This interesting new paper by Chris Kenny et al. compares the newly enacted House plans across the country to sets of randomly generated maps. The bottom line is that the House as a whole is just slightly more pro-Republican than would be expected under nonpartisan redistricting. There’s plenty of gerrymandering but it mostly cancels out.
Congressional district lines in many U.S. states are drawn by partisan actors, raising
concerns about gerrymandering. To isolate the electoral impact of gerrymandering
from the effects of other factors including geography and redistricting rules, we
compare predicted election outcomes under the enacted plan with those under a
large sample of non-partisan, simulated alternative plans for all states. We find that
partisan gerrymandering is widespread in the 2020 redistricting cycle, but most of
the bias it creates cancels at the national level, giving Republicans two additional
seats, on average. In contrast, moderate pro-Republican bias due to geography and
redistricting rules remains. Finally, we find that partisan gerrymandering reduces
electoral competition and makes the House’s partisan composition less responsive
to shifts in the national vote.