Book excerpt: Penguin Specials are designed to fill a gap. Written to be read over a long commute or a short journey, they are original and exclusively in digital form. This is John Gapper's foray into the world of rogue traders. Unlike most bankers, they are household names: Nick Leeson of Barings, Jerome Kerviel of Societe Generale, John Rusnak of Allied Irish Bank. And now the 31 year-old Kweku Adoboli, who allegedly ran up $2.3bn in losses at UBS. These are the men who have bought banks to their knees and global financial systems to a halt. Each time the banks declare themselves to be innocent victims of a fraud. But why do traders keep on committing apparently senseless crimes, with little benefit apart from higher bonuses and a risk of ending up in prison? And why do banks, which should have learned the tricks of the traders, keep being deceived? In this Penguin Special, the Financial Times' associate editor John Gapper unlocks the mystery by delving into the evolutionary risk-taking instincts of both humans and animals - from yellow-eyed junco sparrows in Arizona to honey-bees. He reveals how banks encourage their traders to evade risk limits, and shows how the rogue traders merely mimic the strategies used by their firms to seem more profitable than they really are. A rogue trader is often an outsider who starts in a lowly role and gambles with a bank's money in a bid to become a star. Gapper traces patterns of behaviour and personality that could be used to catch them before disaster strikes. But do the banks really want to? And are the rogue traders just the symptoms of a financial system gone rogue?