About a fifth of the global bond market trades at negative yields. FT statistics journalist Federica Cocco explains why, and what the attraction is for investors. See if you get the FT for free as a student (http://ft.com/schoolsarefree) or start a £1 trial: https://subs.ft.com/spa3_trial?segmentId=3d4ba81b-96bb-cef0-9ece-29efd6ef2132
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