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However, separated twin studies can also be problematic. The environments of separated twins may not actually be that different from each other for the following reasons:

  • The twins shared a similar prenatal environment before they were born.
  • Adoption agencies may tend to place twins in similar households.
  • Since they are similar in appearance and in genetically inherited abilities, the twins may evoke similar responses from people around them.

As in other types of studies, trait similarities in separated twins may be due to both similar genes and similar environments.

Experiences and Behavior

Experiences affect behavior partly because environmental stimulation forms and maintains neural connections. For example, psychological research shows that babies need consistent, loving contact with a caregiver in order to achieve optimal brain development. Neglected babies, lacking attention and physical contact, experience unpleasant emotions that are not simply transitory. Their experiences determine the development of their neural connections. Similarly, soldiers on active combat duty can suffer mental damage from the continuously stressful environment, even if they never experience physical injury.

Adoption Studies

In adoption studies, researchers compare adopted children to their biological parents and to their adoptive parents. Adopted children share more genes with their biological parents. The children’s living environments, however, more closely resemble the environments of their adoptive parents. When adoptive children resemble their biological parents more than their adoptive parents with respect to a certain trait, researchers can hypothesize that the trait has a genetic basis.

Interaction of Genes and Environment

In conducting all these types of studies, researchers have found that while genes influence psychological traits, they don’t act alone. Highly influential environmental factors also play a major role. These factors include:

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