Causal Effects of Board Diversity on Firm Performance in Nigeria: Panel Evidence from Listed Firms

Despite the growing scholarly attention on corporate governance in sub-Saharan Africa, the causal mechanisms through which board diversity shapes firm performance in Nigeria remain insufficiently understood. This study examines the causal effects of board diversity — operationalised through gender diversity, ethnic diversity, age diversity, educational diversity, and nationality diversity — on firm performance, measured by Return on Assets (ROA), Tobin's Q, and Earnings per Share (EPS), using an ex-post facto research design and an unbalanced panel dataset of 151 firms listed on the Nigerian Exchange Group (NGX) over the period 2011 to 2025. To address endogeneity, unobserved heterogeneity, and simultaneity bias, the study employs the System Generalised Method of Moments (SYS-GMM) estimator alongside Fixed Effects (FE) and Random Effects (RE) panel regression as robustness checks. Eight control variables are incorporated: firm size (FS), industry type (INDT), leverage (LEV), profitability (PROF), growth opportunities (GO), auditor type (AUDT), board size (BS), and board independence (BI). The findings reveal that gender and educational diversity exert statistically significant positive causal effects on ROA and Tobin's Q, whereas ethnic diversity exhibits a non-linear (inverted U-shaped) relationship with firm performance. Age diversity and nationality diversity yield mixed results contingent on industry type. These results are consistent with resource dependence theory and the upper echelons perspective, while offering partial support for critical mass theory. The study contributes novel panel-causal evidence from a frontier market context and provides actionable policy implications for regulators, institutional investors, and boards in Nigeria and comparable emerging economies.