Selective migration as a bias in modernization studies: Premigration differences in morphology and blood pressure...

Premigration differences in anthorpometry and blood pressure are examined for evidence of selective migration for biological characteristics. We conducted a 5-years follows-up of the migration status of 99 American Samoans 15-30 years old who had been previously examined in 1981 and found that 33 had migrated. Analysis of covariance of the baseline anthorpometry and blood pressures of the nonmigrants (n = 58) and those who subsequently migrated (n = 29) indicates that selective migration does occur among young American Samoan adults. Migrants of both sexes tended to be younger and leaner and had significantly lower mean blood pressures than nonmigrants even before migration. Selective migration of lean individuals with lower blood pressures may contribute to the lower blood pressures of migrants from American Samoa living in Hawaii or to the heterogeneity in other Samoan populations. These results are one of the few examples of biological selectivity demonstrated to date and illustrate the dangers of assuming that the effects of selective migration are negligible in studies of migration and health.