If its "it is", its "its" is "it's"
[This blog post has time travelled. It is from a previous blog that predates the very existence of this one.]
We often don't pay much attention to the humble apostrophe. It serves its purposes so well that it just sort of blends into the background, unnoticed and unloved. But it lies at the epicenter of an interesting and common cognitive bug.
The apostrophe serves several purposes (in English, anyway).
First, an apostrophe can indicate elided (left out) letters in contractions. When we lose the "o" in "does not", it is replaced with an apostrophe: "doesn't." Similarly "cannot" becomes "can't," and so on. The utility of this in modern English is debated; some styles prohibit contractions at all in formal writing. After all, "doesn't" replaces a mere two characters (the "o" and a space) with one (the apostrophe itself), and the savings in other contractions are similarly small. If the goal is saving space, abbreviations, acronyms, and other more powerful linguistic tools are better choices.
There are environments where saving a character or two matters a lot: the canonical example is print newspapers, whose narrow columns are prone to unfortunate line breaks. But for most writing, most of the time, the contraction is a stylistic choice. Where it really shines is conveying dialog; however much we may differ in expressing ourselves in writing, nearly everyone uses contractions in speech. It's difficult not to: the science fiction character Data from Star Trek supposedly cannot (or does not) use contractions, but the writers of the show accidentally included them with some frequency despite this. In writing, both fiction and nonfiction, contractions help convey how people actually speak.
Apostrophes help with dialog in other ways: they can convey a sort of inside-a-word pause like glottal stops or syllabic breaks, either when transcribing a word of foreign origin like "Sahai'a" or "Xi'an," or for ex-contractions where the original form has vanished from usage entirely, such as "o'clock." Science fiction writers love this for "alien" names; there are books where every other character's name is something like "K'th'q'dp'tl'nt'sr'thik." (For real-world speech which uses sounds atypical in other languages--such as that of the San people--all sorts of punctuation may be waylaid and repurposed for transcription.)
Probably the most common use of the apostrophe, however, is to show possession. The "girl's pet," the "shop's inventory," the "dog's bone." This use is so common that it's often misused, ala Dave Barry's joke that the use of an apostrophe is to indicate to the reader that an "s" is coming up.
And the misuses are interesting to me.
I'm a writer. Directly and otherwise, a significant portion of my income has always derived from my ability to convey information in written form; it's safe to say that I do this for a living. And yet, I cannot seem to get "its" and "it's" right in my first drafts--I'm so consistently wrong that checking it is the first item on my personal proofreading list. Heck, this article is literally about the use of "it's" and "its", and I've had to correct the mistake twice already.
Luckily for my writing ego, I'm not alone. I consume a lot of writing as well as producing it, and a surprisingly large percentage of the writers use the wrong form frequently. A quick google indicates that it's a problem for a very large number--perhaps even a majority--of "casual" writers of English. (It is, ironically, one of those mistakes made much less frequently by non-native speakers.)
It's not that I don't know the rule, and I suspect it's not that other writers don't know the rule, either. We teach this distinction in roughly third or fourth grade in the US.
My suspicion is that what we're looking at here is a conflict in how our brains deal with rules in general.
Grammar is hard. The rules of English grammar alone are enough to fill a very long book, and--largely because English has borrowed from so many other languages over time--the rules often have exceptions:
*"I" before "E," *
Except after "C,"
Or when sound is "long a",
As in "neighbor" and "weigh,"
None of which helps a bit,
With "weird" "foreign" words, like "counterfeit."
As an amusing(?) aside, a recent study found that the "'I' before 'e'" rule in English is actually wrong slightly more often than it's right. The exceptions don't just prove the rule, they outnumber it.
If we had to pin down every rule, exception, and usage for everything we say or write, we'd never get to convey anything. So we rely on sentence patterns, experience, and rules of thumb to get us through. These "rules of thumb" are simpler rules that--hopefully--fail us rarely enough that it doesn't matter. We can keep enough of these simpler rules "in our heads" to help us through day to day conversations and writing, and either live with or deal with the exceptions in a separate pass or with separate tools.
It's obvious that the particular rule set is at least somewhat personalized. Consider the sentence:
"Ameilia has less apples than Feifei."
