Directed by Hill Goldsmith, Professor of Psychology and Investigator at the Waisman Center

Frequently Asked Questions

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Why are you studying twins?

Twin research participants help science identify the importance of genetic and environmental influences for brain and behavior development. Identical twins share 100% of their genes and fraternal twins share, on average, 50% of their genes. We collect many different kinds of measurement and compare large groups of identical twins and fraternal twins. The degree of similarity or difference between twin groups tells us something about genetic and environmental influences. For instance, genetically identical twin pairs are much more similar in height than fraternal twin pairs. Identical and fraternal twins tend to be equally similar or dissimilar in weight. Thus, there is more genetic influence on height.

My twin and I are nothing alike, but the doctor said we are identical. How can that be?

When someone says twins are identical, they mean “genetically identical.” Genetically identical means a single egg was fertilized to form one zygote which then divided into two embryos. Twins can share 100% of the same genes and have different likes and dislikes, different favorite colors, or behave in different ways. This is because not everything is due to genetic influence.  Twins might develop different preferences to help people distinguish one twin from another. The environment is very important in shaping  people’s likes, dislikes, talents, and behaviors.

How many twins have participated in your studies?

Nearly 12,000 twin pairs are included in our studies. Mothers, fathers, and non-twin siblings have also participated in our research.

Do your studies include fraternal twins?

Yes, we include all types of twins in our studies.  It is important for us to be able to compare both identical and fraternal twins to understand child development.

What have you learned so far?

Our research spans many different topics from infancy through early adulthood. See our publications and newsletters for content that interests you.

I’m a twin. Can I participate in your research?

Thank you for your interest! We are not collecting data at this time. Call or email us to update your contact information to learn about future studies 866-230-2560 (toll free), 608-265-2674 (local) or email us at

Do you share phone numbers with other organizations or companies?

No.  All contact information is strictly confidential.

We were in your studies several years ago and moved out of Wisconsin. Can we still participate?

Yes! Most parts of our research are completed by telephone, internet, or mail.