Foil
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Character Role Analysis
Ted and Joanna
In 1979, men and women had clearly prescribed cultural roles that are a little more, uh, prehistoric than they are now. In the simplest terms, men were providers who brought home the bacon. Women were the ones who cooked the bacon, at home, probably with a rug-rat or two at their ankles.
Ted and Joanna are foils for 99% of the movie because they disagree about these gender roles. In the beginning of the movie, Ted's totally into it. He goes out and climbs the corporate ladder at his ad agency; Joanna stays home and takes care of Billy. All's well in Ted's world. Joanna, on the other hand, is like "Nope, I'm more than just Ted's wife and Billy's mom—and I'm gone."
By the end of the film, Ted and Joanna have changed places ideologically, but they're still on opposing sides of the whole late '70s gender roles issue. Joanna thinks she deserves primary custody of Billy, even if she left for 18 months, by virtue of being his mother. Ted, meanwhile, is where Joanna was at the beginning of the movie. He doesn't think there's any reason why a woman is a better parent, or is supposed to be the primary caregiver, just because she's a woman.
The Kramers don't get on the same page until the very last scene of the movie when they both agree that what's best for Billy is calling Ted's apartment home base.