I was sitting on the bus when I noticed a slight teenage girl. Her black backpack and duffel were arranged around a lamp-post in a homey circle with a cardboard sign soliciting spare change. I noticed the girl not because she was homeless, but because she was frightened and defensive. She was standing, arms up half defensive, half placating. Two large punks (whose outfits and painstakingly spiked mohawks I normally would have admired) were clearly harassing her. The verbal taunts quickly became a physical assault.

I remember wondering what they were fighting about, trying to imagine how it was possible a girl so tiny could ever do anything that should provoke a reaction like that, trying to reconcile my previous good image of punks with these assholes.

At no point did I think about stepping in or hope for someone else to do something about the child being held in a head lock by a man while a woman punched her.

When the woman next to me stood up and said “Why isn’t anyone doing anything?,” it took me a few seconds to understand the question the way I do now, as a plea for someone to do something to stop a clearly horrible thing. My mind filled with answers like “they don’t know who’s to blame” or “what’s actually going on,” answers that don’t really matter or make sense.

About 30 seconds after the attack started someone did step in, thankfully.

The bus was full of other people who sat silently watching the attack. The girl was surrounded by a crowd, some of whom barely glanced up from their phones. We were all bystanders.

I knew the bystander effect was very powerful, but I didn’t understand why. I always imagined I would understand what was going on but just pass the responsibility for dealing with on to someone else, that overcoming the bystander effect was simply about being willing to step up and take responsibility. But the part of my brain that recognized that this was a dangerous situation acted in the best way it knew to protect me – by keeping me completely uninvolved. I couldn’t recognize the situation as something that I could or should get involved with. I couldn’t really even recognize it as “bad.” I felt a strange sense of distance from the situation.

I haven’t been exposed to many situations where an intervention is called for, but the ones I have, I’ve acted. I don’t know why I became a bystander this time, but I hope it doesn’t happen again.