To prepare the sample analysed here, we used the demographic data available for each individual to create a ‘geographic origin’ that represents a single location from which the individual’s very recent ancestry is derived. Where possible, we based the geographic origin on the observed country data for grandparents. We used a ‘strict consensus’ approach: if all observed grandparents originated from a single country, we used that country as the origin. If an individual’s observed grandparents originated from different countries, we excluded the individual. Where grandparental data were unavailable, we used the individual’s country of birth.
