First, causality is difficult to establish in the absence of true controlled experimentation. In other words, since we cannot assign people randomly to married and unmarried groups at the outset, any difference between the groups in outcome may be the result of selection, rather than treatment, effects. For example: If married women are more likely to be dissatisfied, it may be because the marriage made them so (treatment effect) or because dissatisfaction-prone women are more likely to choose marriage (selection effect).