This design involves twins discordant for specific environmental factors or exposures, and twins discordant for disease outcomes or measures of morbidity.
Select twin pairs who differ (the most) in exposure
Carlin, J. B., Gurrin, L. C., Sterne, J. A. C., Morley, R. & Dwyer, T. (2005). Regression Models For Twin Studies: A Critical Review. International Journal of Epidemiology, 34, 1089-1099
Hopper, J. L., & Seeman. (1994). The Bone Density Of Female Twins Discordant For Tobacco Use. The New England Journal of Medicine, 330(6), 387-92.
Goldberg, J., & Fischer, M. (2005). Co-twin Control Methods. Encyclopedia of Statistics in Behavioral Science.
Gurrin, L. C., Carlin, J. B., Sterne, J. A. C., Dite, G. S. and Hopper, J. L. (2006). Using Bivariate Models to Understand between- and within-Cluster Regression Coefficients, with Application to Twin Data. Biometrics, 62, 745–751.
Scurrah, K. J., Kavanagh, A. M., Bentley, R. J., Thornton, L. E. and Harrap, S.B. (2015). Socioeconomic position in young adulthood is associated with BMI in Australian families. Journal of Public Health, 38(2), e39-e46..