The library literature addressing the role of bias in collection development emphasizes a philosophical approach. It is based on the notion that bias can be controlled by the conscious act of believing in certain values and adhering to a code of ethics. It largely ignores the psychological research on bias, which suggests that bias is a more complex and subtle phenomenon. Bias is not always conscious and cannot always be consciously controlled. Countering bias requires an understanding of both its conscious and unconscious influences on selectors and the selection process. This article examines the psychological research that has been conducted on bias and shows how it applies to selectors and the selection process in collection development.