When I asked the question "do you dress up for Halloween", I received a couple of unexpected answers. From the comments:
Manjari: They don't sell costumes for adult women that aren't bedroom wear.
Alice: …the costumes are all too trashy these days. Sexy Ragdoll? 70's Afroqueen? Gimme a break.
Looking at most of the costume pictures I found – I didn't do extensive research but still – I think we're on to something. Look at these:
This one is listed under "Women – Occupational":
2 Piece Black Fire Woman Costume

What's her occupation, stripper? This looks like, um...what's the word? Porn. Plus that is OBVIOUSLY a white lady (I know, bad joke).
This is under "Women – Period":
3 Piece Indian Girl Costume W/ Tomahawk

Again – this looks like something from "Zack and Miri Make a Porno". "Poke-Her-Hontas", anyone? Plus the proper term is "native American", everyone knows that.
Even the pic I used on the "do you dress up for Halloween" is clearly meant to be suggestive, although the costume covers far more skin. Compared to some of the Fire Lady and the Tomahawk Queen, it's practically a burka.
The men's costumes from the same store (Ricky's in NYC) are much less revealing.
After an admittedly quick perusal I found only one semi-nekkid costume:
Borat Mankini Green Lycra

The others are mostly clothed -- police officer, etc.
Now, as a guy, sure, I find the porno lady costumes pleasant to look at. And I'm not going all moral majority on you. But who is buying these things? Am I off base here? And when did this trend start?
Plus, won't those women be chilly? It is October, after all.
Source: Ricky's Halloween, Babble commenters
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