At what point does my family tree stop doubling and start decreasing?

Your family tree will not double forever; it will eventually start to decrease due to the limitations of the available population. As you trace your ancestry back through generations, the number of potential ancestors grows exponentially. However, this doubling process comes to an end around the time when the population of your ancestors reaches a certain size.

Before railways, most communities were small and isolated, making it difficult for people to find marriage partners outside of their immediate social circle. This led to a higher likelihood of intermarriage within the same community. As a result, the number of unique ancestors in your family tree stabilises once the population size of your ancestors reaches a certain point.

For example, in the 1700s, most ancestors likely came from a village of a few hundred people. Before that, nearly all ancestors would have been related due to inbreeding. In extreme cases, such as the Scottish island of St Kilda, the population remained small for centuries, limiting the potential for unrelated partners.

The availability of unrelated partners is more important than the overall population size when it comes to the size of your family tree. Factors like geography and social practices can influence which segment of the population is available for marriage. The Ptolemaic dynasty of ancient Egypt serves as an example, where the royal family practiced intermarriage for centuries to preserve their bloodline. Despite marrying close relatives, they still had a relatively small number of direct forebears in their family tree.

In summary, your family tree does not continue to double in size indefinitely. Instead, it reaches a limit determined by the size of the earlier population and the availability of unrelated partners. Once you encounter the same individual as a common ancestor, your family tree will stabilise and stop growing exponentially.