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The counterfactual approach to causality has become the dominant approach to understand causality in contemporary social science research. Whilst most sociologists are aware that unobserved, confounding variables may bias estimates of causal effects, the issues of overcontrol and collider bias have received comparatively less attention. In particular, widely used practices in research on intergenerational mobility require conditioning on variables that are endogenous to the process of the intergenerational transmission of advantage. I review four of these practices from the viewpoint of the counterfactual approach to causality and show that overcontrol and collider biases arise when these practices are implemented. I use data from the German Socio-Economic Panel Study (SOEP) to demonstrate the practical consequences of these biases for conclusions about intergenerational mobility. Future research on intergenerational mobility should reflect more upon the possibilities of bias introduced by conditioning on variables.