In a survey of attitudes to disputed usages in English, respondents were presented with three sentences and asked the following questions about them: “Is it acceptable in English today, would you use it yourself? If so, where and when? If not, why not? If you think the sentence is unacceptable, why would that be the case? Do you ever hear (or see) people using it? What kind of people? Do you object to anyone using it?”
One of the three sentences was “I could of gone to that party”. This use of a modal verb + ‘of’ in writing is well known. But notice that one of the questions in the survey was “Do you ever hear people using it?” (my emphasis).
When you look at what people have written, you can see beyond any doubt whether they’ve used ‘could have’ or ‘could of’. But what happens when you hear them speak? They might say:
1 /kʊd həv gɒn/
2 /kʊd hæv gɒn/
3 /kʊd æv gɒn/
4 /kʊd ɒv gɒn/
If you hear 1, 2 or 3, you can conclude that they’ve said ‘could have’. If you hear 4, you can conclude that they’ve said ‘could of’. But 1, 2, 3 and 4 are all unusual, in fact – especially 2, 3 and 4. (/kʊd ɒv/ is somewhat more likely without a following participle: “I could of.”) Much more usual are 5 and 6:
5 /kʊd əv gɒn/
6 /kʊd ə gɒn/
In both these cases, 5 and 6, you can’t draw any conclusions about whether a speaker said ‘could have’ or ‘could of’, since /əv/ and /ə/ are weak forms of both ‘have’ and ‘of’.
So, for ‘could of gone’, the question “Do you ever hear people using it?” is, mostly, impossible to answer. Nevertheless, in the results of the survey, reported in English Today 116 (December 2013), 19% of respondents thought that the ‘could of gone’ option is acceptable in informal speech, and therefore, presumably, 81% thought not. One of them reports hearing ‘could of’ used “very often”. It would be interesting to know what these respondents based their judgements on, since they can’t have based them on what they actually hear.
Perhaps there’s a clue to the answer in some of the comments they made. Here’s a sample:
“I would not object out loud to anyone using it but I would note their lack of education and file them as of a lower class than me.”
“In their spoken English these words [‘have’ and ‘of’] sound the same. […] I would guess the speaker would be lower class and at secondary level maximum as far as education is concerned.”
“A truly horrible example of chav-speak at its worst.”
These respondents appear to be making inferences about class and education from what they hear. But is it possible that the direction of inferencing is actually the opposite, and that the perception of a speaker as being ‘lower class’ or ‘uneducated’ – perhaps on the basis of appearance, behaviour, pronunciation or use of English in general – tends to prompt the interpretation of their /kʊd əv/ as ‘could of’?
It’s also interesting that the survey questions prompt respondents to think in terms of what’s ‘acceptable’ or ‘unacceptable’. The notion of ‘acceptability’ in discussions of language is commonplace, but what does it really mean? If you get sold a product or a service that you think is unacceptable in some way, you can generally complain about it and ask for a refund or a replacement, etc. If you’re marking a language exam, you’ve got a set of marking criteria that tell you what’s acceptable and what isn’t, and you allot or withhold marks accordingly. But in everyday life, what can someone do if they think something someone else says or writes is, on purely linguistic grounds, unacceptable or even objectionable?

When a survey asks people about a usage’s “acceptability” without allowing for different registers, it foregrounds (and maybe implicitly privileges) formal standard English and leaves no room for non-standard variation. “Appropriateness” might be a more constructive criterion for a survey to explore.
Usage manuals note that would of, could of, etc. are used in writing to convey dialect or lack of education (“an unlettered persona”, as MWDEU puts it). Since I started looking out for the usage I’ve seen would of & co. in a lot of books from a wide range of authors, including Sylvia Plath, Patrick O’Brian, Dashiell Hammett and Carson McCullers. I disliked it at first, but I’ve gotten more used to it.
Stan: Thanks. So when these authors put ‘would of’, ‘could of’ etc. into their characters’ mouths, they aren’t necessarily saying “This character uses the strong form of ‘of’ in these constructions”; they’re saying, rather, “This character is uneducated, or a ‘dialect’ speaker.”
If so, it’s rather like the use of the spelling ‘wot’ for ‘what’ in comic strip speech bubbles. I can’t see how ‘wot’ can represent a different pronunciation from ‘what’, so presumably it’s just intended to contribute to the characterisation of the speaker as uneducated, unsophisticated, etc.
Jonathan: Right. Probably in some cases it’s an attempt at authentic ‘eye dialect’, but in others it could be just a shorthand device for indicating a certain characteristic. There’s no way to know for sure what they intend by it, short of asking them.
For you and for me “wot” evidently sounds the same as “what” – but not everyone speaks (or hears) with the wine/whine merger (https://en.wikipedia.org/wiki/Phonological_history_of_wh#Wine.E2.80.93whine_merger). My guess is that the “wot” spelling at least historically and for some writers refers to a real phonetic distinction with social connotations.
“Woz” for was is an analogous example that’s independent of the wine/whine merger (which, incidentally, my dialect doesn’t have).
It is, or any phonetic referent is obscure (to me at least). Sorry for the mistaken assumption about your dialect – I don’t see how I leapt to that conclusion from your comment, but evidently I did.