Indeed, "there is an interaction between the two kinds of resources of a firm-its personnel and material resources-which affects the productive services available from each" (Pen- rose, 1959: 76). Moreover, because of causal ambi- guity, path dependencies, and social complexity, competitors should find it harder to duplicate an advantage when it results from a bundle of valu- able, firm-specific resources (Teece et al., 1997).