Relative poverty still matters.

Although great reductions of childhood pov erty have been achieved across the world, it remains a substantial cause of poor health and health inequalities. The commitment expressed in the fi rst Sustainable Development Goal “to end poverty in all its forms everywhere” is therefore crucial to ensure that the next generation has the best chance of leading healthy lives. However, what does an end to poverty mean? Sophie Wickham and colleagues shine much needed light on this longstanding and politically heated debate. A common distinction is made between absolute and relative poverty. Absolute poverty involves establishment of an absolute minimum threshold of income that is necessary for survival. By contrast, relative poverty is usually defi ned as having 60% of the median income within a country. Wickham and colleagues investigated the eff ect of movement into relative poverty on childhood socioemotional behavioural problems (measured with the Strengths and Diffi culties Questionnaire) and maternal mental health. Drawing on the UK’s Millennium Cohort Study, the authors identify families who were not in poverty and did not have poor maternal mental health at baseline. They then compared outcomes for families who moved into poverty for the fi rst time with those who did not, fi nding marked increases in the risk of adverse childhood outcomes and maternal psychological distress. Methodologically, this study marks an important step forward in the understanding of a fundamentally important determinant of health. By contrast with previous research, the authors have explicitly sought to understand the causal relationship between movement into poverty and health. Restriction of the analytical sample to those who are not in poverty and do not have poor mental health at baseline limited the potential for reverse causation in their analysis. The focus on transitions into poverty, rather than comparison of exposure measurements taken at a single timepoint as occurs in most epidemiological studies, is a major strength. The authors attempt to explore the role of maternal mental health in mediating the mechanism between poverty and adverse childhood socioemotional development. Here, the study is arguably at its weakest, with, at most, modest attenuation observed and potential residual confounding could not be addressed through the study design. Furthermore, implications of this part of the analysis are unclear. Interventions that target maternal mental health are unlikely to be adequate to mitigate the adverse eff ects of poverty on children. What next for research into the health eff ects of entry into poverty? Ecological evidence suggests that trends in relative poverty are important for mortality, but many large gaps remain in the evidence base. Although this study has identifi ed some of the short-term eff ects of poverty, eff ects are likely to continue throughout the rest of the life course. The focus on relative poverty by the authors, as assessed in relation to median income, is timely. Rather than defi nition of poverty on the basis of income alone, the UK Government suggested a new defi nition involving measurement of other aspects that relate to a child’s future life opportunities, namely on the basis of worklessness, unmanageable debt, poor housing, parental skill level, access to quality education, parental stability, and parental health. The subsequent legislation included proposals to stop measuring income, therefore ending the longstanding assessment of incomebased relative poverty. Although evidence suggests that addressing some of these other aspects of social disadvantage such as worklessness could help to reduce health inequalities, this latest study by Wickham and colleagues reiterates the need to continue assessment of material circumstances and, in particular, income. Socioeconomic measures are not interchangeable. Choices about what is measured can far too easily result in the blame for poverty moving from the most powerful in society to those being adversely aff ected: measurement choices can stigmatise and provide reasons for inaction. Although the legislation to drop measurement of relative poverty was ultimately defeated within the UK after opposition from the House of Lords, defi nitions used to monitor poverty are subject to continued debated within the UK and elsewhere. This latest study has important policy implications. Given its unique focus on transitions into poverty, this research provides an indication of the scale of benefi ts to public health that could be realised by prevention of children from moving into poverty. Much existing policy and research has focused on how best Lancet Public Health 2017