"Estha had always been a quiet child... Estha occupied very little space in the world."

This quote exemplifies the symbolic nature of Estha's muteness, and it being the result of a decision - thus making it selective mutism. Selective mutism is a condition, prevalent in children, wherein a child capable of speech is unable to speak to given situations or people.

This "condition" is relevent not only to Estha, but features in several characters and situations as a catalyst for the plot of the novel.