One obvious limitation of modeling gene-environment interaction in this way was that it constrained investigation to environments that fell into natural groupings (e.g., married/unmarried; urban/rural) or it forced investigators to create groups based on environments that may actually be more continuous in nature (e.g., religiosity). In the first extension of this work to quasi-continuous environmental moderation, we developed a model that allowed genetic and environmental influences to vary as a function of a continuous environmental moderator and used this model to follow-up on the urban/rural interaction reported previously (Dick et al. 2001). We believed it likely that the urban/rural moderation effect reflected a composite of different processes at work. Accordingly, we expanded the analyses to incorporate more specific information about neighborhood environments, using government-collected information about the specific municipalities in which the twins resided (Dick et al. 2001). We found that genetic influences were stronger in environments characterized by higher rates of migration in and out of the municipality; conversely, shared environmental influences predominated in local communities characterized by little migration. We also found that genetic predispositions were stronger in
