Bystanders are more likely to intervene in low ambiguity, insignificant consequence situations than in high ambiguity, significant consequence situations. Latané and Rodin (1969) suggested that in ambiguous situations, bystanders may look to one another for guidance, and misinterpret others' lack of initial response as a lack of concern. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.