From my casual surveys, about half the people who hear that cringe internally, and the other half don't give it a second thought. The difference--one I think interesting enough to have its own article--is whether or not your personal rule set includes "use 'fewer' rather than 'less' for countable items."
So, back to "it's" and "its." We have a few rules that apply here:
Rule 1: Use an apostrophe followed by "s" after a singular noun or pronoun to indicate possession. (And an "s" followed by an apostrophe for plural ones.): "Bob's hat," "one's speech," "the city schools' funding," "dogs' rights."
Rule 2: Use an apostrophe for left-out characters in a contraction: "do not" == "don't," "is not" == "isn't," "it has" == "it's."
The problem is that there are exceptions.
Rule 3: We don't use apostrophe-s for pronouns, such as "his" for "he," "their" or "theirs" for "they," or "hers" for "her."
Rule 4: Rule (1) doesn't apply If the plural form doesn't end in "s", instead, we pretend it's singular: "women's rights," not "womens' rights."
Rule 5: Rule (3) doesn't apply if there's not a specific possessive form of the pronoun: it's "one's thoughts," not "ones thoughts."
Worse, there is at least one case where the correct form depends on usage:
Rule 6: The form of "they" in the sentence "That's their dog" is different from the one in the semantically identical sentence "That dog is theirs." "Theirs" is a possessive pronoun in this case, but "their" is a pronoun-derived possessive adjective. Which is an incredibly subtle distinction, but--weirdly--one that native speakers almost never get wrong, even though probably fewer than one such speaker in a hundred could tell you why. (And I'm not that one person; I looked it up.)
So, that's a lot of rules so far. But why do people have trouble with "it's" and "its," but not the others, even in cases like rule (6) that are, on the face of it, much harder?
I'd argue it's because your brain has brought too many rules into play. Most possessive pronouns or adjectives have a form different from their base pronoun form: "His," not "hes"; "my," not "Is"; "their," not "theys"; and so on. Rule three only really matches two pronouns whose possessive and base forms differ only by the added "s": "her/hers" and "it/its."
Some quick googling indicates that there's actually a fair amount of folks out there that incorrectly use "her's," but nowhere near the level of "it's." I suspect that it's because "her" is a complex usage-based pronoun/adjective, similar to rule (6) above.
Rule 7: The feminine pronoun varies a lot based on usage:
"her" as a pronoun, but only as the object of the sentence: "I like her."
"hers" as a possessive pronoun: "That book is hers."
"her" as a possessive adjective: "Amy ate her lunch."
"She" as the pronoun if it's the subject of the sentence: "She likes me."
In other words, if you use she/her/hers without further thinking about it, you're almost certain to get it wrong. So your brain has prioritized this rule as one to be applied first when you're using feminine pronouns. Perhaps more relevantly, you've probably internalized simple rule 8:
Rule 8: "Her's" is never correct in any usage.
Because English doesn't make contractions from nouns used as objects of sentences, there's no contraction form of "her." "She has" or "She is" are both contractable--"she's" in both cases--but never "her has" or "her is."
At this point, you're probably sick of all these rules. And that's sort of the point; when your brain is trying to construct a sentence, it wants to get on with communicating, not go down checklists of grammatical minutia. And maybe more importantly, your brain evolved to listen, and to speak. Writing is comparatively modern, and even among those of us that do it professionally, we do a lot less of it than speaking and listening. In spoken English, there's no distinction between "its" and "it's" at all.
So, when faced with the decision for a form of "it," some of our brains pretty much give up after rules (1) and (2). In my experience, which rule gets applied tends to differ between people: I will incorrectly use "it's" when I mean "its," but almost never the reverse, so my particular grey matter gives up after rule (1). I see others who primarily err the other direction.
Why do non-native speakers have less trouble with this? Assuming that this statement is true (I have only anecdotal evidence to support it), my suspicion is because they're always bogged down in rules. Without a lifetime of internalizing patterns, you're basically forced to construct sentences by brute force, and so you're likely to spend more time with the internal checklists. It would be interesting to know whether or not this error starts to creep in as non-native speakers approach native fluency; I have no idea whether it does or not.