What makes an adult? An investigation into the psychology of modern adulthood

Adulthood is the longest phase of life, during which individuals often make their most substantial contributions to society. Traditional psychological models of lifespan development define adulthood by the attainment of socio-demographic milestones such as career, marriage, and parenthood. Yet, these milestones – and the models that rely on them – are often out of reach and even irrelevant for adults today. This thesis explores definitions of modern adulthood, proposes a new taxonomy for describing adulthood, and tests when and why people identify as adults, in three publications.
A meta-analysis (paper 1) summarised over 30 years of research using the markers of adulthood scale (k studies = 40, Ntotal = 17,465). Findings showed that the socio-demographic milestones of marriage, parenthood, and career were endorsed by meta-analytic proportions of 24%, 23%, and 58% respectively, calling for a re-definition of contemporary adulthood.
Using the theory construction model, I developed CARES (paper 2), a descriptive taxonomy consisting of five psychological qualities that describe adulthood: Cognitive maturity (confidence in knowledge); sense of Ageing (realisation that life is finite); self-Reliance (feeling in control of life); Eudaimonia (living in alignment with oneself); and Social convoy (network of social relationships). The CARES taxonomy has conceptual and methodological foundations in the literature, maps cognitive, affective, and behavioural development in adulthood, and forms the basis of adults’ psychological well-being.
A cross-sectional survey study (paper 3) of UK residents aged 18-77 (Mage = 39.20, N = 722) identified psychological characteristics to define adulthood, introduced and validated the psychometric CARES scale, and tested predictors of subjective adult status including age, attainment of socio-demographic milestones, and attitudes towards adulthood.
Overall, this thesis introduces a new perspective on adult psychological development, which will inspire future research on the psychology of adulthood and further our understanding of adults’ well-being and mental health.