I Used To Know This Stuff

I use to know how to use apostrophes, but after two decades of marking (two decades' marking?) I've lost the sense of flavour.

Take this:

• "1970s gender expectations"

The author suggests an apostrophe, but I think they want:

• "1970's gender expectations"

but we're not just talking about 1970 - which pedants might argue isn't actually in the 1970s - we're talking about the whole decade:

• "1970s' gender expectations"

which looks a little wrong, and since I suspect the 1970s is singular rather than plural, a case might be made for:

• "1970s's gender expectations"

which is horrible. As far as I can make out from Fowler II, "1970s" is treated an adjectival noun, and thus doesn't necessarily need one.

• "1970s gender expectations"

may do, after all. It doesn't help that I need an MLA styleguide for this - assuming it answers such a question.

I could just rewrite:

• "gender expectations of the 1970s"