As far as I can tell, this originates from David Ogilvy in Confessions of an Advertising Man in 1963, where he states
'On the average, five times as many people read the headline as read the body copy. When you have written your headline, you have spent eighty cents out of your dollar.'
I don't have a copy of the book, so I can't see the context and see if he was saying this based on his experience, or if he ran any tests to see if this was true. Nor can I easily find anybody that has put this to the test.
Are we really blindly repeating what someone said 50 years ago in an entirely different medium?
How many of us look to find the source of a statistic or the context?