WHEN HALL OF FAME quarterback Johnny Unitas put the finishing touches on his career in 1973, walking away from the NFL with an average of 191 passing yards per game over 18 seasons, he likely could not have conjured the numbers Patrick Mahomes would one day produce.

The Kansas City Chiefs star, who's in his eighth season, has an NFL-best career mark of 293.3 passing yards per game.

From the record-breaking salaries being paid to quarterbacks and wide receivers to rules that are skewed in favor of protecting offensive players, so much in today's NFL reflects an emphasis on the passing game.

All of which makes it rather surprising that the first month of the 2024 season has seen a continuation of an undeniable trend: Passing numbers are down, again. For the fourth straight season, passing totals -- which had seen consistent and substantial growth over the past two decades -- are declining.

To explain the trend, ESPN sought the opinions of coaches and players across the NFL and examined decades of statistical data. What that produced was a complicated picture that points to multiple factors. They include the changing skill sets of today's players, a dramatic drop-off in the experience levels of starting quarterbacks and the league's evolving defensive tactics.

"I do think that the sample size is small right now, but there's been a shift," Tennessee Titans coach Brian Callahan said.

Through four weeks, NFL teams were averaging 202.8 passing yards per game -- down 47 yards per game compared with 2020. During the past 10 seasons, teams averaged 240.4 passing yards per game over the same four-game stretch.

This season's four-game output has NFL teams on pace to average the fewest passing yards since 2001, when Rams quarterback Kurt Warner would go on to win Most Valuable Player with 4,830 passing yards. An example of how significant the passing trends had been is that at the time, that was the second-most passing yards in a season in NFL history. Now it ranks as the 28th most in a season.

And yet, the league is now experiencing a shift, which includes a decline in touchdown passes. The 156 touchdown passes through four weeks is the fewest since 2008.

"It is something that we looked into, because it is noticeable," Philadelphia Eagles coach Nick Sirianni said, "and it is a really big difference."

IT'S DIFFICULT TO quantify what impact quarterback readiness has had on these trends, but it's hard to imagine it hasn't played a role.

"I'd be curious to know what those numbers were four or five, six years ago, and who those starting quarterbacks were then and the experience they had versus the quarterbacks now," Chiefs offensive coordinator Matt Nagy said. "Maybe I'm way off, but it could be something there.

"There's a lot of young quarterbacks, and it's a tough position, and so maybe it's a combination of both."

Nagy's instincts appear spot on.

According to ESPN Research, the past two seasons have seen a significant drop in the level of experience among NFL starting quarterbacks. In 2023, the average number of career starts by opening-day starters was 63.1. This season, it was 66.6.

As recently as 2022, the average number for opening-day starters was 82.8. From 2015 to 2022, the yearly average was 81 or more previous starts for seven of those eight seasons.

The reasons for this are not hard to trace. The departures of all-time greats like Tom Brady, Matt Ryan, Ben Roethlisberger and Philip Rivers in the past four seasons have created greater instability at the position. It also means younger quarterbacks are trying to replace them. And there are few things defenses love more than facing inexperienced quarterbacks.

Washington Commanders rookie Jayden Daniels has captured the nation's attention with a dazzling start to his career, but he's very much the exception to the rule.

Through the first four weeks, the three rookie quarterbacks to start games this season have combined for seven touchdown passes and nine interceptions. No. 1 overall pick Caleb Williams of the Chicago Bears and No. 12 pick Bo Nix of the Denver Broncos are averaging 5.6 and 4.8 yards per pass attempt, respectively, after four games.

Seven of the 32 opening-day starting quarterbacks this season were selected in the past two drafts. Three of them -- Williams, Will Levis of the Tennessee Titans and Bryce Young of the Carolina Panthers -- ranked in the bottom four in QBR through four weeks (Young has since been benched in favor of veteran Andy Dalton). Four of them -- Levis, Nix, Williams and the Colts' Anthony Richardson -- were among the leaders in interceptions.

Also of note: Levis, Williams and C.J. Stroud, the Houston Texans' second-year star, are among the most sacked quarterbacks through four weeks. Pass protection plays a distinct role in sacks, but so does quarterback decision-making.

"It's just a tragedy that we're forcing these rookies to play early, but the reality is the only reason why we are is because we've dumbed the game down, which has allowed them to play," Tom Brady, the seven-time Super Bowl winner, said on "The Stephen A. Smith Show." "It used to be thought of at a higher level. We used to spend hours and hours in the offseason, in training camp, trying to be a little bit better the next year.

"But I think what happens is it discourages the coaches from going to deep levels, because they realize the players don't have the opportunity to go to a deep level. So, they're just going to teach them where they're at."

How much of what Brady outlined is responsible for the drop in passing yards is likely up to the individual. But youth and inexperience, according to one current quarterback, is a variable.

"I would think that's more of a factor than what the defenses are actually doing," Colts backup Joe Flacco, a 17-year veteran, said.

DEFENSIVE COACHES WEREN'T going to stand around and watch as the offensive explosion took hold in the NFL.

For years, defensive tactics have been evolving to handle the emphasis on passing. Rules addressing issues like the protection of defenseless receivers, the interpretation of pass interference/illegal contact and how quarterbacks are protected while in the pocket have handed offenses significant advantages.

