Family income trajectories and adverse outcomes in late adolescence: Evidence from the UK Millennium Cohort Study

BACKGROUND: Low family income and poverty are linked to adverse outcomes in childhood and adolescence. However, the role of early-life family income trajectories in shaping late-adolescence outcomes remains unclear. METHODS: Using longitudinal data from the UK Millennium Cohort Study and group-based trajectory modelling, we construct family income trajectories from infancy to early adolescence. We investigate associations of these trajectories with overweight/obesity, underachievement of GCSE educational qualifications, habitual smoking, socio-emotional behavioural problems, and cognitive impairment at age 17, using logistic and ordered logistic regressions, adjusting for confounders. An optimal trajectory is defined as one that does not significantly increase risk for any outcome and reduces risk for some. This optimal trajectory is used as the reference for estimating average marginal effects, ratios of average risk, and population attributable fractions. RESULTS: We identify four income trajectories: low (46.0%), lower-middle (34.2%), upper-middle (15.6%), and high-declining (4.1%), with the upper-middle trajectory being optimal. Compared with this trajectory, the low and lower-middle trajectories show higher risk of GCSE underachievement. The low trajectory additionally shows higher risk of smoking and socio-emotional behavioural problems. The high-declining trajectory, with declining income after the 2007-08 financial crisis, shows higher risk of smoking. DISCUSSION: Early-life income trajectories shape adolescent health and development. Most UK children experience non-optimal trajectories, which are linked to multiple adverse outcomes by age 17. Sustained policy efforts could help mitigate income-related inequalities in adolescent development across high-income countries.