Recent studies indicate that single mothers experience unusually high levels of psychological distress. The purpose of this paper is to compare rival explanations for these high levels. Four hypotheses are tested: (1) the psychological well-being of single mothers, relative to married parents, declines over time; (2) changes in psychological status are related to the incidence and prevalence of life events and chronic strain; (3) single mothers respond more negatively to life events and strain than married parents; and (4) greater exposure to stressful events and chronic strain can account for the relative decline in psychological well-being among single mothers over time. Longitudinal data taken from the Michigan Panel Study of Income Dynamics are used to test each of these hypotheses. The results indicate that the high level of distress among single mothers is due primarily to the ongoing strain associated with low income. This conclusion suggests that policies aimed at raising the income of single mothers would not only improve conditions for children in single parent households but would also enhance the mental health of single mothers. (RDN)