Behavioral psychologists argue that mutism is a child's adaptive response rather than pathological.

A child who has an innately shy and hesitating temperament may be more prone to developing an anxiety disorder — this follows the route of how one’s biological basis influence oneself. However, the relational, environmental, and situational experiences of that child will also influence likeliness and course of development of said child’s anxiety disorder.

(Reference: Anstendig)