Demographers recently looked at the Skellefteå region in northern Sweden (since the work of digitizing population data from the 19th century on had already been done) and had access to 5,850 people and their children, grandchildren and great-grandchildren, which numbered in the tens of thousands.
They found that many died without descendants but the key reason was not infant mortality, that was evident but not the largest factor. It was instead low fertility.
Yet today 25 percent don't have children even though low fertility is a solvable problem. The demographic is the same as in the past; the elites and the poorest have the fewest descendants. Today, educated women and poor men represent those two poles.