Good advice from Katedesign.
'Fashion' of course can refer to clothing, but it is also the embodiment of lifestyle that one leads, which is a collection of lots of elements: how people live, where, what they use to live (appliances), their social activity, social expectation and as KD said, the technology at the time.
'Fashion' isn't always positive either. Although it can push boundaries, it can also be trivial, throwaway and purely a thing of the moment that either has no impact on 'life' or that is an essential stepping stone in the process of development/engineering/life style.
(Crimplene and Rayon were advancements in technology (cheaper manufacture, more flexible to design with) but don't get too close to a fire!)
It isn't necessarily always just about/for women either.
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Advertisements are obviously a 'snapshot' or recording of history and so by recording events/styles in their very nature.
Not sure where I'm going with this, but as with all dissertation topics they are 'open' and offer obvious routes - i.e 'discuss fashion advertising' . But they also offer you a chance to look at them from a different angle which hopefully is where the marks are!!
Perhaps you should be looking at the role of the poster/the advert/the illustrator/the photographer as they would have been the 'recording' artist so to speak?