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Isolation of human populations, leading to genetic divergence, is well documented in island settlements and religious isolates. Demonstration or measurement of isolation is more difficult in mainstream populations. An attempt has been made to assess such isolation by simple methods of aggregate analysis from readily available data. The limitations of the data and methods of analysis are fully appreciated, but it seems that differences between areas are detectable. Hence the methods, simple as they are, warrant further application to other communities. The analyses, both of surnames and of marriage movement data, indicate limited gene flow in Northumberland in the 18th century. Such gene flow as did occur came mainly from neighbouring communities, and there was very little long-distance inflow. There was thus a high degree of population constancy and localization in Northumberland in the 18th century. This could well be a contributory factor promoting the known genetic variability of the area. [ABSTRACT FROM PUBLISHER]