We examine a novel mechanism for structure formation involving initial number density fluctuations between relativistic species, one of which then undergoes a temporary downward variation in its equation of state and generates superhorizon-scale density fluctuations. Isocurvature decaying dark matter models (iDDM) provide concrete examples. This mechanism solves the phenomenological problems of traditional isocurvature models, allowing iDDM models to fit the current CMB and large-scale

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... data, while still providing novel behavior. We characterize the decaying dark matter and its decay products as a single component of "generalized dark matter". This simplifies calculations in decaying dark matter models and others that utilize this mechanism for structure formation.