Do umpires favor pitchers who are of a similar ethnic background? According to an article in the American Economic Review they do. The summary of the study and a link to the original article is at Ars Technica.
Perhaps more significantly, the authors also compare game statistics for matching player/ump combinations in unmonitored ballparks. Here, the numbers are very consistent. For both white and minority pitchers, winning percentages went up by about five percent. Everything else—the number of hits and runs scored against them, the walks they gave up, and the number of home runs hit—all went down (so did strikeouts, although the effect was very small).
We need more eyeballs.