Some pet linguistic peeves are indulged, I find, not for reasons of clarity or grammatical soundness, but out of petty pedantry, habitual curmudgeonliness, or some kind of character disorder. On the other hand, I've been accused – affectionately, I hope – of excessive tolerance in such matters. But I have peeves of my own, one of which is the confusion over its and it's.
Lynne Truss considers this "the greatest solecism in the world of punctuation". In Eats, Shoots & Leaves she writes that it "sets off a simple Pavlovian 'kill' response in the average stickler," and goes on to fantasize – satirically, one hopes – about lightning strikes, hacking, and unmarked graves.
My instincts are less violently judgemental. I don't get wound up over its/it's confusion – but I often wince at it, particularly when it appears in edited prose. So, I imagine, does Roger Ebert:
To summarize the difference: it's is always a contraction of it is or it has. Usually the former. Keep this front and centre, and you'll greatly reduce your chances of ever getting it wrong. Its is the possessive form of it – more fully, the third person neuter possessive pronoun. So you might write of a solitary ant: "It's lost its way."
It's not just students, bloggers and learners who mix up its and it's, but also people for whom words are central to their trade – journalists, broadcasters, reviewers, professors of law, and so on. I even saw a lexicographer slip up. Evidently it's a major source of confusion – a mistake so common as to be virtually normalised. But I'm an editor with a hero-of-Haarlem complex, so I feel duty bound to do what I can. Lawrence Lessig almost put it well:
A look at the causes might shed some light. There are, I suspect, three main reasons for the confusion. One is that its is an exception to a well-known rule: Add apostrophe-s for possession. Hence the ill-advised leap from, say, the dog's tail to *it's tail. Another reason is contagion: the mistaken forms are very prevalent, and their every appearance reinforces the wrongness. The third main reason is that many people don't care.
Apparently, iPhones auto-correct its to it's, which might explain Mr Lessig's lapse. A friend on Twitter thinks this faulty auto-correct feature is responsible for a fair proportion of the confusion on Twitter. She's probably right. It's also worth noting that its and it's have quite a tangled history.
Maybe you'd consider its/it's confusion a negligible matter – the pet rock of pet peeves. It rarely leads to misinterpretation, and it sure doesn't amount to much on a cosmic scale. But careful readers will notice the mistake and consider it a sign of inattention, sloppiness, ignorance, or even illiteracy – especially if it's repeated. So if you value good communication, it's a distinction you ought to make, and make consistently.
That its/it's mistakes occur in the prose of reputable publications and careful writers shows how easily confusion creeps in. But with vigilance and application we can defend ourselves from it. If you're prone to this mix-up, even occasionally, you might want to condition yourself to observe the distinction. Here's a way. For as long as I can remember, I've habitually read erroneous it's as it is. So, for example, in The Guardian recently I saw the following:
Semantically, I inferred what was meant, but in parallel I processed the absurdities ("it is companion book"; "it is environs"). Doing this for years has given me a kind of immunity, I think, by steadily and deeply embedding the rule in my nervous system. I'm very strongly sensitised to it. Accepting it's as its would undermine this conditioning, so I don't.