I truly believe that human beings are hard-wired to be essentially egotistical. Whenever anything happens in our lives, we ask ourselves, either consciously or unconsciously, "what's in this for me?"
In terms of stepping in to render assistance or to react to observed wrongs, therefore, the answer that the person comes up with to that question probably dictates what they do. Many people fear the negative consequences of getting involved, such as being harmed themselves or even just inconvenienced by the need to take time out of their schedules. Many people fear they will be forced to take subsequent action that will jeopardise their reputation, safety or other important personal factors. In such instances, "what's in it for me" is all negative and limiting, and hence the person is probably unlikely to want to get involved.
But for some people, "what's in it for me" might be the chance to right an obvious wrong, to save or assist another human being, to speak up about something they feel passionate about, or any number of other tangible or intangible benefits. Where the likelihood of positive personal outcome outweighs the likelihood of negative outcome, the person is therefore more likely to get involved.
I know this is perhaps a cynical and harsh view of human motivation, and needless to say I don't believe in altruism in the popular sense of the word, but I honestly do believe that we are fundamentally motivated in life by our own best interests, and the extent to which we are seen to be helpful of others is largely dependent on whether our perceived best interests are in line with those of others.
For anyone who is victimised, but particularly for a child, to have the abuse wilfully ignored by others is, I believe, one of the most damaging contextual factors that can turn any potentially traumatic event into a long-term trauma. For a child, I believe it is sometimes irreversibly damaging for the child to interpret this lack of action as a mark of the unimportance of their safety, the fact that they don't matter, have no rights, deserve the abuse, are hateful and shameful people, and any number of other core beliefs about themselves.
I do truly believe that those who stand by and do nothing are as inexcusably guilty, though in a different way, as those who perpetrate the abuse. There is a special place in hell for such people.
Maddog