California’s an outlier in many ways. It’s home to more than 1 in 9 Americans and an economy that ranks among the largest in the world. It’s also the runaway leader among the nation’s most polarized state legislatures, about half of which are more polarized than even Congress, according to an academic analysis conducted by the University of Chicago’s Boris Shor and Princeton’s Nolan McCarty.

But unlike Congress, where polarization and split party control has contributed to the lowest output in decades, states legislatures are different. That’s because all but four are controlled by a single party, according to data collected by the bipartisan National Conference of State Legislatures. Factor in the party of each state’s governor and there are still 37 states where one party controls the legislative and executive branches. Among the 26 states with legislatures more polarized than Congress, all but seven are single-party states.

Of course, measuring polarization is hard. Professor Shor explained the problem in his blog post over at our political science blog, The Monkey Cage — part of an important series they’re doing on polarization:

We haven’t been able to tell in the past, because we haven’t been able to determine just how liberal or conservative state legislators are in all 50 states. One major reason why is that each state is rather unique. Massachusetts Republicans aren’t the same as Texas Republicans; the same is true for each state’s Democrats. Nor do they vote on the same things. These differences mean that measuring ideology – and levels of polarization – is much more difficult for state legislatures than for Congress.

But Shor and McCarty came up with a way to measure polarization, relying on roll call data and self-reported answers to Project Vote Smart’s detailed surveys of state legislative candidates. They defined polarization pretty simply: “The average ideological distance between the median Democrat and Republican in the state legislature.”

What they found was 26 states more polarized than Congress, represented by the dotted line in the chart below, based on averages over a nearly 20-year period through last year. California is a standout for polarization. Rhode Island, Delaware and Louisiana are (relative) model citizens. But, as Shor notes, it may be a reflection of state politics: “In Louisiana, both parties are fairly conservative, and in Delaware and Rhode Island, they are both fairly liberal.”

What’s worse, they found, is that polarization has been getting worse over time. (Check out their post for more.)

Here’s a list of states ranked from most polarized to least and color-coded by party control (blue for Democrats, red for Republicans and no color in states where parties split control):

State Polarization rank
California 1
Colorado 2
Michigan 3
New Mexico 4
Washington 5
Wisconsin 6
New Hampshire 7
Arizona 8
Ohio 9
Minnesota 10
Virginia 11
Missouri 12
Alaska 13
Maryland 14
Texas 15
North Carolina 16
Maine 17
Montana 18
Georgia 19
Iowa 20
Idaho 21
Oregon 22
Utah 23
Connecticut 24
Tennessee 25
New York 26
Vermont 27
Pennsylvania 28
Alabama 29
Wyoming 30
Illinois 31
New Jersey 32
Kansas 33
South Carolina 34
Oklahoma 35
Indiana 36
North Dakota 37
Nevada 38
South Dakota 39
Hawaii 40
Florida 41
Mississippi 42
Massachusetts 43
Kentucky 44
Arkansas 45
West Virginia 46
Nebraska 47
Louisiana 48
Delaware 49
Rhode Island 50