Clue #1: Mary Steenburgen plays a mother figure to Ferrell's characters in both roles.

Elf is about a grown man named Buddy who is raised by elves on the North Pole. Then, one day, he embarks on an adventure to go find his real dad in New York City. In the city, he meets his paternal father, Walter, played by James Caan, and his new wife, Emily, played by Mary Steenburgen. Walter doesn't get along with Buddy, but Emily quickly accepts him into their family.

Steenburgen appears again in Step Brothers, but this time as the mother of Brennan, a big man child who begrudgingly has to accept that he has to share a house with his new step brother, Dale Doback, played by John C. Reilly.

Following the theory, Steenburgen's character leaves Walter, adopts Buddy, changes their names, moves them outside of the city where she then falls in love again, and marries a single father, Dr. Robert Doback, played by Richard Jenkins.