Chapter Twelve is the section of this novel that first makes reference to the sudden change in the relations between Chris and his parents, which occurs between his sophomore and his junior years. Note what the text tells us about this important stage in Chris's life:
Chris's relations with his parents, which had been unusually courteous since his graduation from high school, deteriorated significantly that summer, and Walt and Billie had no idea why. According to Billie, "He seemed mad at us more often, and he became more withdrawn--no, that's not the right word. Chris wasn't ever withdrawn. But he wouldn't tell us what was on his mind and spent more time by himself."
The author tells us that Chris had discovered the facts about his father's previous marriage and his adultery. Clearly, for somebody who possessed such an unyielding approach to what is right and wrong, such a discovery would have impacted Chris immensely. However, the quote from his mother above indicates one of the negative ways in which he handled this information. To not talk at all about it and to become a recluse was not the healthiest way to try and process this information. At least talking to his parents and being open about his emotions would have helped greatly.