The NFL's offensive surge in 2020 has pushed into historic territory. The league has averaged a combined 51.3 points per game, the highest through four weeks of a season since the 1970 AFL-NFL merger.

The previous high was 48.1 points per game, set in 2018.

A variety of factors have contributed to the rise in scoring, including high-level quarterback play and the loss of home-crowd advantage. But the most significant shift has been a dramatic drop in penalty flags, the result of an explicit effort by the league to call only "clear and obvious" fouls.

Retired referee Walt Anderson, now the league's senior vice president of training and development, told NFL.com last month that he does not want officials "all of a sudden to start calling the ticky-tack stuff." Anderson acknowledged that it's what "the NFL likes and what the audience likes."

During the first four weeks of the 2020 season, NFL games have averaged 13.4 flags per game, including those that were declined or offset. That's the lowest per-game average through four weeks in the ESPN Stats & Information penalty database, which goes back to the 2001. The drop has not been minor: The average has fallen 28% from the same period of 2019 and 19.3% from 2017.

The primary cause of the decrease has been a substantial decline in flags for offensive holding. There have been 126 such calls in 2020, the NFL's lowest total since at least 2001. That's a 56.3% drop from 2019, when the league made offensive holding a point of emphasis, and 41.2% from 2018.

In addition to fall in flags for holding, NFL offenses have also benefited from an increase in calls for defensive pass interference. There have been 93 such flags this season, also a high since at least 2001. The average per year from 2001 to 2019, over the same period of weeks, was 75.9.