This commentary reviews the recent history of explanations to crossover paradoxes such as the birth weight and gestational age paradoxes, with a special emphasis on the current proposal by Kramer et al. in this issue of the Journal (Am J Epidemiol. 2014;179(3):361-367). We contend that the causal structure of these paradoxes is essentially identical to that of several well-known selection biases. We then consider the pros and cons of using composite outcomes to circumvent these selection biases.
Keywords: causal diagrams; composite outcomes; paradox; selection bias.