Understanding Behavioral Biases in Wealth Management

James is a financial advisor who has been working with high-net-worth clients for over a decade. Recently, he noticed that his clients frequently hold onto underperforming assets in their portfolios, even when they are presented with evidence that it would be better to sell these assets. This behavior aligns with certain psychological biases identified in behavioral finance. Which psychological bias is James experiencing in his clients' reluctance to sell these underperforming assets?