In chapter 18, the book talks about how Bernard, Helmholtz, and John create their identities. For example, They all say bye to each other as they go to different islands, never to see each other again. This creates their identity because no one ever says bye to anyone because there are no relationships in the hatcheries.
Another example would be when John creates his identity by whipping himself to purify himself. This creates his identity because no one in the hatchery whips themselves because no one knows about purification. This tells us that John doesn't care what others think, he just wants to do what he feels is right.
Lastly, John creates his identity by not stopping the film crew as they make a movie out of him whipping himself. This creates his identity because this shows the readers that he doesn't care that millions of people are going to watch him whip himself, he only cares that he is doing this to purify himself.