
From the 1997 television program Stephen Hawking's Universe http://www.pbs.org/wnet/hawking/html/home.html
Unsourced variant: All of my life, I have been fascinated by the big questions that face us, and have tried to find scientific answers to them. Perhaps that is why I have sold more books on physics than Madonna has on sex. This quote seems to combine the above sentence from Stephen Hawking's Universe with a statement from the Foreword to The Illustrated Brief History of Time: As Nathan Myhrvold of Microsoft (a former post-doc of mine) remarked: I have sold more books on physics than Madonna has on sex.