Those forces which move people, business, and industry towards a centre—accessibility, functional linkages, agglomeration economies, and external economies—thus encouraging the growth of large central places. Porteous in R. Martin, ed. (1999) cites labour market externalities, intermediate services, technological and informational spillovers, and socio-institutional and cultural factors as centripetal forces. Grote (2008) J. Econ. Geog. 8, 2 holds that, with ever-falling transaction costs, centripetal forces decline over time.