Twins Hold Special Role in Medical Science

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Twins hold a special place in medical science by uniquely providing insight into how behavior and genetics affect disease, aging, and response to medical treatments. Through twin studies, scientists understand the interplay between genetics and the environment in ways that the study of unrelated people–or even other types of siblings–is unable to do.

Because identical twins share 100 percent of their genes and fraternal twins share 50 percent, comparison of a twin pair similarity (for measurable traits) can explain the relative role of genes, or the environment, in the causation of that trait.

For example, if identical twins are more similar than fraternal twins for a particular characteristic, then genetic factors may play a more important role in causing that trait. But if fraternal twins are similar to identical twins for a trait, then genetic and environmental factors may be equally important in the trait’s causation, called “etiology.”

This is critical information as we seek to discover, treat, and prevent diseases such as cancer, heart disease, and hypertension. Twin studies also provide important insights for studies about brain aging and addiction.

For the last 17 years, SRI International has managed a registry of twins—both fraternal and identical—who volunteer their time so that researchers can advance understanding of disease, addiction, environmental exposure, and responses to medication. We’re privileged to now have more than 3,000 adult and child twins in SRI’s Twin Research Registry.

Each twin in our registry first completes a form acknowledging their interest in participating in a research study. A registry recruiter will then contact the twin to get basic information, and share details about the study’s requirements. To participate in a research study, both twin pairs must agree. Once the recruiter  receives agreement from both twins, important research can begin.

We’re enormously grateful for our twins, 1,375 of whom have participated in funded studies that have led to important scientific findings and published papers. Let’s take a look at some recent research results:

  • Nicotine addiction: A study of the pharmacokinetics (how the body processes a drug) of nicotine in twins resulted in new biometric estimates of genetic and environmental contributions to nicotine metabolism—a trait related to one’s risk for nicotine addiction. SRI has an ongoing related project about medications commonly used to treat nicotine dependence.
  • Pain management: The latest in a group of SRI studies on drug effects has determined the relative proportion of genetic and environmental factors to pain tolerance. The experiment measured response to heat and cold stimuli to the hand. Analyses of the data also looked at painkillers and drug side effects. Our researchers used bioinformatics to identify genes that may be associated with pain in some medical conditions. DNA from twin volunteers confirmed the connection between pain and those particular genes.
  • Immunology: Studies of the human immune system are increasingly turning to immunological markers for information. We believe that twin studies offer a cost-effective way to determine the proportion of genetic and environmental influences on immunity. For example, we are conducting a series of studies to examine responses to vaccination against seasonal flu viruses.
  • Shingles virus: The twin study design is also being used in a series of studies that will examine variability in responses to the FDA-approved vaccine for Varicella zoster (shingles), a virus that can affect older adults who have previously had chicken pox. A goal for the studies is to increase understanding of how immunity changes with age.

We look forward to sharing more updates about SRI’s twin studies. If you are a twin or a parent of twins, please visit to learn about SRI’s Twin Research Registry community and how your study participation can help researchers gain a better understanding of human health and behavior.

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