[infecto]

While academically you are correct, I am not sure I would say it has never been a good idea. I have a US bias but most portfolios in the US have a significant if not total weighting on domestic markets. Even in my memory of the last 10 years there have been continuous questions on how much international exposure one should have. Most of the time you see something like a 5-10% exposure in international markets which in theory gives you the "best risk adjusted returns" but in reality since the US has been a continuous powerhouse, gives you a lower overall return.