Since its inception in the early 2000s, social media has quickly become a meaningful source of online community for people across the globe. The use of promotional advertising on social media has brought both helpful and potentially harmful exposure to a wide variety of information. This dissertation aims to uncover certain underlying physiological mechanisms, specifically, neural mechanisms, that drive donation behavior in online contexts involving a mediated form of communication. Understanding donation behavior online is vital to make more effective campaigns that reach mass audiences to solve world problems. Focusing on donation behavior as a primary online prosocial action, this dissertation explores the following research question and associated hypotheses, “How do our physiological processes influence donation decision making in varying online contexts, and what types of information influence those decisions?” Although much is known about factors that influence donation behavior, less is known about what physiological processes are correlated with prosocial behaviors, especially in online situations. This approach is taken in an effort to connect psychophysiology with a common, ecologically-valid modern media experience to help understand why people decide to act on social media posts or keep scrolling. Chapter one reviews psychophysiological responses to media and their traditional place in the literature, connecting the seemingly dissimilar fields of psychophysiology and communication. A secondary goal of this review is to establish the importance of measuring physiological responses in communication studies. Next, the history of research on prosocial behavior is discussed, including the operationalization of prosocial behavior to create context for the main study that connects physiological responses to online donation behavior. The second half of the dissertation describes a pilot study (N=155) focusing on social media post characteristics that contribute to online donation behavior. In the 3 x 2 x 2 pilot study, image emotional valence (positive, negative, neutral), popularity (low or high number of likes), and source intentions (label of an advertisement) were manipulated to examine their relationship to online donation behavior. In the main study, a revised design based on the pilot results is presented with the addition of neural fMRI data collection. The ultimate goals of this dissertation are to (1) investigate the physiological correlates of online donation behavior (through emotional appeals and other features of social media posts) and (2) contribute to the literature connecting the communication and neuroscience disciplines. The pilot study revealed that negative emotional appeals influenced people to donate more to causes than neutral and positively-valenced images. Additionally, other features influenced whether people acted on posts, such as the number of likes or a label as an advertisement (source intention). The dissertation involves combining this design with an fMRI study to investigate the neural responses to this facet of decision making online. The results of the studies described in this dissertation are the following: across multiple studies, participants donated the most money to negatively-valenced posts compared to neutral or positively-valenced posts. Other factors, such as the number of likes on a post, or whether the post was labeled as sponsored or unsponsored, presented mixed results (in terms of statistical significance and visual patterns). Additionally, the neural patterns seen when people were viewing negative, positive, and neutral posts, varied by region, with the high emotional posts (positive and negative) showing different activation patterns compared to the neutral posts. Neural patterns mirrored behavioral changes to provide additional support for the observed behavior. Applying a physiological lens to online behavior remains an area that is underserved in the literature, and this is a gap this dissertation seeks to fill. Combining neural data with behavioral findings is performed to reveal that people may be physiologically regulating their responses to the campaigns, and in turn, may take action based on their physical reactions. Ideally, the results will be applied to policy-making and lead to cause marketing efforts to help solve world problems with more effective messaging to the public. / Media & Communication