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?
Today, nearly all 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.
First, 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.
Evidence of genetic influences:
- Twin studies suggest that identical twins IQ's are more similar than those of fraternal twins (Promin & 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).
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 (Reinberg, 2008).