The Globe and Mail reports in its Saturday edition that over the past decade, the U.S. has thumped its international rivals, producing returns that have been more than twice as high as the payoff from stocks in the rest of the world. The Globe's Ian McGugan writes that in the question now is whether Wall Street can maintain the excitement. By global standards, U.S. stocks are now priced at nosebleed heights in relation to their economic fundamentals. The skyhigh expectations for artificial intelligence provide a reason to be cautious about what lies ahead for Wall Street. A trio of AI-related stocks -- Nvidia, Microsoft and Apple -- now make up more than 20 per cent of the S&P 500 Index. All three companies have surged this year because of expectations they will benefit from a mass turn toward AI. The problem is that no one is quite sure how AI will change things -- or, for that matter, if it will change things. Jim Covello, head of global equity research at Goldman Sachs, is among the doubters. He argues that AI is expensive and not as useful in its current state as people think. It still lacks a simple, obvious application that saves time and money for anyone who uses it. AI needs a powerful case for its existence.