What factors determine whether or not a bystander will intervene in an emergency?

Bystander behaviour can be classified into two distinct categories; pro-social and anti-social behaviour. Pro-social behaviour is often referred to as altruism, although this definition is not quite accurate. Altruism is the purest form of pro-social behaviour, and it refers to the act of helping someone, or some cause, with no perceived benefit to the helper. However, there are many people who believe that true altruism is just a concept. For example, some would say that when the Princess of Wales was involved in all her charities, helping aids victims for example, that she had no benefit from this, and was therefore being altruistic. However, apart from the publicity that was generated (which she may or may not have cared about), it almost undoubtedly left her with a feeling of self-worth that she had made a difference, no matter how small or large that difference was….(short extract)