Story highlights More than half of all Congressional districts are safe for one of the two parties

People don't live, work or socialize around others who have different political views

(CNN) If you're like most Americans, you don't live around or socialize with people with whom you disagree with politically. This self-sorting by political ideology is why so many people were shocked Donald Trump won last November ("But I don't know anyone who voted for him") while half(ish) of the country knew it was happening all along.

A new map courtesy of the Cook Political Report shows just how divided we are along partisan lines. Here it is:

The map shows two things: 1) How few competitive districts there are in the country and 2) just how geographically isolated we are, politically speaking.

First, a bit of explanation. The Cook Political Report assigns each of the 435 House districts a Partisan Voting Index (PVI) score that seeks to compare how each seat performs politically against every other seat. A seat with a PVI of R+8, for example, means that the district performed eight points more Republican than the national average in the 2016 presidential election.

Generally speaking, a district with a PVI score between R+5 and D+5 is considered competitive. Seats with a PVI of +10 (or more) for either side are generally regarded as safe.

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