Stock returns re#ect new market-level and "rm-level information. As Roll
(1988) makes clear, the extent to which stocks move together depends on the
relative amounts of "rm-level and market-level information capitalized into
stock prices. We "nd that stock prices in economies with high per capita gross
domestic product (GDP) move in a relatively unsynchronized manner. In
contrast, stock prices in low per capita GDP economies tend to move up or
down together. A time series of stock price synchronicity for the U.S. market
also shows that the degree of co-movement in U.S. stock prices has declined,
more or less steadily, during the 20th century. These "ndings are not due to
di!erences in market size or economy size.1