Why Does S-Backing Happen?
Two of the most fundamental questions about these sound changes are why they happen in the first place, and how they spread to new speakers. So why does S-backing happen? Several linguists have offered opinions. One possibility is that it’s a continuation of the same sound change that turns those T’s into chuh /ʧ/ sounds in words like truck. The S is turning into shuh /ʃ/ to be more similar to the /ʧ/ that comes right after it, a process called assimilation.
However, linguist Michael Shapiro argues against this idea. He points out that the T-to-/ʧ/rule that operates on TR clusters, like those in the word truck, has an exception: It doesn’t happen in STR clusters. If I put an S in front of truck to get struck, I pronounce it as [str]uck, not as s-ch-truck [sʧr]uck. This is true for many S-retractors, too, who pronounce it as shtruck [ʃtr]uck, not as shchtruck [ʃʧr]uck, a fact noted by linguists Richard Janda & Brian Joseph. Shapiro’s solution is to say that the T in the middle of the cluster is irrelevant, and that the S turns into shuh /ʃ/ to be more like the R that comes at the end: After all, both /ʃ/ and R are pronounced with the tip of the tongue pulled back somewhat. But yet another linguist, named Wayne Lawrence, argues that assimilation doesn’t work that way, skipping over some consonants in order to affect others.
The most likely possibility comes from still another linguist, named David Durian, who argues that instead of assimilating to a sound that comes later, the S is assimilating to a sound that comes earlier. In his study, he found that the words that were pronounced with a retracted S most often were those where the STR cluster came after the vowels /i/ or /I/, in words like destroy (deSHtroy) and restrict (reSHtrict). This is a very reasonable kind of assimilation, because these vowels are pronounced with the tongue raised high inside the mouth, and so is /ʃ/—at least, higher than it is for plain old S.
Why Do Pronunciation Changes Spread?
But wait a minute! I hear you saying. What about all those words that begin with STR? This gets into the second question, of why sound changes spread. Janda and Joseph argue that although a sound change begins with some kind of phonetic condition, if it hangs around long enough it begins to be affected by other things.
In this case, speakers seem to have generalized it from STR after certain vowels to STR anywhere—sometimes even across words. In fact, I even heard Michelle Obama, who does a lot of S-retraction, do it in the phrase parents trying, which she pronounced as parent[ʃ] trying. Janda and Joseph note that for some speakers, S-backing shows signs of generalizing even further, to any consonant cluster beginning with S. Examples they’ve heard include [ʃ]till, [ʃ]creen, and under[ʃ]tand.
Pronunciation Has a Social Aspect
In addition to spreading to different phonetic contexts, there’s also a social aspect to how sound changes spread. Durian’s study in Columbus, Ohio found that it correlated with a speaker’s pride in living in or having lived in an urban area, as well as with age: Younger speakers do more S-backing than older ones.
One more linguist, Kathryn Campbell-Kibler, looked at the social aspect from the opposite direction: How do others perceive speakers who have S-backing? Campbell-Kibler found that varying combinations of S-backing, S-fronting (otherwise known as lisping), and so-called G-dropping (for example, fishin’ instead of fishing) could affect whether a speaker was perceived as intelligent or unintelligent, gay or straight, from the American South, or from the country. One commenter on a blog post that I wrote on this subject said that S-backers were perceived as having lower social class in the UK; but here in the United States, another commenter reported hearing it from NPR reporters and on The Real Housewives of Atlanta.
So whatever generalization you’re tempted to make about speakers that use S-backing, don’t take it too seriously, because it’s probably more complicated than you think. And now that you know about S-backing, get ready to hear a lot more of it in your lifetime, as it gains [ʃ]trength and makes [ʃ]trides.