To what does Joan attribute Joe's attraction to Merry in the novel, The Wife?

Merry Cheslin is a woman with whom Joe had an affair during a summer writing conference in 1987. Merry was young and attractive, and she was a talentless writer. Over time, Joan realized that Joe’s affair with Merry was motivated in part by Merry’s lack of talent, as it made Merry unthreatening to Joe’s ego.