2012 was a year in which US markets outperformed the rest of the world over the first 7-8 months, but gave it all back over the final 4 months. The trend can be seen clearly in the chart below showing US stock market cap as a percentage of world market cap (using Bloomberg data).
The Fiscal Cliff drama really hurt US stocks relative to the rest of the world. As most global markets were hitting new 52-week highs in November and December, the S&P 500 was struggling to remain above its 50-day moving average. This year has been a different story, however, and the US has begun to chip away at the losses in share that it experienced in the fourth quarter. In the chart below, the recent low made in US market cap share came at the very end of 2012, and it has been straight up since the start of the year.
At 32.7%, the US still makes up nearly a third of the world's stock market capitalization, but it needs to gain another 1.7 percentage points to get back to its high of 34.4% reached last July.