Localized knowledge spillovers and patent citations: A distance-based approach

The existence of localized knowledge spillovers found by Jaffe, Trajtenberg and Henderson (1993) has recently been challenged by Thompson and Fox-Kean (2005). To settle this debate, we develop a new approach by incorporating their concepts of control patents into the distance-based test of localization (Duranton and Overman, 2005). Using microgeographic data, we identify localization distance for each technology class while allowing for cross-boundary spillovers, unlike the existing literature where localization is detected at the state or metropolitan statistical area level. We find solid evidence supporting localized knowledge spillovers even when finer controls are used. We further relax the commonly made assumption of perfect controls, and show that the majority of technology classes exhibit localization unless hidden biases induced by imperfect controls are extremely large.