This is a lovely experiment, by psychologist Arthur Aron:
"In experiments by Dr. Aron, participants rated themselves and their partners on a variety of traits, like “ambitious” or “artistic.” A week later, the subjects returned to the lab and were shown the list of traits and asked to indicate which ones described them. People responded the quickest to traits that were true of both them and their partner."
The article that this experiment is reported in, Tara Parker-Pope's "The Happy Marriage is the 'Me' Marriage," sensationalizes, or at least misreads, the data it is based on. She draws a false contrast between enduring and "sustainable" marriages, without offering any evidence that there are many enduring but unsustainable marriages to begin with.
Nonetheless, this experiment is an interesting demonstration of the way in which strong marriages shape the couple into one.