(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." These findings are, however, a matter of some dispute. While gerrymandering may not decrease electoral competition in all cases, there are certainly instances where gerrymandering does reduce such competition. One state in which gerrymandering has arguably had an adverse effect on electoral competition is California. In 2000, a bipartisan redistricting effort redrew congressional district lines in ways that all but guaranteed incumbent victories; as a result, California saw only one congressional seat change hands between 2000 and 2010. In response to this obvious gerrymandering, a 2010 referendum in California gave the power to redraw congressional district lines to the California Citizens Redistricting Commission, which had been created to draw California State Senate and Assembly districts by another referendum in 2008. In stark contrast to the redistricting efforts that followed the 2000 census, the redistricting commission has created a number of the most competitive congressional districts in the country. The effect of gerrymandering for incumbents is particularly advantageous, as incumbents are far more likely to be reelected under conditions of gerrymandering. For example, in 2002, according to political scientists Norman Ornstein and Thomas Mann, only four challengers were able to defeat incumbent members of the U.S. Congress, the lowest number in modern American history. Incumbents are likely to be of the majority party orchestrating a gerrymander, and incumbents are usually easily renominated in subsequent elections, including incumbents among the minority. Mann, a Senior Fellow of Governance Studies at the Brookings Institution, has also noted that "Redistricting is a deeply political process, with incumbents actively seeking to minimize the risk to themselves (via bipartisan gerrymanders) or to gain additional seats for their party (via partisan gerrymanders)". The bipartisan gerrymandering that Mann mentions refers to the fact that legislators often also draw distorted legislative districts even when such redistricting does not provide an advantage to their party. Gerrymandering of state legislative districts can effectively guarantee an incumbent's victory by 'shoring up' a district with higher levels of partisan support, without disproportionately benefiting a particular political party.