There are usually a handful of plays that determine the outcome of a football game. Fourth-down plays are almost always part of that equation.

Fourth down is sort of a super-sized, ultra-concentrated distillation of the game itself.

"Something big," said Baltimore Ravens head coach Brian Billick, "is going to happen. Either you're going to covert in a crucial situation that keeps a drive alive, that allows you to score, that makes a difference in the game, that gives your team a huge emotional lift. The reverse of that is you go for it, you take your chances, you get stoned and it's a huge emotional drain for your team.

	“	Everybody's excited about it until the play's run. Then, half are excited and half aren't. ”
		— Bill Belichick on going for it on fourth down

"You pay your money, you take your chances."

The Denver Broncos thought they were being aggressive when they tried a long Jason Elam field goal against the Ravens in Week 4's Monday night game. Problem was, Ravens cornerback Chris McAlister was lurking in the end zone and gathered in the ball that was well short. After a brief hesitation -- and a crushing block from Ray Lewis -- McAlister bolted 107 yards for a touchdown and an NFL record.

The game's chemistry had been altered. The Broncos were never seriously in the game thereafter.

Those sharp swings of emotion are the biggest factors missing in David Romer's analysis of fourth downs. The Cal-Berkeley economics professor was unable to account for momentum in his complicated formula that he says should prompt coaches to go for it more often on fourth down.

"From a numbers standpoint, obviously, (the equation) made sense," said San Francisco 49ers head coach Steve Mariucci. "Fourth downs … that's a different animal. Am I going to go for a fourth-down-and-inches on my own 11-yard-line? There's a certain common sense that needs to take place as to whether you even think about attempting it or not."

Romer has heard this sort of thing before. In fact, he and some associates recently began studying data on momentum swings. After looking at situations when one team receives the ball after a major event -- say, a failed fourth down or a turnover -- Romer said he believes that momentum is not an important factor.

"The numbers seem to indicate that the team that gets the ball tends to let down a little bit," Romer said. "And the team that loses the ball seems to rally some. Actually, I think it might turn out that the momentum shifts the other way than most people think."

The one thing everyone agrees on is that players -- on both sides of the ball -- love the high stakes of a fourth-down call.

"In that state of mind, they're reacting like the fans are," said Miami Dolphins head coach Dave Wannstedt. "Everybody's saying go for it and our players are out there saying go for it, but I think as a head coach you need to really look at the big picture.

"I think our players would go for it on fourth down every time."

That would include, of course, injured quarterback Jay Fiedler. "You definitely get enthusiastic, you get fired up," Fiedler said. "You see the offensive line's eyes light up a little bit more. You want to hit the guy a little bit harder."

Miami teammate Zack Thomas: "If it's a run play and they're trying to get an inch, then that's the best feeling in the world when you stop them. You hit them in the mouth and you get up and that's the best. It gives you a little swagger."

But as New England Patriots head coach Bill Belichick pointed out, half of that enthusiasm subsides as soon as the play is over.

"Everybody's excited about it until the play's run," Belichick said. "Then, half are excited and half aren't."

It's the downside of that momentum swing that scares coaches.

"It can backfire," explained Oakland Raiders quarterback Rich Gannon. "You're at the opponent's 40-yard line and you're feeling good about yourselves. You go for fourth-and-one and you don't get it -- now the team has a short field. They go down the field and score and now the momentum has swung again."

Greg Garber is a senior writer at ESPN.com.