I’ve written before about a comma(,) which muddles meaning, and a comma with restrictive which. The first was in a newspaper editorial, the second in a de Maupassant translation; both were inserted seemingly because of an unfortunate belief in the bogus rule about that and which.
Here, inevitably, is another example, this one in bell hooks’ book All About Love:
Ultimately, though, the authors remained wedded to belief systems, which suggest that there are basic inherent differences between women and men.
The appearance after belief systems of a comma followed by which induces a pause and primes the reader to expect a relative clause about the consequences of remaining wedded to belief systems in general. But that’s not what happens.
First, uninflected suggest implies that the antecedent (what which refers to) is plural, i.e., belief systems, not the (singular) fact that certain authors remain wedded to them. Then the rest of the line shows that the writer is talking about specific belief systems.
So the relative clause is restrictive – the type of belief systems referred to is semantically restricted. Adding a comma makes it non-restrictive, which makes no sense here. I had to reread the sentence to parse it properly, this time ignoring the misleading comma.
Maybe the writer used which rather than that to avoid repeating that (“belief systems that suggest that”), though of course the second that is optional. Or maybe she just preferred which there. Either relative pronoun would have been fine. Adding a comma was not.
Again I’m inclined to think the comma was added by an editor who remains wedded to a belief system* which misinforms them about the grammaticality of which in restrictive clauses in all varieties of English.
Further reading: a discussion among editors on the that/which pseudo-rule. Or see the links above for more detailed discussion.
* Note the complete lack of a comma here. Savour it.