Bystander effect
Overview
The bystander effect refers to the phenomenon where the more people are present at an emergency, the less likely each individual is to help. Prompted by the 1964 Kitty Genovese case, Latané and Darley reported experiments on it in a 1968 paper.
There are three main causes. Diffusion of responsibility ("even if I don't do it, someone else will"), pluralistic ignorance ("no one else is reacting, so it must not be a big deal"), and evaluation apprehension ("it would be embarrassing if I stepped in and got it wrong"). These work together, and in front of a crowd, fewer people act.
The bystander effect online
The bystander effect works online too. Even when you see someone being slandered on social media, if they have tens of thousands of followers, it's easy to think "someone else will report it even if I don't." Even when you see a nasty question published in a question box, you tend to think "someone else will call it out even if I don't speak up."
In anonymous settings, this tendency can get even stronger. Under your real name, there's a risk of being blamed later for "doing nothing back then," but when you're anonymous, no one will know if you just watch. As a result, a structure forms where silence spreads in the face of bullying and harassment online.
Going from bystander to someone who acts
The key to breaking the bystander effect is making it your own problem. Switch your mindset from "someone else will do it" to "if I don't do it, no one will." Knowing that the bystander effect exists is said to increase helping.
In a question box context, small actions can be a start toward breaking the bystander effect: if you see someone receiving nasty questions, send them a message of support, or report the problematic question. When one person acts, it becomes easier for others who see it to act too. Being the first one takes courage, and that's what makes it valuable.