The effect of migration on ages at vital events: A critique of family reconstitution in historical demography

Demographic rates of historical populations have usually been calculated using only data from stayers alone. Can they be extrapolated to the population as a whole? Ruggles has recently pointed out, using both logic and a computer simulation, that stayers experience vital events earlier in life than movers due to migration censorship: those who experience them later in life have often migrated away from the community being studied. We show that stayers do indeed marry and die at younger ages than do movers, using a genealogical database on the American North (1620–1880). These differences are caused, however, both by migration censorship and by genuine differences between the two groups and the places they lived. Therefore changes over time among stayers are not good indicators of changes in the population as a whole because they are affected by changing migration rates. Thus no simple “correction factor” can be extrapolated to estimate the general population; neither stayers (nor movers) constitute a “baseline” or “normal” process: both must be considered together in order to gain an accurate picture of the population as a whole.