From the Sports Economist:
David Leonhardt reports on Justin Wolfers' research with point spreads in today's New York Times. The title of the piece is "Sad Suspicions About Scores in Basketball" -- NCAA basketball, that is.
Mr. Wolfers has collected the results of nearly every college basketball game over the last 16 years. In a surprisingly large number of them, it turns out that heavy favorites just miss covering the spread. He considered a number of other explanations, but he thinks there is only one that can explain the pattern. Point shaving appears to be occurring in about 5 percent of all games with large spreads.
... Smaller favorites -- teams favored by 12 or fewer points -- beat the spread almost exactly 50 percent of the time, showing how good those oddsmakers are at their jobs. But heavy favorites cover in only 47 percent of their games. There is little chance that the difference is due to randomness.