Growing Up in New Zealand is one of the world's most comprehensive longitudinal studies of child development, following a cohort of almost 7,000 children born in 2009 and 2010, alongside their parents and wider families, from before birth through childhood and into adolescence. Longitudinal studies — which follow the same individuals across time, collecting data at repeated intervals — are uniquely valuable for disentangling the complex causal relationships between early life experiences and later developmental outcomes, because they allow researchers to track how characteristics present at one point in time predict or influence characteristics at a later point, rather than inferring causation from cross-sectional snapshots that compare different individuals at the same time.
The cohort recruited into the study was designed to be representative of the diversity of New Zealand's population: approximately 44 percent of the children were from the least socioeconomically advantaged families, 24 percent were Maori, 6 percent were Pacific, 12 percent were of Asian ethnicity, and the remaining 58 percent were New Zealand European or other. This demographic breadth was deliberate: New Zealand's relatively small population and high degree of ethnic, socioeconomic, and geographical diversity make it an unusually informative setting for studying how social conditions shape development, as the range of experiences captured within a single national study is greater than in more homogeneous populations.
Among the most important findings of the study's early waves was the degree to which developmental differences between children were already measurable at the age of nine months, long before school or other formal developmental interventions could plausibly account for them. Children from low-income households, from families with low maternal education, and from some ethnic minority groups showed measurable differences in language development and cognitive assessment scores at nine months that predicted subsequent developmental trajectories. These early differences, the study's researchers emphasise, do not reflect inherent differences in children's capacities but are the product of differences in the environments — quality of early care, exposure to language, nutritional status, and stress — to which children have been exposed from conception onwards.
The study has generated findings on a wide range of topics including physical health, mental health, educational achievement, family structure, and the use of digital media. Among the more striking findings is the relationship between housing quality and child health: children living in cold, damp housing — a significant issue in New Zealand's older housing stock — showed higher rates of respiratory illness, sleep disruption, and missed school days, with consequences that compounded over time as educational gaps widened. The study found that the proportion of children living in inadequate housing was substantially higher among Maori, Pacific, and low-income families, contributing to an already substantial inequity in developmental outcomes that was measurable from the earliest data collection points.
The Growing Up in New Zealand study has had direct policy impact, informing the design of early childhood intervention programmes, housing quality standards, and income support policies by providing policymakers with population-level evidence of the developmental consequences of specific social conditions. The study's finding that developmental inequalities are already established in the first months of life has strengthened the case for investment in the antenatal period and the earliest years, rather than waiting for problems to manifest at school entry. Several government policies introduced during the study period — including the Family Incomes Package that increased financial support for low-income families with young children and the Warm Homes initiative that subsidised insulation and heating for households with young children — were influenced by the study's findings.
As the cohort has grown older, the study has extended its focus to adolescence — a period of heightened risk for the emergence of mental health conditions, risky behaviour, and disengagement from education. The study's data on the childhood precursors of adolescent mental health conditions has attracted particular attention, given the high rates of depression, anxiety, and self-harm among New Zealand's adolescent population. The longitudinal design allows researchers to identify which early life experiences and family circumstances are most strongly associated with later mental health difficulties, enabling earlier identification of children at risk and more targeted early intervention.