What would you do if you witnessed a crime?

From riding the CTA to attending a Bears game at Soldier Field, there seem to be plenty of reminders around Chicago instructing residents and visitors that if they “see something, say something.”

But not everyone abides by that.

Take the May attack on the CTA’s Red Line, when a woman fought off an attacker in an attempted sexual assault—as a passenger boarded the train and left without helping her. The crime sparked outrage and prompted conversation about what bystanders should do if they witness a crime.

When confronted with a crisis, some witnesses become so-called good Samaritans, feeling a moral obligation to intervene or call for help. Others do nothing, out of fear of retaliation or putting their own lives at risk, among other reasons.

Sometimes, witnesses don’t come to the aid of people in emergencies because of a social psychological phenomenon called the “bystander effect,” according to criminology experts.

The “bystander effect” happens for three reasons, according to Arthur Lurigio, psychology and criminal justice professor at Loyola University Chicago:

Diffusion of responsibility, meaning that an individual thinks that because there are multiple witnesses, somebody else will call for help

Social inhibition, which boils down to the fear of misunderstanding the situation and getting embarrassed for intervening

Pluralistic ignorance, which plays into the assumption that the situation doesn’t require intervention because nobody else is taking action.

Lurigio considers the situation that played out on the CTA—particularly the bystander’s lack of action—”unusual.” Research has suggested that the smaller the number of people who witness an emergency, the more likely someone is to respond or come to the victim’s aid.

“Given what we know about the bystander effect, given the ready availability of phones, given the ready availability of another means to call for help, this is an unusual case in the sense the person did nothing,” he said.

If a person intervenes, he has a greater risk of getting attacked than if he had done nothing, but the likelihood is greater of the attacker fleeing, Lurigio said. Simply calling for help requires minimal involvement and minimal risk, particularly in the CTA attack, he said.

“Let’s say he was fearful, he could have dialed 911 or hit a panic button or notified somebody else who was in the subway that there was an attack,” Lurigio said. “The situation and all the parameters begs the question of why this person did nothing.”

Failing to report a crime or failing to intervene is not a crime in itself, according to the Cook County state’s attorney’s office.

“The major exception to this in Illinois is where a responsible adult fails to intervene to protect a child being victimized where the responsible adult, generally a parent, knows of the danger and fails to protect,” said Sally Daly, a spokeswoman for the prosecutor’s office.

Some people feel compelled to act, as was the case with Ray Pulido, whose instincts kicked in last month when he saw a man knock a woman to the ground in Lakeview and take off with her purse. Pulido and his two Peoples Gas co-workers chased down the thief and cornered him in a parking lot, and the man was arrested, police said.

“When the incident happened in front of us, it was a natural reaction for me,” said Pulido, 22, who lives in Logan Square.

Afterward, he said his mother told him it was a brave act yet a foolish one because the purse-snatcher could have had a weapon. He said she asked, “Is a woman’s purse really worth a trip to the morgue?”

But Pulido said it happened so fast that he didn’t think about it.

“If somebody needs help, you go help,” he said.

Chicago Police advise witnesses of a crime to never pursue an assailant but to instead call 911 immediately and remain on the scene when possible.

But some people choose not to get involved at all because they want to avoid testifying on the case in court or they fear the alleged criminal would find out where they live and retaliate against them, said Chuck Sczuroski, master trainer at the National Crime Prevention Council in Washington, D.C.

One way law enforcement is trying to combat the lack of involvement, he said, is by encouraging use of anonymous crime reporting sites including wetip.com

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and crimestoppersusa.com, as well as apps like ICE BlackBox, which allows users to call 911 and record video to send to police. Chicago Police have created the TXT2TIP program for people to anonymously submit tips via text or online.

In addition to reporting the crime, Sczuroski stressed the importance of being a good witness.

“Part of being a good witness is to consciously at the moment the incident is happening to look at something unique—a scar, type of earring, tattoo—something that makes that person stand out, that would be easily noticeable for law enforcement,” Sczuroski said.

Instead of describing clothes, like sweatshirts, which can easily be turned inside out, focus on physical characteristics that are hard to change quickly, such as hair or eye color, hairstyles such as ponytails, facial hair, a limp and even sneakers, which are unlikely to get ditched, he said.

“Sometimes, the best thing you can do is be a good witness,” Sczuroski said.