Understanding Behavioral Effects in Active Equity Investing

John is a portfolio manager at an equity investment firm. He recently read a paper discussing behavioral biases in equity markets, particularly how emotional factors impact investment decisions. After reviewing his portfolio's performance over the past year, he noted that several stocks began to underperform after their prices hit new highs. John suspects that his clients' emotional behaviors may be influencing the way he evaluates these investments. Considering behavioral finance theories, which of the following concepts is most likely at play in this situation?