Randomized control trials are considered the pinnacle for causal inference. In many cases, however, randomization of participants in social work research studies is not feasible or ethical. This paper introduces the co-twin control design study as an alternative quasi-experimental design to provide evidence of causal mechanisms when randomization is not possible. This method maximizes the genetic and environmental sameness between twins who are discordant on an "exposure" to provide strong counterfactuals as approximations of causal effects. We describe how the co-twin control design can be used to infer causality and in what type of situations the design might be useful for social work researchers. Finally, we give advantages and limitations to the design, list a set of Twin Registries with data available after application, and provide an example code for data analysis. [This is the online version of an article published in "Research on Social Work Practice."]