Detailed description of the Behavioural Avoidance Test (BAT):

Participants will be asked to stand in front of a closed door. They will be asked by the experimenter to approach the spider as closely as possibly, but they are reminded they can stop at anytime. Participants will then enter a room in which a table will be located approximately 3 meters in front of them. On top of the table will be a glass terrarium containing a fast moving spider (Huntsman - Holconis). These spiders were chosen because they are all low risk (non toxic) and non-aggressive to humans. In addition, these spiders are also commonly found in homes or gardens, thus increasing the ecological validity of our study. Participants will be asked to approach the spider in a stepwise manner (i.e., in 10 steps). Performance will be scored on a 10-point scale that corresponds with the steps taken during the test; the steps are 0 = refuses to enter room, 1 = stops 3m from terrarium, 2 = stops 2m, 3 = stops 1m, 4 = stops close to terrarium, 5 = touches terrarium, 6 = removes lid, 7 = puts hand in terrarium, 8 = touches inside walls and floor of terrarium, 9 = guides spider over hand which is resting on the floor of the terrarium. For steps 7 to 9 the experimenter will relocate the spider to a larger enclosure with a zippered opening that can be arranged so the spider cannot escape the enclosure and the participant will be provided with a long sleeved shirt and a gardening glove. Once the participant has entered the room, the experimenter does not persuade the participant to continue.

Detailed Description of One-Session-Treatment (OST):

There will be three sessions. Session 1 involves the preparation stage, a pre-treatment interview, pre-treatment measures, and OST. During the pre-treatment interview a clinical psychologist will collect information about the participants fear of spiders and provide a rationale for treatment. The participant will then complete pre-treatment measures, including the BAT (to assess pre-treatment levels of avoidance; see detailed protocol above), as well as self-report questionnaires. After completing the pre-treatment measures, participants will then begin OST. This treatment will strictly follow Öst's manual for specific phobias. It will take a maximum of three hours and has been shown to be highly effective in treating specific phobia, including spider phobia. OST involves the use of a maximum of 4 spiders of varying sizes, from 0.5cm to 5cm, housed in glass terrariums. Two spider species will be used: St Andrew's Cross (Argiope) and Huntsman (Holconia immanis). These spiders were chosen because they are all low risk (non toxic) and non-aggressive to humans. In addition, these spiders are also commonly found in homes or gardens, thus increasing the ecological validity of our treatment. There are a total of five steps and at the start of each step, the experimenter will provide instructions and then model the behaviour for the participant. Each step will be repeated until the participant experiences a reduction of at least 50% in their anxiety, as assessed by Subjective Units of Distress ratings (SUDS; 0-100), or rate the conviction in their belief that a catastrophe will occur as 0 (0-100). Steps 1-4 will be completed with the St Andrew's Cross spiders. Step 1 involves teaching the participant how to catch the smallest spider with a glass and postcard (i.e., placing glass upside down over the spider in the terrarium and to slide the postcard underneath). Step 2 involves touching the spider from behind with the dominant index finger. Step 3 involves letting the spider walk on the participant's hands. Step 4 involves allowing the spider to walk on the participant's legs and body. Steps 1-4 are then repeated with larger St Andrew's cross spiders. After completing this, the participant will be encouraged to have the two largest St Andrew's cross spiders walk on their hands simultaneously. Step 5 is provided in the treatment as an extra step and will be completed only if there is time remaining and if the participant gives consent. Step 5 involves encouraging the participant to complete the final step of the BAT using the Huntsman spider (i.e., guiding the spider over their gloved hand). The experiment ends when all steps are completed, or if the maximum 3 hour time limit has been reached, or if the participant indicates that they do not want to continue. After treatment, participants will be checked to ensure they are not feeling any distress and will be accompanied by the experimenter to Healthscope Pathology to provide a blood sample to verify hormone levels.

Session 2, a follow-up session, will occur one-week later, as per Öst's treatment manual. During this session, post-treatment measures will be taken, including structured clinical interview assessment, self-report questionnaires and BAT.

Session 3 will occur 3 months later and will be a long-term follow-up session. Participants will undergo the same measures as in session 2. Participants will be fully debriefed regarding the aims and implications of the experiment. During this process, participants will be asked if they have any questions or concerns, and will be checked to ensure they are not feeling any distress.