affectr is a an r package for sentiment analysis. The built in dictionaries permit accurate classification of text along three affective dimensions: rationality vs. emotionality, social impact, and valence (positive vs. negative). These dimensions have been found to organize the neural representation of others' mental states in analyses of functional magnetic resonance imaging (fMRI) data (see here). The motivation, construction, and validation of these dictionaries is described here, with code available here. The raw weighted dictionaries are freely available in a portable (csv) format here.