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Sanjay Kinra, Commentary: Can conventional migration studies really identify critical age-period effects?, International Journal of Epidemiology, Volume 33, Issue 6, December 2004, Pages 1226–1227, https://doi.org/10.1093/ije/dyh340
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Schooling et al. examine the association between migration to a more advantaged environment and the subsequent risk of cardiovascular disease.1 Their data show that, compared with those who migrated later in life, those who migrated in the first two decades were at a greater risk of diabetes, hypertension, hyperlipidaemia, and ischaemic heart disease. From this the authors infer that there may be specific vulnerabilities to environmental change during childhood. They further suggest that the increased risk may be mediated through altered trajectories of childhood growth and puberty.
Their study makes an important contribution to the limited body of knowledge that exists on the determinants of the health effects of migration.2,3 Substantial economic migration (from mainland China to Hong Kong) over a short period of time, and the relative ethnic similarity between the migrant and non-migrant populations, offer a valuable opportunity to isolate the contribution of environmental exposures with minimal confounding by genetic differences. The study's findings are limited to some extent by poor characterization of the primary exposure, that is, migration. Migration status was assigned according to the place of birth and the age at migration estimated from current age and the length of stay in Hong Kong. Age at migration was used as a broad proxy measure for environmental exposures before and following migration. Such a binary classification of environment ignores the marked heterogeneity that exists among places at various stages of development.4 It also ignores the possibility of multiple migrations. However, the most uncertain aspect of this research—as indeed of many other such studies of migration—is the appropriate interpretation of the significance of age at migration.
