When Kitty Genovese was murdered, her family and the wider world was told that bystanders watched, but did nothing to intervene. Psychologists tried to explain this callous inaction with a popular theory - the "bystander effect".

Dr Laurie Santos was taught this theory - that most people won't in step help - but talking to Kitty's brother and Lady Gaga's mother she reveals that the "bystander effect" is wrong. People do like helping out, and we get a happiness boost from being kind. So how do we encourage more bystanders to intervene?