While adults can readily report another agent’s false belief, theories of belief processing typically assume that this process requires the inhibition of one’s own salient current knowledge: belief processing involves overcoming an initial “egocentric bias” towards one’s own knowledge. However, evidence for the presence of egocentric bias during tasks in which adults explicitly report another agent’s false belief is surprisingly limited, with some studies providing conflicting results (e.g., Wang & Leslie, 2016; Rubio-Fernandez, 2017), failures to replicate (e.g., Ryskin & Brown-Schmidt, 2014; Samuel et al., 2018) or data that do not clearly support the presence of an egocentric bias (e.g., Back & Apperly, 2010). In this online psychology experiment (n = 267), we used computer mouse tracking to attempt to measure, in adults, egocentric bias during an unexpected transfer false belief task. Mouse tracking allows researchers to measure the online competition between different response options when one makes a decision, and thus has the potential to reveal attraction to response options that reflect participants’ own knowledge during a false belief task. Participants viewed video scenarios in which an agent had either a true belief (“TB-scenarios”) or a false belief (“FB-scenarios”) as to the location of a toy. In each video, the agent first watched the toy hidden in one of two cups. The keys were then moved to the other cup either in the agent's presence (TB-scenarios) or absence (FB-scenarios). At the end of each video participants used a mouse to answer questions presented on the screen by moving the mouse from the bottom centre of the screen to click on one of two response boxes located in the top left and right of the screen. Key experimental questions required participants to answer: “Which cup does HE think the toy is in NOW?” (belief question); “Which cup do YOU think the toy is in NOW?” (reality question); “Which cup do YOU think the toy was in FIRST?” (memory question). Answers to all questions were always either "Red" or "Blue", and the location of the response box corresponding to these two possible answers remained fixed for participants across all trials. On each trial participant response accuracy, response time, time taken to first move the mouse, and time-stamped mouse coordinates as they made their response were recorded. Participants received 12 experimental question trials (2 of each Scenario x Question combination). Participants did not receive any practice trials of these experimental questions: combined with the low number of total trials, these conditions should reduce the impact of any practice effects on participants' performance. Critically, on belief questions only on FB scenarios the alternative incorrect answer reflects participants’ own knowledge (if the agent thinks the toy is in the "red" cup, then the participant will know that the toy is in fact in the "blue" cup). In contrast, on belief questions following TB scenarios the correct answer reflects participants' own knowledge. For both reality and memory questions, however, irrespective of scenario the correct response is consistent with what participants’ themselves know. If there is an egocentric bias towards one’s own knowledge when processing another's false belief, then one would predict a statistical interaction between question-type and scenario. One would predict that any differences between question-types (e.g., in errors, response times or mouse path deviation towards the incorrect answer) to be greater following FB-scenarios relative to TB-scenarios. We did not find statistically significant interactions of question and scenario in our data.This project investigates the fundamental cognitive processes underlying our ability to understand other people's beliefs about the world, specifically when those beliefs are different to our own. Human beings have been described as "egocentric creatures": even as adults we often assume that other people share our perceptions, desires and knowledge about the world. However, the exact cognitive processes that lead to such errors in adults, in particular when thinking about other people's beliefs, are currently not well understood. Research within psychology has traditionally focused on the egocentricism of children under 5 years of age. Young children show a profound difficulty in reporting that another person has a belief that differs from current reality. Imagine your friend watches you put some chocolate in the kitchen cupboard, and the leaves the room. While she is out, you then move the chocolate into the fridge.

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