The connection between mental imagery and feelings of presence within a film has not yet been investigated in sex research in relation to observational stance (imagining oneself as either a spectator or participant while viewing a film). Several studies have shown that people who take a participant stance when viewing a sexual film are more likely to report greater subjective sexual arousal (SSA). Research on observational stance has also found that viewing a preferred stimulus is predictive of taking a participant stance. Despite this, very few studies have allowed participants to select their own stimuli. The procedure for this study involves presenting sexual films that were researcher-selected or participant-selected to women and men, while continuously measuring their SSA. Information is collected about observational stance and vividness of mental imagery via questionnaires. Given the existing literature, we expect to find that: 1. Greater mental imagery ability will be associated with adopting a participant stance. 2. The relationship between mental imagery ability and taking a participant stance will be stronger for participant-selected sexual stimuli than for researcher-selected sexual stimuli. 3. Taking a participant stance will be associated with greater SSA. 4. The relationship between taking a participant stance and SSA will be stronger for participant-selected sexual stimuli than for researcher-selected stimuli. Broadly speaking, this project is the first to examine the relationship between mental imagery and observational stance for sexual stimuli, and is among the first to allow participants to self-select stimuli. Results of this project will encourage the development of standardized procedures for providing participants with optimal sexual stimulation.