The study of global social phenomena raises several difficulties that are not commonly encountered when research questions are posed at less comprehensive levels. Problems of arising from aggregation, diffusion, and path dependence, to name three, plague any attempt at answering questions that are posed at a global level. Ultimately, all these problems arise from the fact that there is only one world available for us to study, and when the world itself is the scope of the analysis, additional worlds are not available for validating initial results.
— Global Social Change: Historical and Comparative Perspectives
by Christopher Chase-Dunn, Salvatore J. Babones