As a Music Educator (including Guitar Education) of well over 30 years, I believe that Spaw has it right! I am wondering where this "Sunday paper" got their figures. DMcG also makes a very good educated guess about likely equal proportions. The difference being that the American parents of the "Baby boomers" especially in the late 1950's-60's, & the "Baby boomer" parents seem to be more likely to spend $$ on non-necessities to make their children "happy." Thus, there may be somewhat more Americans who at one point or another had a guitar.
   There is one popular theory which maintains that about half of the guitars bought by these same American parents are simply being recycled through many hands who, as Spaw aptly put it: "screwed around with it" & then got dusgusted with painful stinging fingertips & the fact that they didn't sound exactly like the recording of their fav band, before the it got passed on to other hopefuls via garage/yard sales, pawn shops, sleepless parents of teenagers, etc.
    Another theory suggests that wives of guitar luthiers & fathers-in-law of guitar players romp about the countryside grabbing up all the old, cheap, unused excess guitars & burn them in bonfires at their gatherings. One version of this theory purports that ex-wives
& soon-to-be-ex-wives (& aforementioned fathers-in-law) of guitarists with few gigs & no "real job" buy them up for the purpose of lying in wait for the aforementioned guitarists in order to "El CaBong" them. Visi d'arte. Visi d'amoure.