Dear Donald, 

I think this paper is useful for you. The researchers' results from SEM revealed that basic moral sensitivity in bullying did not relate  to pro-bully behavior but

related to outsider and defender behavior, mediated by moral disengagement

in bullying.

Robert Thornberg and Tomas Jungert, Bystander behavior in bullying situations: basic moralsensitivity, moral disengagement and defender self-efficacy, 2013, Journal of Adolescence, (36), 3, 475-483.

Dear Donald:

As Lütfye say, a response to your question cannot forget the classical study by Latané and Darley (1970) on the murdering of Katherine in a New York street at a certain night.  Even though such murdering had been seen by many New Yorkers who saw it from the windows of their homes, no bystander left his/her home to help the woman who was being spanked by a murder. Accepting that bystander behavior depends on several variables. such as his/her mood, culture, moral development, and so forth, Latané and Darley stressed that bystander's behavior greatly depends on the number of bystanders at hand.

If, for example, one is in need of help, the greater the number of bystanders who see the needy person is, the less likely to be helped one is. Why is this the case? According to Latané and Darly's perspective, this is the because of a psychological process they called responsibility diffusion. In other words, when there are many bystanders observing a murdering, for example, each bystander thinks that one of them will intervene. As a result, one's responsibility to intervene is attributed to somebody else and, in the end, nobody intervenes.

So, the number of bystanders in a given situation has found to be a predictor of bystander intervention behavior. 

I hope I have got your question and that this helps.

Best regards,