This study provides the first rigorous estimate of the costs to U.S. taxpayers of high rates of divorce and unmarried childbearing both at the national and state levels. Among its findings: Even programs that result in very small decreases in divorce and unwed childbearing could yield big savings for taxpayers. The report is organized as follows: Section I explains why policymakers may have an interest in supporting marriage. Sections II and III explain the methods used to estimate the taxpayer cost of family fragmentation by using evidence about the relationship between family breakdown and poverty. Section IV reveals the national estimate of the taxpayer cost. Estimated costs for individual states are found in appendix B.(Author abstract modified)