The private museum has become a phenomenon of the 21st century. There are some 400 of them around the world, and an astonishing 70% of those devoted to contemporary art were founded in the past 20 years. Although private museums have been accused of being tax-evading vanity projects or 'tombs for trophies, ' the picture is far more complex. Georgina Adam's investigation into this extraordinary proliferation, based on her recent visits to over 50 private...