Malcolm Gladwell: Why underdog David often beats Goliath

David and Goliath: it's the biblical tale of how an underdog took down a giant which has been repeated throughout history.

It's also the title of Malcolm Gladwell's provocative new book looking at why longshots succeed much more than we expect.

The BBC's Katty Kay spoke with Gladwell about his findings on why we don't always understand advantages and disadvantages.