Similar security measures are in place at M&T Bank Stadium. Ravens senior vice president for public and community relations Kevin Byrne said season-ticket holders are sent a "fan creed" that stresses good behavior. The creed is read at the start of each home game, and fans have created a tradition of yelling one of the lines: "Don't be a jerk."

"Our goal is to create a safe and energetic atmosphere at our homes games, where fans of all ages can enjoy the event and help us have the best home-field advantage in the NFL," Byrne said in a statement. He declined to provide information on the number of security personnel present at the games.

Statistics provided by the city-run OpenBaltimore website show that since 2008, 67 assaults were reported at or near Camden Yards and 57 were reported at or near M&T Bank Stadium.

Bader said in a statement that the Orioles are "saddened" by the incident that left Fortese injured. "Our thoughts remain with Matt and his family."

Meanwhile, longtime family friend Jeff Minnichbach has created a website, mattfortese.com, to help raise money for Fortese's medical expenses. He hopes to raise $10,000, but Fortese's bills are expected to far exceed that.

People across the country have responded, Minnichbach said, and donations have been received from as far away as Alaska.

"Matt's just a happy-go-lucky guy," Minnichbach said. "He is always laughing about anything and everything."

The reason for fan aggression is multifaceted, said Paula Parker, an associate professor at East Stroudsburg University, and Rick Grieve, a Western Kentucky University professor. Both are consultants for the Association for Applied Sport Psychology.

Fans most likely to cause problems are those who tie their identity to the team. Such fans can see their team's supporters and a rival's as "us and them."

Put those personalities in a stadium full of people, where they might feel they have the cover to act poorly, and trouble can emerge, according to Parker and Grieve.

"When you're a fan in a crowd, there is a depersonalization, you become part of the crowd, your moral compass isn't centered on you, but on the group," Grieve said.

Alcohol is often a factor as well, Parker and Grieve said.

Stadiums began taking precautions years ago to limit alcohol consumption at sporting events, said David Jernigan, director of the Center on Alcohol Marketing and Youth and an associate professor at the Johns Hopkins Bloomberg School of Public Health.

Baseball stadiums across the country stop selling beer after the seventh inning. Many limit alcohol purchases to two per person per transaction, and most don't allow fans to bring alcohol into the ballpark, he said. Memorial Stadium first banned fans from bringing beer in 1985.

"In a highly charged emotional setting, adding alcohol is like throwing gas on a lit fire," Jernigan said. If the Orioles wanted to take further precautions, management could raise the price of alcohol or stop vendors from selling in the stands, he said.

Eric Strawderman, an Orioles season-ticket holder the past three years, said there's nothing wrong with razzing a rival team's fan, so long as it's done in good spirits.

Strawderman, 26, added, "No one should go to a ballgame and leave in an ambulance."

Baltimore Sun reporters Jessica Anderson and Andrea K. Siegel contributed to this article.

ywenger@baltsun.com

twitter.com/yvonnewenger