Don is 39 at the beginning of the novel, and he evolves and grows tremendously. His original intention with the Wife Project questionnaire is to eliminate all incompatible women, who would otherwise waste his time. He is extremely confident about what he wants in a partner, but he spends little time thinking seriously about what he has to offer in a relationship.

When Rosie rejects him, he realizes that he has deliberately cultivated the identity and mannerisms of an eccentric person. He originally adopted a clownish persona to head off people laughing at him for things that he couldn’t help. Now that he’s grown socially and matured, through his interactions with Rosie and the world she’s opened up to him, he understands that he can change—the social mannerisms that govern people’s reactions to him are under his control. He can manipulate his behavior to be more acceptable. He does not have to be a fool. This realization is the climax of Don’s character development. Don grows up at this moment, though it took him 39 years.

By the end of the novel, Don is focused on what he wants to give in a relationship, instead of on what characteristics and qualities his partner must have.