Knowledge producers conducting research on a particular set of questions may respond to supply and demand shocks by shifting resources to a different set of questions. Cognitive mobility measures the transition from one location to another in idea space. We examine the cognitive mobility flows unleashed by the influx of Soviet mathematicians into the United States after the collapse of the Soviet Union. The data reveal that American mathematicians moved away from fields that received large numbers of Soviet émigrés. Diminishing returns in specific research areas, rather than beneficial human capital spillovers, dominated the cognitive mobility decisions of knowledge producers.

Many economists believe knowledge production generates positive spillovers among knowledge producers. The available evidence, however, is mixed. We argue that spillovers can exist along three dimensions: idea, geographic, and collaboration space. To isolate the key channel through which knowledge spills over, we use a unique data set to examine the impact of a large post-1992 exodus of Soviet mathematicians on the output of the nonémigrés. Although the data reveal strong competitive effects in idea space, there is evidence of knowledge spillovers in collaboration space, when high-quality researchers directly engage with other researchers in the joint production of new knowledge.