Isonymy, inbreeding, and demographic variation in historical massachusetts

AbstractRandom isonymy and inbreeding in a population may be affected by demographic variation, particularly population size and migration. Total levels of isonymy and inbreeding are also affected by cultural behaviors, which may themselves be affected by demographic factors. This paper investigates the relationship of demographic variation and inbreeding components (total, random, nonrandom) in five towns in north‐central historical Massachusetts, 1741–1849. Estimates of exogamy rates (the proportion of individuals who are migrants) were derived from 6,038 marriage records, and population sizes were obtained from census data. Inbreeding components were computed using the surnames of 5,791 marriages. A total of 72 marriages are isonymous, giving an estimated total inbreeding coefficient of 0.0031. Demographic and inbreeding measures were computed for 33 samples defined on the basis of town and marriage cohort. The interrelationship between demographic variation and inbreeding was tested using rank order correlation over all 33 samples. Total inbreeding within samples shows no significant correlation with any of the demographic variables. Random inbreeding within samples shows a significant negative correlation with population size, and nonrandom inbreeding shows a positive correlation with population size. Various explanations for these correlations are offered. The opposite influences on random and nonrandom inbreeding cancel one another. These results demonstrate the complex interaction between cultural factors, demographic variation, and genetic structure in human populations.