Identical Twins Hint at How Environments Change Gene Expression

Monica and Erika Hoffman stand barefoot, side by side near a sign that reads “Twin Studies Center” at California State University at Fullerton. Their glasses removed, both have auburn eyes, softly jutted chins, light freckles, and perky noses. Both wear black shirts and small sparkly earrings (Erika’s are flowers, Monica’s, bows). The identical twin sisters turned 39 the day before this lab visit.

“You are the 101st twin pair we’ve had in this study,” Nancy Segal, a shrewd, spirited professor in a sequined black hoodie, tells them, as a cluster of graduate students shadow her through the halls. Segal, a fraternal twin herself, is a walking Wikipedia of twin science. She specializes in evolutionary psychology and behavioral genetics, and has studied thousands of twins and their families around the world. Nearly three decades ago, Segal founded the Twin Studies Center to learn what twins like the Hoffman sisters—far from the same, despite appearances—have to teach us about the complex interplay of forces that impact our health and shape who we are.

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