We developed a new scale measuring sexual aversion towards a specific person, which we will use to measure incest aversion specifically towards co-participants. To develop this measure, we first came up with items measuring willingness to engage in intimate behaviours that people may be comfortable when doing it with a partner but not with a sibling. These items ranged in severity (e.g., from "Hugging" to "Sexual Intercourse"). A full list of items are provided below. This scale will first be piloted online. Online volunteers will be randomly asked to think of either their romantic partner, or their sibling. Participants will be asked a few questions about this person to ensure they are thinking of someone specific (e.g., What is your partner/sibling's age? What is your partner/sibling's sex?). For participants who do not have a romantic partner or a sibling, they will be asked to imagine they have one. The scale will be reduced in items based on the data from the pilot. This will be based on the following criteria favouring items that 1) best discriminate between participants randomly asked to think of a romantic partner or a sibling, 2) best discriminate between high and low scorers on the overall scale (i.e., produces a good distribution of responses), and 3) load onto a similar factor with other items when run in a Principal Components Analysis. ### Items Please rate how comfortable or willing you would be at engaging in the following behaviours with your co-participant on a 7-point scale. [1 = Extremely Uncomfortable/Not Willing, 7 = Extremely Comfortable/Willing] 1. Play fighting together. 2. Hugging. 3. Eating food from the same plate or drinking from the same glass. 4. Tickling each other. 5. Sleeping in the same bed. 6. Holding hands while walking down the street. 7. Sharing a toothbrush. 8. Giving each other massages. 9. Eating a romantic dinner together by candlelight. 10. Discussing sexuality. 11. Watching nudity on TV together. 12. Changing outfits in the same room. 13. Discussing sexual fantasies. 14. Running into each other naked in the bathroom. 15. Playfully flirting. 16. Cuddling on the couch. 17. Bathing together. 18. Sending sexy messages to each other. 19. Kissing on the lips. 20. Masturbating together in the same room. 21. Oral sex. 22. Sexual intercourse.