Social influence/social identity

Typified by thoughts such as "My friends don't seem to have found an issue with it, so I guess it's OK" (or conversely "We just don't stand for that here.")

Social identity is a person's sense of who they are based on their group membership(s) e.g. our friendship group, nationality, class, sports team, sub-culture. We are influenced by others in our groups. If they do not take action, we are less likely to. If they intervene, we're more likely to support that action.

Gendered norms and expectations, cultural differences, 'group think', and lad culture culture can all contribute. Additionally, we may not feel the same connection with a victim we don't identify as closely with.

Diffusion of responsibility

Typified by thoughts such as "There's so many people around, someone else will surely intervene" (or conversely "I'm the only one here; if I don't do something, no-one will")

The more people there are about, the less likely we are as individuals to help someone. That's not just about there being more people to help; each member of the group feels a decreased responsibility to act. They expect that someone else will take actions, or already has.

We feel justified in not acting when we're part of a larger group of people who have made the same decision, and we feel less conspicuous in that decision.

Fear of retaliation

Typified by thoughts such as "They might get angry, or violent, if I challenge their behaviour." or "I'll never get that job if I make a fuss." (or conversely "Someone could get hurt if this isn't dealt with this safely.")

A bystander may have a legitimate fear of negative consequences or retaliation if they intervene. Those fears might be for their short-term safety, or long-term impact on their relationships or future prospects. Common fears relate to the possibility of physical and/or emotional harm, reputational damage, loss of/damage to relationships, and withdrawal of opportunities.

Audience inhibition

Typified by thoughts such as "I'm going to get laughed out of here if I say something." (or conversely "I don't know what other people think.")

People are often inhibited from helping due to fear that other bystanders will evaluate them negatively for their actions if they intervene. They might anticipate being embarrassed, or fear being ostracised by peers.

Audience inhibition is particularly impactful in public situations, when we don't know the people around us very well and find it harder to anticipate their response (or we catastrophise). It can also have a strong influence on interventions within a group whose views or actions are known, if your intervention goes 'against the grain' and challenges the norms of that group.

Pluralistic ignorance

Typified by thoughts such as "It seems like I'm the only one that has a problem with it." (or conversely "I know my colleagues/friends are just as fed up of this behaviour.")

In social psychology, pluralistic ignorance is a situation in which a majority of people in a group reject something (privately), but go along with it because they assume incorrectly that most other people accept it.

In this way, many of us feel much more alone with our concerns than we really are. Because we're less likely to speak out about it, the assumption survives when really our concerns are shared by others. Something can seem to be a 'norm', and goes unchallenged, despite it being rejected by the majority of people in the group.