Abstract: Twelve studies involving 27,482 participants met the inclusion criteria. Interventions were either mandatory or voluntary and included up to 10 discrete components in varying combinations. All but one study took place in North America. Although we searched for parental health outcomes, the vast majority of the sample in all included studies were female. Therefore, we describe adult health outcomes as 'maternal' throughout the results section. We downgraded the quality of all evidence at least one level because outcome assessors were not blinded. Follow-up ranged from 18 months to six years. The effects of welfare-to-work interventions on health were generally positive but of a magnitude unlikely to have any tangible effects. At T1 there was moderate-quality evidence of a very small negative impact on maternal mental health (standardised mean difference (SMD) 0.07, 95% Confidence Interval (CI) 0.00 to 0.14; N = 3352; studies = 2)); at T2, moderate-quality evidence of no effect (SMD 0.00, 95% CI 0.05 to 0.05; N = 7091; studies = 3); and at T3, low-quality evidence of a very small positive effect (SMD −0.07, 95% CI −0.15 to 0.00; N = 8873; studies = 4). There was evidence of very small positive effects on maternal physical health at T1 (risk ratio (RR) 0.85, 95% CI 0.54 to 1.36; N = 311; 1 study, low quality) and T2 (RR 1.06, 95% CI 0.95 to 1.18; N = 2551; 2 studies, moderate quality), and of a very small negative effect at T3 (RR 0.97, 95% CI 0.91 to 1.04; N = 1854; 1 study, low quality). At T1, there was moderate-quality evidence of a very small negative impact on child mental health (SMD 0.01, 95% CI −0.06 to 0.09; N = 2762; studies = 1); at T2, of a very small positive effect (SMD −0.04, 95% CI −0.08 to 0.01; N = 7560; studies = 5), and at T3, there was low-quality evidence of a very small positive effect (SMD −0.05, 95% CI −0.16 to 0.05; N = 3643; studies = 3). Moderate-quality evidence for effects on child physical health showed a very small negative effect at T1 (SMD −0.05, 95% CI −0.12 to 0.03; N = 2762; studies = 1), a very small positive effect at T2 (SMD 0.07, 95% CI 0.01 to 0.12; N = 7195; studies = 3), and a very small positive effect at T3 (SMD 0.01, 95% CI −0.04 to 0.06; N = 8083; studies = 5). There was some evidence of larger negative effects on health, but this was of low or very low quality. There were small positive effects on employment and income at 18 to 48 months (moderate-quality evidence), but these were largely absent at 49 to 72 months (very low to moderate-quality evidence), often due to control group members moving into work independently. Since the majority of the studies were conducted in North America before the year 2000, generalisabilty may be limited. However, all study sites were similar in that they were high-income countries with developed social welfare systems. The effects of WtW on health are largely of a magnitude that is unlikely to have tangible impacts. Since income and employment are hypothesised to mediate effects on health, it is possible that these negligible health impacts result from the small effects on economic outcomes. Even where employment and income were higher for the lone parents in WtW, poverty was still high for the majority of the lone parents in many of the studies. Perhaps because of this, depression also remained very high for lone parents whether they were in WtW or not. There is a lack of robust evidence on the health effects of WtW for lone parents outside North America.

Plain-language summary: We found 12 studies involving 27,482 participants that compared groups of lone parents in WtW interventions with lone parents who continued to receive welfare benefits in the normal way. All of the studies were at high risk of bias because the staff who collected the data knew when respondents were in the intervention group. In some studies, lone parents who were not in the intervention group were affected by similar changes to welfare policy that applied to all lone parents. We used statistical techniques to combine the results of different studies.These analyses suggest that WtW does not have important effects on health. Employment and income were slightly higher 18 to 48 months after the start of the intervention, but there was little difference 49 to 72 months after the studies began. In a number of studies, lone parents who were not in WtW interventions found jobs by themselves over time. It is possible that effects on health were small because there was not much change in employment or income. Even when employment and income were higher for the lone parents in WtW, most participants continued to be poor. Perhaps because of this, depression also remained very high for lone parents whether they were in WtW or not. All but one of the studies took place in the United States or Canada before the year 2000. This means it is difficult to be sure whether WtW would have the same effects in different countries at other times.