The conventional reason for hyphenating words that temporarily work together as a single adjective is to avoid ambiguity.
Generations of young writers and editors have been advised by sadder but wiser colleagues that they should swim well clear of a man-eating shark. On the other hand, a man eating shark is likely to have a bag of chips dripping with salt and vinegar, and if you have the understandable desire to nick one as you saunter by, he seldom has teeth sharp enough to inflict a fatal wound — although there’s a growing school of thought that he’s justified in trying.
It’s sound advice. All the same, The Chicago Manual of Style concedes: “Where no ambiguity could result, as in public welfare administration or graduate student housing, hyphenation is not mandatory…”.
My question is: how often does the context of a sentence really leave room for significant ambiguity?
Consider these sentences.
A surfer was attacked by a man-eating shark near the beach yesterday.
At a restaurant, a man eating shark complimented the chef.
If the hyphen were removed from the first sentence would readers really be confused? Would they think a man eating a shark had either the ingenuity or perverse desire to attack a surfer at the same time?
More likely, it’s the man in the second sentence — eating shark in the grammatically prescribed manner — who is likely to make readers giggle when they recognise the possible ambiguity.
Here’s another example. In my second paragraph above, many writers would jump on sadder but wiser colleagues and hyphenate sadder-but-wiser with manic satisfaction.
Would hyphenation make the sentence more readable? It would take a properly designed survey of actual readers to settle this question (which no prescriptive grammarian is ever likely to do), but hyphenation may sometimes make reading more difficult.
For example, when a compound modifier falls at the end of a line of justified type, it might have to be hyphenated again if it runs onto the next line. An extra hyphen can produce grotesqueries that look like this:
Generations of young writers and editors have been advised by sad-
der-but-wiser colleagues that they should swim well clear of a man-eating shark.
Editing applications provide ways of fixing a bad break, as copy editors call such ill-placed hyphens, but when every compound modifier must be conscientiously hyphenated, this can consume an unconscionable amount of editorial time, with dubious gains for the reader.
Another problem is that many writers are actually confused about what constitutes a compound modifier. Their typical response is to hurl hyphens into the breaches and hope they fall in the right places.
Here are examples that I encounter constantly:
an 80 year-old man
an 80-year old man
For the record, it should read an 80-year-old man, since man is the noun being described and 80-year-old is the single adjective formed by the preceding words.
Would anyone be substantially confused if the phrase read an 80 year old man?
Writers should think a bit before a rule becomes a knee-jerk response, especially when they’re uncertain of how it should really be applied.