Can historical demography benefit from the collaborative data of genealogy websites?

A growing number of websites offer users the possibility of building family trees. This article analyses the data collection and entry work of these users and how their results may be used in historical demography to further knowledge on past generations. To that end, the results obtained on the Geneanet website are compared with those established in the literature, concerning the entries of 2,457,450 French or French-origin individuals who lived in the 19th century. The comparison shows a considerable bias in the sex ratio, with women underrepresented. Fertility is also substantially underestimated. Regarding mortality, the data (compared with historical values) underestimate the mortality of men up to the age of 40 and that of women up to the age of 25, after which age it overestimates both. Lastly, the wealth of spatial characteristics contained in the family trees is also used to produce new data on internal migration in the 19th century.