This paper compares two different approaches empirically to control for unobserved characteristics when estimating the effect of marriage on male and female earnings: the longitudinal and the twins approach. The estimates were obtained by exploiting the longitudinal dimension of a large sample of Swedish twins, so that longitudinal and twin-based estimates could be obtained in the same sample. The two approaches lead to different conclusions both regarding the role of unobserved characteristics in the cross-sectional earnings–marriage relationship and the effect of marriage on earnings. The paper investigates three potential explanations of this difference.