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They say:
there is little work published in law that would be of direct relevance to research in psychology, and each of these fields do a good job of citing within their disciplines. This leads to strong internal coherence and recognition that creates strong similarity in the co-citation network and thus dense clusters […] Sociology, on the other hand, has a different structure. We have traded network centrality for density […] Instead of a field devoted to a comparatively narrow niche of the social science idea space, sociology has opted for being a truly general discipline.