Building on previous research that has shown that extraversion is the strongest Big Five personality predictor of effective leadership the present study employed meta-analytic procedures to examine the independent effects of the two main aspects of extraversion—agency and affiliation—on leadership outcomes (transformational leadership and leadership effectiveness). Results showed that it is specifically the agentic aspect of extraversion that has a positive impact on leadership whereas the affiliative aspect is unrelated to transformational leadership and negatively related to leadership effectiveness. Additionally we demonstrate that assessing extraversion in terms of these two aspects leads to substantial improvements in predictive validity relative to broad measures of extraversion. Limitations associated with common method bias and modest effect sizes notwithstanding our findings inform theory on specifically why it is that extraverts are perceived as better leaders than introverts. We also discuss practical implications for how to select and develop leaders in organisational settings and outline the types of organisations in which agency measures are likely to be most useful for selection purposes.