Dimensional Funds produced a series of videos answering common client questions and concerns for use by advisors like us. This video is entitled: Why Should We Invest Internationally?
With the recent superior returns in US stocks, I think the most significant quote from the video was:
From 1973-2015, the S&P 500 performed better than a globally diversified index portfolio on an annual basis 17 times.
It performed worse 26 times.
I recommend watching the video to understand better why the diversification of including foreign and emerging market stocks is used in portfolio construction.