As a result, NFL teams leaned into the passing game. By 2020, the final season with 16 regular-season games, the NFL had witnessed a 16.6% increase in passing yards when compared to 2000.

The answer from defensive coordinators was to play schemes that responded to the phenomenon. It's possible the league has reached a tipping point where coverages that feature two deep safeties are beginning to have a more pronounced effect on passing success.

"I mean, that's the league right now," New York Jets quarterback Aaron Rodgers said. "It's a lot of umbrella coverages, a lot of soft quarters coverages and two-high [safeties].

"That's what we saw a lot of in Week 1 and Week 2. There wasn't a lot of single-safety. When there was, we took a couple shots. But the league is really trending towards more umbrella defenses."

Said Jacksonville Jaguars quarterback Trevor Lawrence: "Even the teams that have typically been single-high [safety] teams are playing more shell, which just changes your passing attack a little bit. Not to say you can't attack that still, but it definitely changes than if it's [single] safety."

The data backs them up.

Quarterbacks have faced coverages featuring two deep safeties on 45.9% of dropbacks this season, according to ESPN Analytics/NFL Next Gen Stats. That's the highest rate since the data was first tracked in 2016. It continues a steady increase in this area; six seasons ago, in 2018, the rate was 35%. Through four weeks, the Vikings, Bills, Jaguars and Commanders were all playing split-safety coverages on more than 60% of quarterbacks' dropbacks.

In theory, those coverages are intended to prevent chunk plays in the passing game, which are fundamental to being a highly productive offense. Last season, eight of the top 10 teams with the most pass completions of 20 yards or longer made the playoffs.

"Teams are going to make you drive the entire field," Mahomes said.

These tactics haven't necessarily created a huge increase in rushing yards, even with the extra defender dedicated to deep coverage. Rushing yards have only seen a slight increase in recent seasons. But it has influenced the quicker, shorter passing game that is now commonplace across the NFL.

"You can still throw the ball down the field versus two-high," Flacco said. "But I think it maybe affects the mindset of the playcaller."

Consider: Even while passing yardage is seeing historic declines, completion percentages continue to climb. In the 1980s and 1990s, it was rare to see completion rates well above 60%. The average completion rate in 1980 was 56.2%. In 1990, it was 56%. In 2000, it had barely budged, increasing to just 58.2%.

This season, quarterbacks are completing 661% of attempts, putting 2024 on track to have one of the highest figures in league history. It sounds counterintuitive -- passing yards are down even while more passes are being completed -- but it speaks to the impact coverages are having on quarterbacks.

Teams have taken stock and throwing check downs and shorter attempts. It's further demonstrated in the steady decline in air yards per pass attempt, which have fallen from 8.1 in 2017 to 7.5 last season. So far in 2024, the average is 7.0.

There's another consequence to all of this: Scoring is down. Offensive points per game has dropped in each of the past three seasons and is on pace to do so again.

"There are opportunities out there, you just have to execute it at a higher level," Mahomes said. "[But] there's not as many as maybe I've had previously."

WITH THE PROLIFERATION of dual-threat quarterbacks, many of whom grew up playing in spread offenses and were groomed in 7-on-7 competitions, playing the position in 2024 looks decidedly different than it once did.

More dual-threat quarterbacks means more quarterbacks are running the ball. That hasn't necessarily led to more overall rushing attempts, but what's changed is who is running the ball.

"When you look across the league, there's a higher number of running quarterbacks, so now you're going to call more types of plays where they end up running the ball," Flacco said.

In the past two seasons, teams are using quarterback-designed runs at a rate that obliterates previous numbers. From 2006, the first season for which data is available, to 2022, the highest year on record, designed runs by quarterbacks increased by 102%. Designed runs by quarterbacks this season are also on pace to be among the highest numbers ever produced.

There is arguably more incentive to attempt such runs because the higher use of split-safety coverages translates into fewer defenders near the line of scrimmage. That gives an advantage to offenses, especially when the quarterback must be accounted for in the run game.

Along those lines, 15 of the top 25 single-season rushing efforts by quarterbacks have been registered in the past 10 seasons by a new generation of players at the position.

"A lot of people look at rushing stats and they think it's just [running backs] running the football," New Orleans Saints coach Dennis Allen said. "But it's quarterbacks being able to run with the football on zone-read plays. ... And they're gaining yards on those things, making you have to play 11-on-11.

"And then you have the athletic quarterbacks that are gaining yards scrambling, which goes down as rushing yards."

This might add some context around the fact that pass attempts through four weeks were down 8.1% compared with the same period from last season.

But even with all the reams of data available, there's one question for which the answer remains elusive: What comes next?

"The league," Cleveland Browns offensive coordinator Ken Dorsey said, "is evolving every year, every week."

ESPN reporters Sarah Barshop, Rich Cimini, Turron Davenport, Mike DiRocco, Alaina Getzenberg, Paul Gutierrez, Tim McManus, David Newton, Daniel Oyefusi, Marc Raimondi, Kris Rhim, Adam Teicher, Kevin Seifert, and Katherine Terrell contributed to this report.