Account for David's increasing fear in The Chrysalids.
David's fear continually increases through the progress of The Chrysalids on account of different occurrences at points in his life. One such occurrence are the two incidents involving Sophie. First, her foot gets caught and to release it, David removes her shoe, noticing that she has six toes. Afterward, Sophie's mother interrogates him and then begs him to keep their secret: Sophie is a Deviant and a Blasphemy according to Waknuk law.
Then one day when he and Sophie are playing in a creek, he ill-advisedly encourages Sophie to go barefoot and Allan comes along and sees Sophie's footprint. Sophie and her family flee, but are caught. Sophie is punished and banished.
These incidents cause David to realize the import of the Definition of Man: "...on each foot, five toes...." The Definition declares that any Deviation is not human though it may seem to be human. David has nightmares and realizes that death can could come to a Deviant he knows and recognizes to be human, despite the definition. These thoughts all begin to apply to himself, as well, as his deviant telepathy becomes a greater part of his life.
Another such incident revolved around his Aunt Harriet's death. She had given birth to a baby girl who was a Deviant. She had turned to her sister, David's mother, for help. All she received was reproach and accusations of evil from David's mother and father. It was then that David turned to praying earnestly that his deviation be removed from him. These incidents and others all contributed to David's growing fear, especially as Petra kept getting into trouble and exposing the telepaths by her screams of terror.