Research in modularization of product families reveals numerous individual cause and effect impacts of modularity on the firm. While many modularization methods are present in the literature, an overall combined picture of the interrelations of modular product structures on the firm is yet missing. This makes the construction of an economic business case for modularity difficult. The aim of this work is to show a causal network of how modular product structures affect a firm's economics through modularity's impact on the firm's activities over its products lifecycle phases. Therefore we introduce a literature based impact model on the basis of three main cause and effect node types, being (1) modularity property causes leading to (2) product life phase effects, leading to (3) economic impacts. We show how modularization decisions on a product family structure can impact activities in the product life cycle differently and the resultant economic outcomes. The cause and effect model is a combined summary of the corpus of knowledge found in the literature, highlighting for example that modularity can have an initial negative delaying effect on the development time for a platform followed by a positive reduction in product variant development times, and can a positive impact on reduced manufacturing costs. For validation the model was evaluated by industrial experts against nine case studies of major product-family-wide design projects, marking effects that were experienced and observed in their modularization projects.