The Iron Age of Mainland Southeast Asia began in the fifth century bc and lasted for about a millennium. In coastal regions, the development of trade along the Maritime Silk Road led to the growth of port cities. In the interior, a fall in monsoon rains particularly affected the Mun River valley. This coincided with the construction of moats/reservoirs round Iron Age settlements from which water was channelled into wet rice fields, the production of iron ploughshares and sickles, population growth, burgeoning exchange and increased conflict. We explore the social impact of this agricultural revolution through applying statistical analyses to mortuary samples dating before and after the development of wet rice farming. These suggest that there was a swift formation of social elites represented by the wealth of mortuary offerings, followed by a decline. Two associated changes are identified. The first involved burying the dead in residential houses; the second considers the impact of an increasingly aquatic environment on health by examining demographic trends involving a doubling of infant mortality that concentrated on neonates. A comparison between this sequence and that seen in coastal ports suggests two interconnected instances of rapid pathways to social change responding to different social and environmental stressors.