The rising cost of U.S. health care has reduced the share of compensation that is taxable by Social Security. Between 1960 and 2010, non-taxable employer premiums for worker health plans increased from 1 percent of employee compensation to 7 percent. We use international data to examine the determinants of trends in health care spending and the reasons that the U.S. experience has differed from that of other high-income countries. In 2010, the share of U.S. gross domestic product devoted to health care was 7.2 percentage points higher than the share in other rich countries. We document the growth of this gap in the past five decades. Much of it developed between 1980 and the mid-1990s, though we also find another episode of outsized growth in the early 2000s. We identify six countries, including most of Scandinavia, which have seen a slowdown in health spending growth. These were also countries that had higher-than expected health spending, given their average incomes, in the 1960s and 1970s. The slowdown in health expenditure growth may simply reflect a reversion of their spending toward the OECD mean. We find no mean reversion in U.S. health spending growth. Our review of other literature suggests that the current excess in U.S. health costs is mainly traceable to higher prices for health care goods and services. Compared with other OECD countries, the United States has been slow to develop institutions or global budget constraints that restrain the pace of growth in health costs.