The heritability of the degree of residential area urbanization in twins and their siblings in the Dutch population was examined. The postal code was known for 6879 twins and 2724 siblings registered with the Netherlands Twin Register and born between 1940 and 1983. Using data from Statistics Netherlands (Centraal Bureau voor de Statistiek, 2001), these postal codes could be related to residential area characteristics, including urbanization level. The degree of urbanization was assessed on a 5-point scale: very heavy, heavy, moderate, low and not urbanized. Genetic model-fitting was carried out in three age cohorts: young adults (born 1975 to 1983), adults (born 1965 to 1974) and older adults (born 1940 to1964). Twin and sibling resemblance in urbanization level was expressed in polychoric correlations. These correlations decreased from the youngest cohort (.66 to .86) to the oldest cohort (.20 to .58). In all 3 age cohorts, genetic factors did not contribute to familial resemblance. The influence of common environment decreased in importance from the young cohort (70% to 83%) to the old cohort (46% to 47%) and was lower in women than in men in all but the oldest age cohort. This study did not replicate Australian findings of a genetic contribution in the older cohorts; common environmental factors and, increasingly with age, unique environmental factors determine where the Dutch live. Future studies in European and other populations will reveal whether these results are specific to the Dutch population.
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