Complementarity is the supposed idea/tendency that in a relationship, the two people will complete what is missing in the other. In more common words this is “opposites attract.” The book gives the example of loners pairing with socializers, risk-takers pairing with the cautious, and so forth.
Although the book does not agree that this is the case in real life, media leads us to believe that it is. One example I thought of was the movie Enchanted.
In the real movie (above is just the trailer), both Prince Charming and eventually Patrick Dempsey both are chasing after Giselle. Prince Charming is a fantasy character, is perfect and is just like Giselle. But [SPOILER ALERT], Giselle ends up with Patrick Dempsey who is very different from her. He’s rougher, he’s real life and he’s from New York. Fairy tales and New York just don’t mix that well. The movie provided the option of a guy just like her (Prince Charming) and a complementarity guy (Patrick Dempsey). As can be seen from this example from Enchanted, complementarity is spread by the media even if it does turn out to be false.