In recent years a growing interest in gender diversity has arisen in the business context, and a significant number of research studies have analysed its effect on corporate performance. Nonetheless, these studies have not led to conclusive results, perhaps owing to the complexity of the relationship. The present work focuses on analysing two possible justifications of the contradictory effects of gender diversity on company performance: the business context and the optimum size of the female presence in decision-making bodies. The results obtained show that when working conditions and academic background are similar, women achieve better performance in sectors traditionally dominated by men. Moreover, to take the best advantage of gender diversity it is recommended that boards of directors should be balanced or have a slightly higher female presence.