Chapter10 - Opposite-sex twins in medical research

Abstract

Studying sex or gender differences for any human trait or condition by comparing males and females from opposite-sex twin pairs is a powerful way to protect from bias arising from uncontrolled familial factors (both genetic and nongenetic) while controlling for age and year of birth. In this chapter, we briefly introduce this study design for medical research and provide examples of its application in studies of physical and mental health. We also explore evidence suggesting that sharing the mother’s womb with an opposite-sex cotwin might produce sex differences for certain health outcomes—with implications for the interpretation of opposite-sex twin studies. Finally, we argue that the investigation of sex or gender differences of opposite-sex twin pairs offers the opportunity for more precise insights into the role of such differences in determining health and disease, both at the individual and population level.