Why do we say "You Scratch My Back and I'll Scratch Yours"?

Meaning:

I'll do something for you, if you do something for me.

In Latin: Quid pro quo ("something for something")

Background:

Sources say that this expression traces back to Montaigne - presumably by which they mean Lord Michel Eyquem de Montaigne, one of 16th century France's leading philosophers who is known for popularizing the essay as a literary format and for using casual anecdotes to express serious intellectual insights.

It is first attested in the USA in Artemus Ward, His Book, written by American humorist Charles Farrar Browne (1834-1867) using the pen name of his character Artemus Ward, an illiterate rube with Yankee common sense. Browne published the first of the Artemus Ward series in 1858, in The Plain Dealer (Cleveland, Ohio). In collected form, the series achieved great popularity in both the U.S. and Britain.

Brown is considered America's first stand-up comedian and was very popular as a speaker and writer. He died tragically young of tuberculosis.


To the Editor of the——

Sir—I'm movin along—slowly along—down tords your place.  I want you should rite me a letter, sayin how is the show bizniss in your place.  My show at present consists of three moral Bares, a Kangaroo (a amoozin little Raskal—t'would make you larf yerself to deth to see the little cuss jump up and squeal) wax figgers of G. Washington Gen. Tayler John Bunyan Capt Kidd and Dr. Webster in the act of killin Dr. Parkman, besides several miscellanyus moral wax statoots of celebrated piruts & murderers, &c., ekalled by few & exceld by none.  Now Mr. Editor, scratch orf a few lines sayin how is the show bizniss down to your place.  I shall hav my hanbills dun at your offiss.  Depend upon it.  I want you should git my hanbills up in flamin stile.  Also git up a tremenjus excitemunt in yr. paper 'bowt my onparaleld Show.  We must fetch the public sumhow.  We must wurk on their feelins.  Cum the moral on 'em strong.  If it's a temperance community tell 'em I sined the pledge fifteen minits arter Ise born, but on the contery ef your peple take their tods, say Mister Ward is as Jenial a feller as we ever met, full of conwiviality, &the life an sole of the Soshul Bored. Take, don't you?  If you say anythin abowt my show say my snaiks is as harmliss as the new-born Babe.  What a interestin study it is to see a zewological animil like a snaik under perfeck subjecshun!  My kangaroo is the most larfable little cuss I ever saw.  All for 15 cents.  I am anxyus to skewer your infloounce.  I repeet in regard to them hanbills that I shall git 'em struck orf up to your printin office.  My perlitercal sentiments agree with yourn exackly.  I know thay do, becawz I never saw a man whoos didn't.

Respectively yures,               
A. Ward.

P.S.—You scratch my back &Ile scratch your back.

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