Changes in family structures, such as the composition of households with respect to size, age and gender, can have an impact on poverty rates and the income distribution more generally. We analyze the impact of changing family structures on the income distribution among adult Costa Rican women between 1993 and 2009, using decomposition methods. There was a general increase in the share of family structures associated with lower incomes (singles with dependents) until 2001. After 2001, this trend reversed for women at the upper end of the income distribution, while it continued for women at the lower end. Correspondingly, we find a general negative effect of changing family structures on incomes of adult women until 2001, and an inequality-increasing effect after 2001. The change in trends might be due to a law coming into force in 2001 and which mandated DNA tests for presumptive fathers unwilling to recognize their children.