After the Seattle Seahawks thrashed the San Francisco 49ers 29-3 a few weeks ago, some disgruntled 49ers fans blamed the loss on the handicap of playing the game at Seattle’s CenturyLink Field. The stadium’s pair of huge, domed canopies -- designed to shield fans from inclement weather -- cover 70 percent of the seats and reflect sound back into the stadium.

That peculiar acoustic quality is exploited to the hilt by Seattle’s home crowd, which has become known in the NFL as the team’s “12th Man” due to its propensity for roaring so loudly that other teams can’t hear the quarterbacks’ signals.

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But Seattle's stadium is far from the only one visiting teams dread. Multiple studies by sports statisticians shows that in every other major professional and college sport across the planet, from NBA basketball to international cricket, there’s a pronounced home-field or home-court advantage.

In a 2010 paper, University of Rochester social psychologist Jeremy P. Jamieson, who did a meta-analysis of studies on both team sports and individual contests such as golf, tennis and boxing going back more than six decades, calculated that those who are competing at home tend to win slightly more than 60 percent of the time.

“The home field advantage is definitely real,” Jamieson said in an interview.

Why the home team wins more often than not, however, has proven to be a trickier question to answer. Traditional explanations have blamed everything from the rigors of travel and the distraction of being harassed by raucous fans, to home teams’ familiarity with and ability to exploit the anomalous quirks of their buildings.

In the old Boston Garden where the NBA Celtics won a string of championships in the 1950s and 1960s, for example, the trademark checkerboard wood floor was rumored to have dead spots where the ball didn’t bounce as well. The spots were thought to cause opponents to lose control of their dribbles and commit turnovers.

In baseball, "you had the old Polo Grounds in Brooklyn, where the left and right field were short and center field was very deep," noted Bradley D. Hatfield, a sports psychologist at the University of Maryland. "If you have a stadium like that, you could have an athlete who isn't necessarily the best power hitter, but who learns from playing there all the time to put the ball down those lanes. It's a question of familiarity and players' strengths matching the field."

But scientific researchers also have attributed varying amounts of the home-field advantage to physiological and psychological effects upon both the players and the officials who supervise the contests.

The home-field advantage isn’t the same in every sport, according to University of Chicago economist Tobias J. Moskowitz’s and sportswriter L. Jon Wertheim’s, 2011 book "Scorecasting: The Hidden Influences Behind How Sports are Played and Games Are Won." They report that the sport with the most extreme home-field advantage is Major League Soccer in the United States, where between 2002 and 2009, home teams won 69.1 percent of the time.

Nearly as lopsided is NCAA men’s basketball, where home teams won 68.8 percent of the contests between 1947 and 2009. In college football, between 1869 and 2009, the home team had a winning percentage of 63 percent.

Perhaps the most impressive home record in history was achieved by Michael Jordan’s 1995-96 Chicago Bulls, who won all but two of their 41 home games, on their way to an NBA-record 72-10 regular season record. 

Major U.S. professional sports showed only a slightly smaller advantage for home teams. In the NBA, they win 60.5 percent of the time, while NFL teams won 57.3 percent of the time at home, and NHL home teams prevailed 55.7 percent of the time. Major League Baseball, where home teams win 53.9 percent of the time, is the most evenly-matched of them all.

So why do home teams win most of the time? One explanation may be that, for whatever reasons, performing in front of a friendly crowd tends to kick home team players’ hormonal systems into high gear.

A 2003 study by neuroscientists Nick Neave and Sandy Wolfson from the U.K.’s Northumbria University found that salivary testosterone levels in British professional soccer players were significantly higher before a home game than an away game. The effect was more pronounced if the home team players perceived the visitors as an “extreme” rival than merely a “moderate” one that they didn’t feel as strongly about besting.

The Seahawks delivered a humbling loss to the 49ers (29-3) at CenturyLink Field in Seattle.Troy Wayrynen/Corbis

The British scientists theorized that the home team’s hormonal surge is an evolutionary holdover, related to animal species’ territoriality -- that is, the tendency to vigorously defend their ranging area against intruders. Indeed, some studies have shown that animals defending their turf tend to have a home-field advantage, even if they are smaller than the intruders.

Sports fans seem to believe pretty strongly in the notion that they can influence the game by creating visual or aural distractions. The spectators at NBA arenas, for example, often flutter streamers and wave colorful objects behind the backboard while an opposing player is shooting free throws.

Whether such distractions work on highly trained, focused professional athletes is a matter for debate.

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Moskowitz and Wertheim report that NBA visiting teams shot exactly the same free-throw percentage over the past 20 years -- 75.9 percent -- that home teams achieved. And in deadlocked NHL games decided by shootouts, in which  each team gets to pick three players to shoot one-on-one at the other team’s goalie, visiting teams’ 50.6 percent winning rate was actually higher than the 49.6 percent that home teams managed.

Those examples aside, a deeper dig into individual athletes’ statistics shows that even among the most talented, being on the road does seem to affect performance negatively.

Miami Heat superstar LeBron James, for example, is slightly less accurate from the field when away (.542) than he is at home (.592) and makes a lower percentage of free throws (.714 vs. 785).  He also tends to make slightly more turnovers (3.3 away, vs. 2.7 at home). While those are slight differences, in a close game they could mean the difference between victory and defeat.

Part of the explanation, Jamieson says, may be that playing in front of an unfriendly crowd can cause subtle but possibly significant degradation of the complex skills that athletes have developed through extensive practice. It’s not the distractions, necessarily, but the crowd’s jeering when a player makes a mistake.

The negative feedback, he says, can raise a player’s stress level, and cause a player to try to compensate by concentrating on specific parts of his baseball swing or his basketball jump shot.  When players do that rather than simply performing the entire skill as practiced, it’s a recipe for disaster, he says.

“When you do performance monitoring -- that is, trying to do something carefully so that you don’t do it poorly -- you’re basically trying not to lose. Instead of thinking about winning, you’re trying to avoid being booed.”

There’s also evidence that officials are influenced psychologically by the home crowd’s emotional response to plays.

A 2007 study by Harvard University researcher Ryan Boyko, which looked at 5,000 English Premier League soccer matches involving 50 different referees, found that away teams tended to score fewer goals and give away more penalties, which suggests that officials made calls in home teams’ favor.

Boyko also found that the effect was less pronounced when officials were more experienced, suggesting that they developed a resistance to the crowd’s psychological influence over time. Even so, Boyko calculated that for every additional 10,000 people in the stadium, the home team’s advantage increased by 0.1 goals.

Sports psychologist Hatfield, however, noted that not every individual athlete benefits from home field advantage.

Star players may benefit from familiarity with the stadium and feed off the energy of the crowd, he said, but less-skilled and less-experienced players actually may perform worse at home, because of anxiety about meeting the home crowd's expectations.

"If an athlete isn't a seasoned veteran," Hatfield said, "he may actually play better on the road."