This study considers the dilemma faced by employees every time a colleague requests knowledge: should they share their knowledge? We use adaptive cost theory and self-efficacy theory to examine how individual characteristics (i.e., self-efficacy and trait competitiveness) and situational perceptions (i.e., ‘busyness’ and perceived competition) affect knowledge sharing behaviours. A study was conducted with 403 students who completed a problem-solving exercise and who were permitted (but not required) to respond to requests for knowledge from people who were doing the same activity. Our results suggest that people who perceive significant time pressure are less likely to share knowledge. Trait competitiveness predicted perceived competition. This and low task self-efficacy created a sense of time pressure, which in turn led to people feeling ‘too busy’ to share their knowledge when it was requested. Perceived competition was not directly related to knowledge sharing. Implications for research and practitioners are discussed.