Mass media content often propagates through social channels, for instance through shares on social media. In these social spaces, message effects interact with social forces like social influence to impact behavior and attitudes which has important implications for large-scale media effects. The abundance of online data about sharing patterns has enabled detailed descriptions of these processes but commonly used methods are less well suited to understand the psychological processes that facilitate sharing decisions. To address this knowledge gap, this dissertation used functional magnetic resonance imaging to study processes occurring in propagation chains where communicators shared New York Times health news articles with receivers through Facebook messages. Results from four empirical studies support a parsimonious framework, suggesting that communicators integrate considerations of the expected self-related and social outcomes of sharing into an overall signal of the value of sharing a piece of content which directly impacts their choices. To this end, Chapter 2 demonstrates the involvement of neural activity in regions associated with self-related, social, and value-related processing in sharing decisions made by individual communicators. Chapter 3 shows that the extent of neural value-related activity in response to these articles is significantly related to population-level sharing behavior of hundreds of thousands of real-world online New York Times readers and that neural valuation mediates the effects of self-related and social processing on choice. Chapter 4 demonstrates that these key processes are relevant across sharing contexts, namely when communicators are faced with different audience sizes. Yet the measures used here still showed insightful context-sensitivity through modulation of signal intensity. Finally, Chapter 5 discusses neural communicator-receiver coupling of activity in key regions of interest associated with valuation, self-related and social processing as a facilitator of information transfer between communications and receivers. Significant coupling suggests that central processes identified in communicators may propagate through social interaction and impact secondary receivers. In sum, this dissertation offers a detailed, parsimonious framework of the neural and psychological bases of sharing decisions and thus constitutes progress in scientific efforts to optimally account for and utilize social forces in the design of large-scale message campaigns and interventions.