‘Survival of the fittest’ aptly describes the work environment. Employees and the organisations for which they work are therefore required to have various skills sets to afford them a competitive advantage in the job market. This is one of the many reasons why private and public organisations make use of the assessment center, and specifically the behaviour observation exercise to evaluate and select future personnel. Although the behaviour observation exercise provides the rater with rich information regarding a candidate’s skills, rater errors that are often inadvertent can result from a rater’s inherent subjectivity. One such error, central to this study, is Introversion/Extraversion bias. This type of bias plays out when raters rate candidates with personality types similar to their own more favorably than other candidates when the candidate’s degree of introversion or extraversion should not be considered relevant to the selection criteria. This study aims to explore the effect of Introversion/Extraversion bias on the scores of behaviour observation exercises performed during a leadership assessment center in a security environment. The sample consists of 103 participants (14 raters and 89 candidates) all belonging to the same security organisation. The researcher conducts a cross-sectional, non-experimental field study. Candidate as well as rater Introversion/Extraversion preferences are measured by the Jung Personality Questionnaire (JPQ). The scores of two behaviour observation exercises are used to explore the interaction effect between rater Introversion/Extraversion and candidate Introversion/Extraversion. Point-biserial correlations, independent t-tests as well as a one-way ANOVA are used to test the hypotheses. No interaction effect is identified between rater Introversion/Extraversion and candidate Introversion/Extraversion, indicating that raters did not score candidates with similar personality types to their own more favorably. However, the results indicate that extraverted candidates were rated higher by both introverted and extraverted raters and are consequently perceived to have performed better in both behaviour observation exercises. The study postulates that the nature of the exercises, which require high levels of engagement with fellow team members (a typical strength of extraverts), is one of the main contributors to the perception that extraverted individuals are better performers. The results of this study not only contribute to the lacuna in research on the topic, but also to the development of an unbiased behaviour observation exercise within this security organisation. Copyright 2011, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: De Beer, E 2011, The influence of introversion/extraversion bias on leadership assessment with behaviour observation, MCom dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-03302012-154446 / > C12/4/136/gm / Dissertation (MCom)--University of Pretoria, 2011. / Human Resource Management / unrestricted