Can historical demographics benefit from collaborative data from genealogy sites?

Summary Sites that offer their users to reconstruct their family tree online are flourishing on the Internet. This article analyzes the work of collection and data entry carried out by these users and how it could be used in historical demography, in order to complete the knowledge of past generations. To do so, the results obtained from the Geneanet database are compared with those known from the literature, and concern the records of 2,457,450 French individuals or individuals of French origin who lived in the 19th century. A significant bias in the sex ratio (under-representation of women) is thus highlighted. Fertility is also strongly underestimated. As for mortality (in comparison with historical values), these data underestimate male mortality up to about 40 years of age and female mortality up to 25 years of age, and then overestimate it. Finally, the wealth of spatial characteristics contained in the family trees is also exploited to produce new data on internal migration in the nineteenth century.