Scandinavia and the World

Comments #9839780:

Old habits die hard 16 8, 4:14am

@Yeehaw0 Not really. In eastern Europe or Turkey for example that was not an issue. People more or less figured out that when closely related people make a baby, it ends up a mess. They didn't knew the exact reason why like we do today, but they figured out how to at least avoid it.
Usually when they ran out people to marry, young people from close range villages would travel to find a spouse they are not related to, or even fully move out and form new villages. Also shifting populations due to better land offers(that's basically how the US was formed, and that is just one example), big merchant groups that would settle for long in a different country, nomadic groups running from war(this is why ex Yogoslavia is such a hot ethic mess of a map), rape during wars or kidnaping if you want to get into the dark side of it would all pretty much assure new blood often enough to avoid. Even the kings and nobleman would avoid to marry family because the "blue blood" idea was not as popular here.
Sure, there were still isolated people or ultra nationalistic ones that refuse the notion of marring anyone they would consider "outsiders". However they were not enough to be a huge incest problem. Not everyone was the damn Austrian aristocracy were their family tree was more of a stump.
I think it really depends of what corner of the world you are thinking of, hence why the down votes. The comment was a bit too general.