Some political science research suggests that, contrary to common belief, gerrymandering does not decrease electoral competition, and can even increase it. Some say that, rather than packing the voters of their party into uncompetitive districts, party leaders tend to prefer to spread their party's voters into multiple districts, so that their party can win a larger number of races. (See scenario (c) in the box.) This may lead to increased competition. Instead of gerrymandering, some researchers find that other factors, such as partisan polarization and the incumbency advantage, have driven the recent decreases in electoral competition. Similarly, a 2009 study found that "congressional polarization is primarily a function of the differences in how Democrats and Republicans represent the same districts rather than a function of which districts each party represents or the distribution of constituency preferences."