Do people change their behaviour just because other people do? In his new book 'Positive Linking' (http://www.faber.co.uk/work/positive-linking/9780571279203/) economist Paul Ormerod explores how networks - models for human society's interactions - impact on economic policy and behavioural trends, explaining along the way the '50 Shades of Grey' phenomenon, and why Manchester United has become globally successful whilst near neighbour Rochdale AFC is forever floundering.