Senate Majority Leader Mitch McConnell (R-Ky.) speaks at a news conference Thursday about the passage of the Tax Cuts and Jobs Act. (Aaron P. Bernstein/Reuters)

University of Wisconsin political science professor Barry Burden raised an interesting point on Twitter this week. Maybe, he theorized, the Senate was growing more polarized because it was welcoming more members of the hyperpartisan House into its ranks.

In theory, that makes some sense. We can use partisanship data from VoteView to see how the chamber has indeed grown more polarized, with bigger positive values indicating a more conservative voting history and a larger negative number a more liberal one. (There’s a lot more to these figures than just that, but for our purposes, it will suffice.)

Both the Senate Democratic and Senate Republican caucuses have moved away from the middle over the past 10 congresses. (We are currently in the 115th Congress, which will run for 2017 and 2018. The 107th Congress was in 2001 and 2002.)

We can also see that the Republicans, in particular, have grown more conservative and now match the House Republican caucus on that metric.

So it makes sense. Conservative members of the partisan House get elected to the Senate, and the Senate grows more conservative.

But actually, it’s senators without experience in the House who are making the Senate more polarized.

Here is the partisanship score of every senator who served in any of the congresses from the 107th to 115th and who also served in the House. We’ve included their House tenures as well, which, in some cases, stretch back to the 1950s.

Notice the two senators we’ve flagged at the top: Sen. Jeff Flake (R-Ariz.) and former Oklahoma senator Tom Coburn. They are the most conservative members of recent senates.

But now let’s look at those senators who didn’t serve in the House. There are much more partisan senators in the mix, from both parties.

Mike Lee (R-Utah), Ted Cruz (R-Tex.), Rand Paul (R-Ky.) and Joni Ernst (R-Iowa) came to the Senate from outside of Congress. So did Elizabeth Warren (D-Mass.) and Kamala D. Harris (D-Calif.). (Former West Virginia senator Carte Goodwin only served for a few months in 2010.)

If we combine these two charts, the density of the darker-colored, never-served-in-the-House senators is clearly further from the middle on the Republican half of the diagram.

Averaging those who did and didn’t serve in the House, we see that this isn’t just true of Republicans. For both parties, it’s the former members of the House who, in recent years, have been less partisan than their colleagues.

For the Republicans, the average senator who had served in the House had a partisanship score of 0.42 in the 107th Congress and 0.47 in the 115th. Among those Republicans who didn’t serve in the House, the average spiked from 0.32 to 0.51 — an increase of nearly 60 percent.

Before the 112th Congress, it is the case that former members of the House were the more conservative members of the Senate Republican caucus. After, that’s no longer true.