If you’re a native speaker of English, you are no doubt familiar with two meanings of since, which I’ll refer to as the “time” usage (1a) and the “reason” usage (1b):

(1a) Bob Patel has owned and operated the beach motel since 1983.
(1b) Carton says they will first escort his sister home, since he wants Mr. Cruncher to come with them [...]

It’s odd, though, because I keep seeing people insist that only one of these common usages should be accepted. The first, as far as I’m aware, is accepted without complaint in all quarters. But the reason-usage certainly raises some hackles. For example, Jesse Kornbluth writes:

“SINCE and BECAUSE [are] not synonyms. ‘Since’ only refers to time: ‘Since August, he’s been in a funk.’ It cannot be used to suggest causality: ‘Since he’s depressed, we never call him.'”

I bolded the end of that complaint, because it’s obviously untrue; since certainly can be used to suggest causality. Kornbluth just did so. He means that it oughtn’t to be used in this way, of course, but for someone who subtitled his piece “Ten usage and grammar errors that could (or should) cripple a career”, Kornbluth is being surprisingly cavalier about his modals.

I digress. The point under debate here is whether since is acceptable in the reason-usage. Let’s start by noting a prominent writer whose career Kornbluth figures could or should have been crippled by his usage of since: Shakespeare. From The Comedy of Errors (via the OED):

Since that my beautie cannot please his eie,
Ile weepe what’s left away.”

And it’s not just Shakespeare. This reason-usage of since is antedated to the mid-1500s in the OED. Paul Brians and Bryan A. Garner (in the Chicago Manual of Style) track it back at least to the 14th century. So older English writers didn’t see a problem with it. Most modern writers don’t either; if they did, Kornbluth wouldn’t have anything to complain about.

In fact, though this is a persistent myth, I’m having a heck of a time finding major sources pushing for it. None of the usage guides on my shelf mention it, not even the ones that seem to be composed entirely of unanalyzed pet peeves. The MWDEU notes that this is a newer complaint, and one that seems to replace an older preference for since over because in this context.

I suspect the rule has come from stylebooks. The American Psychological Association’s stylebook, for instance, bans the reason-usage, and reports that this is the fifth most violated rule in their book. The Guardian also bans it, though the AP and Chicago Manuals don’t. The Economist appears to embrace it; I see no entry on since, and the guide itself employs the reason-usage in a discussion of stanch and staunch.

For stylebooks that do ban the reason-usage, the stated concern is primarily one of ambiguity between the time- and reason-usages. Sometimes that’s a valid concern. Compare these two sentences:

(2a) Since you left, I haven’t eaten; I’m still stuffed from our meal.
(2b) Since you left, I haven’t eaten; you took the forks with you.

These start off ambiguous, and in some situations that could be bad. But these ambiguities are pretty restricted. Since has to introduce a clause (not a time/date as in (1a)), and it generally has to be in a past tense (not the present as in (1b)). Furthermore, the effect of the ambiguity is often small. For instance, consider the ambiguity in this sentence from the MWDEU:

“In a second term, Carter might have moved the course of government toward the left, but since Reagan won the election the nation’s political movement has been toward the right”

I have a hard time distinguishing between the two meanings in this sentence. I suspect that since is intended to hit a midpoint between correlation and causation here, a sort of each-influenced-the-other situation; Reagan wouldn’t have been elected without some rightward shift, but the rightward shift wouldn’t have taken off without Reagan’s election, either. This fits with the intuitions of both the MDWEU and Garner, who note that since expresses causation more mildly than because does.

If you’re worried about the ambiguity, go ahead and avoid since in place of because. No one’s going to get mad at you for not using since. And when ambiguity is intolerable, maybe it makes sense to avoid it. But in general, English users haven’t encountered much trouble from this tiny ambiguity over all these centuries since its emergence. So don’t mistake it for a rule of English — and since it isn’t one, don’t judge others for using since in this way.

Summary: Since can be used with more or less the same meaning as because, although it’s less emphatic about the causal relationship. This can be slightly ambiguous, but only under certain conditions. You can avoid it if that concerns you, but it’s perfectly acceptable to use since in place of because.

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