. . . have a little knowledge of demographics and statistics.
I had not heard about the project and am not clear about the premise - the 200 miles triangle business.When I google about the Presidents, I get sites about the project of a 12-year-old.(I'm not disparaging a 12-year-old, or doubting her conclusions.I'm sure there is enough info on the web to trace the lines she did.)That most U.S. Presidents descend from Plantagenet kings is nothing new, and most of those who made unsuccessful bids for the office, too.Somewhere I read that, in most cases that have been documented, the candidate with the closer royal connection wins.(Close is relative.It is usually the case, for example, of a 23 great grandfather versus 22 great grandfather.)
So is the theory that "SOME or MOST" Americans have one or more lines tracing back to this Copenhagen/London/Paris population?Or that "SOME or MOST" Americans have ALL or most of their lines tracing back?
The first premise is not difficult at all to believe.On the second one, more specifics are needed.
It would not be unreasonable to expect that someone from 800 years ago, having an average number of offspring, would have millions or even hundreds of millions of descendants today.
As for Japan, I am not very familiar with historic migration patterns within Japan and marriage practices.My impression is that populations within Japan stayed relatively isolated from each other, until more recently.This is certainly true (still) for some of the more remote areas.So, no, just because the Japanese royal family goes back in time, there is no reason to conclude that they are related to a larger portion of the population.