Do the filmmakers not realize that an international super-spy is too busy to be alternately curling or straightening her hair all the time? All those materials would not fit in a modest, helicopter ready travel bag. Almost more unbelievable than Asgardians. Sub-question: did they not have hair ties in the exploitative Russian baby spy program?
closed as primarily opinion-based by DVK-on-Ahch-To, System Down, The Fallen, K-H-W, HorusKol Feb 3 '14 at 4:11
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Ok, this seems like a bit of a strawman question, but I'll answer it anyway.
Firstly, lets look at the Black Widow on screen, and assume that the release dates of the movies follow real time.
- 2010 - Iron Man 2: Natasha needs to infiltrate Stark Industries posing as a lawyer and personal assistant to a CEO who is a known egotist. Looking good is a prerequisite.
- 2012 - The Avengers: The first we see Natasha is coming of an infiltration of an upper-echelon Soviet Military event. Looking good is a prerequisite. On top of that, we can assume the events of The Avengers take place over a few days, so she mightn't have had time to "reset" her hair - some hair dos can be easier to maintain than remove.
- 2014 - Captain America: The Winter Soldier: So far we've only seen the trailer, but in the airdrop before Steve Rogers jumps out, they have a casual office talk about setting him up with someone in the typing pool. Normal office banter (albeit at 10,000ft). This is the first we see of Natasha being 'casual' or 'normal' (as opposed to her saving the world, and her hair is either loosely tied back or out in the official stills.
On top of that, its 4 years between her being on screen, so when not on an espionage mission, a few hair changes doesn't seem excessive.