Preindustrial human societies living in ancient Central America apparently varied considerably in their vulnerabilities to the sudden massive stresses caused by explosive volcanic eruptions. The focus is on ancient complex societies and volcanic eruptions in the Zapotitán Valley during the past two millennia in what is now El Salvador, within a contextual and comparative framework of other eruptions and societies from other areas of Central America. Comparing eruptions and their societal effects indicates that there was a threshold of magnitude of these eruptions beyond which the society affected did not recover, and reoccupation was by a different ethnic group. An example is the TBJ (Tierra Blanca Joven) eruption of Ilopango where the original inhabitants never recovered to reoccupy the area. In contrast, the smaller Boquerón eruption devastated the eastern half of the Zapotitán Valley, but recovery was achieved by the indigenous society. The concept of “scalar vulnerabilities” is introduced, where physical factors such as the magnitude of eruptions are compared along with cultural factors such as variation in organizational complexity and institutionalized hostility. Simpler egalitarian societies in this sample were more resilient to sudden massive stresses than were the more complex societies. Simpler societies relied less on intensive agriculture, staple crops, fixed facilities, redistributive economies, and hierarchical institutions. Cultural factors such as chronic political hostilities can greatly increase the vulnerability of societies at any point on the simple-to-complex range. It is argued here that assessing a society's vulnerability must include the magnitude of stress, the complexity of the society (including the related factors of adaptation, demography, and the “built environment”), and the political landscape.