Perhaps the greatest cause of trauma in the novel is Hassan's rape, which is a direct result of his earlier defense of Amir against the wealthy bully Assef. Hassan defends Amir by aiming a slingshot at Assef so that he will leave Amir alone. Assef gets revenge by trapping Hassan in an alley while he is kite running and raping him as two of his friends hold him down. Amir witnesses the rape but is too frightened to interfere.
The first consequence of this trauma is that the joy goes out of Amir's life because of his guilt at being a bystander and doing nothing to prevent his friend's rape. In chapter 8, for example, Amir, who has finally won his father's approval with his kite battling, derives no joy from an outing with him. Amir states:
That should have been fun. ... I finally had what I'd wanted all those years. Except now that I had it, I felt as empty as this unkempt pool I was dangling my legs into.
This dissociation from life is typical of trauma, as is Amir's inability to sleep. He wakes up one night, haunted by the rape, which now has taken a dominant hold of him:
"I watched Hassan get raped," I said. ... A part of me was hoping someone would wake up and hear.
The second consequence of the rape is that it destroys the friendship between Amir and Hassan. Amir no longer wants to be around Hassan, who is a reminder of his failure to intercede. Amir is so distressed by Hassan's presence that he frames him for theft so that he and his father will be sent away.
The story is told from the point of view of Amir, so we don't have access to what Hassan is going through, but we learn from Amir that his friend's eyes now look "tired," and his face looked older than it had before. We later learn that Hassan's desire was to take one of Assef's eyes out in revenge, an uncharacteristically violent wish for Hassan and an indication of the depth of his trauma. This wish is finally achieved by Hassan's son after Hassan's death.
Amir carries the trauma of his betrayal of his friend with him to America until he returns to Afghanistan and meets up with Sohrab, Hassan's son, and achieves some healing through adopting the boy.
In chapter 11, you can find some of the consequences of the trauma of the forced exile of Baba to America. Amir explains that adjusting to the culture gives Baba an "ulcer." Baba also reacts violently when he is asked for an ID when writing a check in a local grocery where he has been shopping for two years. Amir comes running to see.
On the floor: oranges, an overturned magazine rack, a broken jar of beef jerky, and shards of glass at Baba's feet.
Baba can't understand why he is not trusted, as he would be in his native land. The violent reaction shows the depth of his pent-up emotions.