Most diseases arise not purely through genetic abnormalities nor purely through environmental causes, but as "complex" conditions brought about by the combined effects of genetic susceptibility factors, nongenetic experiences and exposures, and bad luck. Finding simple models capable of both characterizing such joint effects and providing new insight into pathogenesis remains an ongoing challenge in etiologic epidemiology. Additive null models can capture certain pure forms of independent etiologic effects in studies of rare conditions and can be useful for predicting possible effects of interventions. The concept of exposure modification is here proposed as useful, particularly in thinking about biologic interactions between exposures and genetic variants. Openness to parsimonious joint models and the insights they can provide is key to advancing our understanding of etiology.