Why do humans bully, why do some do it and others allow it? Are bullies born or do they learn their bullying? Mike Williams speaks to anthropologist Christopher Boehm about links between the bullying behaviour of our ape ancestors and our own behaviour. He also speaks to author Helene Guldberg about the challenges defining the term as well as performance poet Shane Koyczan about his experience being both bullied and being a bully.(Image of a teenage boy bullying another boy. BBC Copyright/Corbis Royalty Free)