Freakonomics did a great podcast about this a few years ago as well: http://freakonomics.com/2011/10/10/where-have-all-the-hitchhikers-gone-a-new-freakonomics-

Freakonomics did a great podcast about this a few years ago as well: http://freakonomics.com/2011/10/10/where-have-all-the-hitchhikers-gone-a-new-freakonomics-radio-podcast/ http://freakonomics.com/2011/10/10/where-have-all-the-hitchh...