Moe, Larry, and Curly
Moe, Larry and Curly
The answer has more do to do with where you learned to write and where you are being published than with grammar itself. Much punctuation usage is a matter of publication style, not general grammar rules. Beware! The border between the two can be a dangerous zone, as borderlands so often are.
In the case of comma usage within a series of elements, it's all up to publication style. U.S. newspapers, magazines, and periodicals mainly follow the AP Stylebook, which prescribes no comma before the and in series such as the one that started this sentence. U.S. book publishers, following the Chicago Manual of Style, usually prescribe the use of the serial comma after the second element and before the third. That's not true in the UK, where book publishers generally do not use the serial comma. It's often known as the Oxford comma there, because Oxford University Press is one of the few book publishers that does use it.
Why this should be so is a matter of some conjecture. Personally, when I reflect upon the old days of hot metal type and narrow newspaper column widths, I suppose that every character counted and eliminating commas left more room for letters; thus, the journalists' disdain for the serial comma. I have no hard evidence for this. However, consider http://twitter.com, the latest digital communication craze, and its iron-clad 140-character format. As one struggles for maximum compactness of communication, it is very evident how lack of space could persuade one to permanently drop a punctuation mark (and to come up with some extremely strange abbreviations).
The reason adduced for the superiority of serial comma usage - no matter what Mrs. Stearn taught you in the tenth grade - has to do with clarity of meaning, the "prime directive" of grammar and punctuation. The perhaps apocryphal book dedication, "To my parents, Ayn Rand and God," makes clear the dangers of omitting the serial comma. Authors may on occasion be divas, but few actually claim to be semidivine. The demigod author derives from an easy misreading: The two elements after the comma can read as an appositive to the first element. (In other words, author Ayn Rand and the Supreme Being are the parents.) Other examples exist, especially when a modifier applying to the first element or two does not necessarily apply to the third: Think about studying medieval history, modern politics, and economics. To which era do your economic studies belong? Does modern modify both politics and economics, or only politics?
So, good reasons exist for preferring the use of the serial comma, but decades of publication style and preference may argue against it. The best rule is to pick one way and stick to it for each published piece, for it is variations from pattern that will divert the readers' attention to the punctuation rather than the substance. No one wants a reader to stop and note, "They put a comma here but not here; why is that?" Unless, of course, that reader is a reader of this blog, who now knows the answer.