One of the most startling post-war effects in the story is the distinct absence of Larry, the Keller's other son. Chris has returned from the war alive, but Larry is missing, and his mother refuses to believe that he is dead. She holds out hope that Larry will return and looks for signs everywhere.
For example, when Ann Deever comes to the Kellers for a visit and she sleeps in Larry's old room, that night a tree that was planted in the yard in memory of Larry is blown down by a strong storm. It also happens to be the month of Larry's birth. So Kate Keller believes that the signs indicate that Larry is trying to communicate that he is still alive and that the family should not lose hope.
Kate Keller is so emotionally distraught over losing her son in the war, that she can't move forward and see that Chris and Ann are in love. She continues to refer to Ann as Larry's girl, and won't hear of Chris going out with Ann. In fact Chris is the one who invited Ann to the house because he wants to propose to her.
Kate has the neighbors convinced that her superstitions are true. Frank does Larry's horoscope to help support Kate's desire to believe that her son is alive.
The post-war retrospect in the play allows Joe Keller to reflect on his actions at the end of the play and consider that his decision to ship the cracked cylinder heads caused the death of many pilots and that his son, Larry committed suicide because he could not live with the thought that his father was a murderer.
Once Joe Keller realizes that his own son killed himself because of what he did, now, after the war is over and he saved the factory, and it is still prospering, which was the whole reason why he shipped the faulty parts in the first place, he feels compelled to take responsibility for his crime. He decides to exact the ultimate punishment on himself. He goes into the house and commits suicide by shooting himself.