NFL teams are breaking all the rules this year

Halfway through the NFL season, and some of the longstanding football beliefs seem half-baked.

The past nine weeks have put some rules of thumb to the test, some because of surprising trends, others due to statistical curiosities.

Now that every team has at least eight games in the books, here are eight theories under heavy pressure:

Defense wins championships

That was certainly the case in the most recent Super Bowl, with Denver’s dominating defense roughing up Carolina’s top-ranked offense. But lately, teams with some of the best defenses have been struggling. Check out the records in the last five games of the teams with the top five defenses: Arizona (2-2-1), Baltimore (1-4), Minnesota (2-3), Denver (2-3) and Houston (3-2).

Conversely, the teams with the top five offenses have been rolling during the same span: New Orleans (4-1), Atlanta (3-2), Dallas (5-0), Washington (3-1-1) and Oakland (4-1).

Beware the rookie quarterbacks

The Cowboys have won seven in a row with rookie Dak Prescott at the helm. He has thrown for 12 touchdowns with two interceptions, and has run for four scores. Philadelphia rookie Carson Wentz has come back to earth after a 3-0 start, with the Eagles losing four of five.

In all, five rookie quarterbacks have started games this season, the others being Cleveland’s Cody Kessler, New England’s Jacoby Brissett and Denver’s Paxton Lynch. Seattle’s Trevone Boykin and Kevin Hogan of the Browns have also thrown passes in games. Seems like the only rookie quarterback who hasn’t played is No. 1 overall pick Jared Goff of the Rams.

Kickers are getting too good

That’s why the league moved extra points back, right? Well, kicking miscues are a weekly occurrence this season. Second-round pick Roberto Aguayo has made every field goal an adventure in Tampa Bay. Arizona’s Chandler Catanzaro and Seattle’s Stephen Hauschka both made and missed field goals in overtime of a game that ended in an ugly 6-6 tie. And Pittsburgh’s Chris Boswell had the worst onside-kick attempt in memory Sunday, when he tried a nifty soccer move and barely knocked the ball off the tee.

NFL TV ratings are bulletproof

Ratings have plunged this season, causing significant concern within a league that unfailingly mints money. Theories abound for the swoon: viewers have turned their attention to the election; the concussion issue has hit home; too many game days (and not enough good games); and residual anger over players kneeling during the national anthem. Lots of questions, but no firm answers yet.

Penalties will kill you

They haven’t done in the 7-2 Raiders, who have a league-leading 94 flags. That’s 23 more than Denver, which is second on that list. Does 23 sound familiar? The Raiders were flagged an NFL-record 23 times at Tampa Bay in Week 8 … and won.

You need to win the turnover battle

It sure helps. In fact, that’s typically the most telling statistic (aside from final score) when it comes to determining wins and losses. But there are some strange turnover differentials out there at the midway point. Buffalo (plus-eight), Philadelphia (plus-six) and New Orleans (plus-four) are near the top of the list, and they’re a collective 12-13, with none having a winning record. The New York Giants and Houston are tied near the bottom at minus-seven, yet both are 5-3.

In the end, it’s about outscoring your opponent

No question. But some of the point totals are head-scratchers. Houston is atop the AFC South at 5-3, but has been outscored by 30 points this season. Philadelphia has outscored opponents by 57 points but is in the NFC East cellar at 4-4, and San Diego is last in the AFC West despite a 21-point scoring advantage.

With the NFL’s competitive balance, nobody will go 0-16 again

It could happen with Cleveland. The Browns are 0-9, with a tough remaining schedule: at Baltimore, Pittsburgh, New York Giants, Cincinnati, at Buffalo, San Diego, at Pittsburgh.

The 2008 Detroit Lions, the only team to finish 0-16, are keeping the champagne on ice — and they’d be happy to share it.

sam.farmer@latimes.com