Why there are no black senators

In the public debate over Blagojevich's appointment of Roland Burris to the vacant Senate seat, reference is often made to the fact that there are currently no black U.S. Senators. The reason for this is nicely elucidated in a thoughtful analysis today at Fivethirtyeight.

It's not simple racism or bias; the discrepancy reflects population demographics. The analysis begins by looking at the House of Representatives, where there are 39 African-American representatives.

The districts these 39 Congressmen serve, however, are not very representative at all. All 39 contain a higher percentage of African-Americans than the population as a whole, ranging from Keith Ellison's district in Minneapolis, which is just barely more black than the national average, to Jesse Jackson Jr.'s on the South Side of Chicago, which is 68 percent African-American. About 64 percent of the members -- 25 of 39 -- come from districts that contain an outright black majority. The districts are also much more Democratic than the country as a whole, with an average PVI [Partisan Voting Index] of D +25; only Sanford Bishop's district in Georgia, which has a PVI of D+2, is anywhere close to the national average.

The graph above shows the % chance of having a black congressman (vertical) versus the % of the population in the district that is black (horizontal). The chance of having a black congressman reaches 50:50 when the black population reaches about 40%.

States that elect Senators have a more diverse population than districts that elect representatives, and there are no states with black populations over 40%. Only six states have more than 25% of their voters African American, and these are all in the culturally conservative Deep South, where the Partisan Voting Index is shifted toward Republicans. There are other factors, of course, many of which are discussed at FiveThirtyEight.