Reading about Japanese history, this passage caught my attention:
It is notable that in almost every case in which the shogun really counted he proved to have come to the top through irregular, “outside” channels through adoption and not through birth, youth, and adolescence in the Great Interior (Ōoku) of the Edo Castle. To a remarkable degree this was true in daimyo houses as well; innovative and strong individuals were usually adopted into the main line. Not only that: the shoguns who made a difference frequently had mothers whose plebeian origins refreshed the Tokugawa blood line.
Marius B. Jansen. The Making of Modern Japan, Part 2, Chapter 3
An observation that, I suspect, is very generally applicable.