Is investing 100% in an S&P 500 ETF smart?

 Q: My friend recently said that I should dump all my Canadian and international ETFs, and invest instead in an S&P 500 ETF for my Couch Potato portfolio. I was worried that holding only U.S. stocks wasn’t diversified enough, but his answer is that the S&P 500 is geographically diversified because it’s full of multinational corporations. Since the S&P 500 performs so much better in the long run than Canadian or international markets, I’m tempted to follow