such as family, education, enriched social environments, and peer groups have all been linked to differences in IQ. For 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 parents. Research also suggests that parents expect older children to perform better on a variety of tasks, whereas later-born siblings face lesser task-focused expectations.


Ceci, S. (2001). Intelligence: The Surprising Truth. Psychology Today, 34(4), 46.

Kramer, M.S., About, F., Mironeva, E., Vanilovich, I., Platt, R.W., Matush, L., ... Shapiro, S. (2008). Archives of General Psychiatry, 65(5), 578-584. doi: 10.1001/archpsyc.65.5.578.

McGrue, M., Bouchard, T.J., Iacono, W.G., & Lykken, D.T. (1993). Behavioral Genetics of Cognitive Ability: A Life-Span Perspective. In R. Plomin & G.E. McClearn (Eds.), Nature, Nurture, and Psychology. Washington, DC: American Psychological Association.

Plomin, R., & Spinath, F. M. (2004). Intelligence: Genetics, Genes, and Genomics. Journal of Personality and Social Psychology, 86(1), 112–129.