John Wyndham uses David's throwaway remark to poke fun at how a fanatical attachment to dogma can undermine not only our sense of humor, but also our capacity for rational judgment. David's father is such an anti-mutant extremist that the very idea of his son having a third hand—even if meant solely in jest—is simply too horrible for him to contemplate.

In totalitarian societies, such as the one portrayed in The Chrysalids, even humor can be dangerous; a seemingly harmless joke can be interpreted as a revelation of subversive attitudes that threaten the regime's control. Humor is often used as a way of releasing tension—a vital consideration in any society, but especially in one where people are under the control of a ruthless dictatorship. Yet even this safety valve is denied to David by his father. As a true believer in the dominant creed, he cannot allow his son to joke about something which is so fundamental to his warped worldview.

This incident indicates the absurdity of the situation in which David is living, as one careless remark comes to be misinterpreted so grotesquely. David hurts his hand on a splinter, and goes to his mother who binds it for him, annoyed because she is busy. David comments that he could have done it himself if he had a third hand. His father hears this remark and reacts in his typical extreme way. Note the way that David tries to protest his innocence:

"I only meant if," I protested. I was alarmed, and too confused to explain that I had only happened to use one way of expressing a difficulty which might have been put in several ways.

Religious extremism has been taken to such absurd lengths that this incredbly innocent remark is able to be interpreted as asking to be a mutant and as a sign of blasphemy. We are presented with a world where conforming to the norm is so vital and important that even casual remarks can be taken as signs of sin and wishing to be something else.