Numerous large-scale studies have now shown that psychological distress rises steadily from early adulthood to middle age before declining and then levelling off in old age (e.g. Blanchflower & Oswald, Reference Blanchflower and Oswald2008; Stone et al. Reference Stone, Schwartz, Broderick and Deaton2010). Lang et al. (Reference Lang, Llewellyn, Hubbard, Langa and Melzer2011) examined data from over 100 000 people to provide the first substantive evidence that age-related changes in psychological distress differ as a function of income. Specifically, distress, psychiatric diagnoses and the use of psychiatric medicine were found to peak sharply in midlife primarily amongst those in the bottom 20% of the income distribution. These observations are consistent with findings showing that financial pressures and finance-related subjective stress peak in midlife (Almeida & Horn, Reference Almeida, Horn, Brim, Ryff and Kessler2004). The less well off could therefore be at particularly high risk of financial strain at this time. In addition, a lack of financial resources during middle age may generate a spiral of stress in other aspects of life (e.g. interpersonal tension, sleep and work difficulties).

If the findings of Lang et al. (Reference Lang, Llewellyn, Hubbard, Langa and Melzer2011) are shown to be robust (e.g. to period and cohort effects) they may shed new light on contradictions in existing conceptualizations of mental health inequalities over the lifespan. At present, two competing hypotheses propose that: (i) mental health inequalities increase continually over the lifespan (cumulative disadvantage hypothesis), and (ii) mental health inequalities converge with age potentially as a result of selective mortality amongst the disadvantaged (age-as-leveler hypothesis). Support has been gathered for both ideas (e.g. Dupre, Reference Dupre2007; Yang, Reference Yang2008).

Interestingly, Lang et al. (Reference Lang, Llewellyn, Hubbard, Langa and Melzer2011) find evidence that disparities in distress widen in the first half of life and are attenuated following middle age. It follows that those examining the trajectory of socio-economic differences in distress in samples of young adults may observe findings which are compatible with the cumulative disadvantage hypothesis, as illustrated in Fig. 1. Support for the age-as-leveler hypothesis could tend to occur in samples of older adults, as illustrated by the post-midlife section of Fig. 1. Those who test for linear (rather than a quadratic) interactions between age and socioeconomic status in predicting mental health amongst adults of all ages may observe parallel lines (as the inclines and declines in distress amongst the disadvantaged will cancel each other out) and no interaction effect.

Fig. 1. Curvilinear relationships between age and psychological distress for those with low (–––) and high (– – –) income.

Further research will help evaluate the merit of these predictions. However, it is clear that studies examining income-related differences in psychological distress do need to incorporate quadratic age effects. The consideration of such U-shapes may act as a catalyst for developing a deeper understanding of trajectories of mental health inequalities over the lifespan. A first step in this direction will be to establish if the U-shape in distress and well-being identified in numerous studies, rather than being normative, tends to occur centrally amongst an economically disadvantaged subset of the population.