Congressional district maps were last redrawn after the 2010 Census  right when the tea-party wave handed Republicans increased majorities in state legislatures, which meant control in more states over the new district lines. Thanks to technology, it's easier than ever for one party to create new districts that maximize its seat share. In North Carolina in 2016, for example, Republican candidates received 53 percent of the total votes for the House, but won 10 of the 13 congressional seats. The results have been most pronounced in swing states; the report's authors Laura Royden and Michael Li estimate that GOP-drawn maps in North Carolina, Michigan and Pennsylvania have netted Republicans between two and three seats on average in each state. (Some Democrats have played games as well: Royden and Li also estimate that Democratic gerrymandering in Maryland and Massachusetts cost the GOP a few seats.)