Lucas (1988) hypothesised that human capital externalities explain persistent productivity growth and become manifest via interactions between workplace colleagues. Consistent with the first part of this hypothesis, Fox and Milbourne (2006) concluded that an increase in the average level of human capital in Australian economics departments raised the research productivity of departmental members. This paper tests the robustness of this finding by using a direct, rather than a proxy, measure of human capital and confirms the existence of human capital externalities within Australian economics departments. But we go further by investigating the second part of Lucas’ hypothesis. Whilst there are numerous ways in which departmental colleagues may interact, we investigate whether the externality becomes manifest via co-authorship. We find no evidence that this type of interaction significantly enhances research productivity, especially for higher quality outputs.