How do psychologists study the influences of heredity and environment on psychological traits?

Today, researchers generally agree that heredity and environment have an interactive influence on intelligence. ... Heredity places an upper and lower limit on the IQ that can be attained by a given person. The environment determines where within these limits the person's IQ will lie.  The two most basic influences on social behavior are genes (the chemical instructions that people inherit from their parents' DNA) and the environment (all other, non inherited factors). Contrary to a common misconception, genes do not cause behavioral or personality traits, they only influence them.  These studies suggest that genetic factors underlie about 50 percent of the difference in intelligence among individuals. ... It is likely that a large number of genes are involved, each of which makes only a small contribution to a person's intelligence. Intelligence is also strongly influenced by the environment.

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