Psychology has traditionally seen itself as the science of universal human cognition, but it has only recently begun seriously grappling with cross-cultural variation. Here we argue that the roots of cross-cultural variation often lie in the past. Therefore, to understand not only how but also why psychology varies, we need to grapple with cross-temporal variation. The traces of past human cognition accessible through historical texts and artifacts can serve as a valuable, and almost completely unutilized, source of psychological data. These data from dead minds open up an untapped and highly diverse subject pool. We review examples of research that may be classified as historical psychology, introduce sources of historical data and methods for analyzing them, explain the critical role of theory, and discuss how psychologists can add historical depth and nuance to their work. Psychology needs to become a historical science if it wants to be a genuinely universal science of human cognition and behavior.
