In nineteenth-century science, the emotions played a crucial role in explaining the social behavior of animals and human beings. Beginning in the 1890s, however, the first American psychologists, resolutely parsimonious in method, dismissed affective experience as intellectually imprecise. Yet in practice, feelings continued to influence at least one research setting: animal experiments. Laboratory reports, although focused on learning, became a repository of informal observations about the animals' temperaments and moods. When American psychologists began to reexamine the emotions between the world wars, they drew on this empirical legacy in animal studies. They also devised a conceptual approach to emotion consistent with their expectation of experimental precision.