Partnered individuals live longer, healthier lives. It has been hypothesized that both social causation (partnership benefits) and health selection may explain this association. Since much of this literature is focused in the U.S., comparative studies of the potential impact of policy on the causation and selection components of this association have been scant. Using comparable data from the U.S. Panel Study of Income Dynamics and the Canadian Survey of Labour and Income Dynamics, we test the selective and causal relationships evident during entrance into partnership. We use fixed change-point analysis with multilevel models (MLM) to fit trajectories of change in both Canada and the U.S. to understand the role of both health selection and partnership benefits. In Canada, partnership benefits were evident, while health selection was only marginally significant. In the US, health selection was prominent in both men and women, but partnership benefits were not significant. We argue that the differences in the extent of defamilialization of social policy between the two countries may impact the way and extent to which people choose partners and benefit from those partnerships.