Based on market capitalization I think an allocation should be 55% S&P 500, 30% Developed non-US markets, 15% emerging markets.

International Stocks are undervalued compared to the USA. They have under-performed the S&P for the last 10 years. This will not always be the case. I think international stocks beat the USA stock market over the next 10 years. This occurred in 2000-2010 and will repeat at some point.

I know some are getting worried about the S&P 500 being so high now, this is a way you can take some S&P 500 off the table and re-allocate to relatively undervalued international stocks.