This paper draws upon the animal spirit, hubris, and narcissism theories to conduct an empirical study investigating the behavioural determinants influencing mergers and acquisitions (M&A). While M&A transactions have shown a trend of value destruction for acquiring firm shareholders, they have simultaneously demonstrated value enhancement for shareholders of the acquired firms. The repercussions of this value erosion extend beyond shareholders, impacting pension funds, employees, customers, suppliers, government tax revenues, and banks within the economy. Utilising a long-event window methodology, the research examines cumulative abnormal returns (CARs) for acquirers’ shareholders over a three-year post-completion period, focusing on Australian acquisitions from 1990 to 2006. The findings highlight animal spirit, hubris, and narcissism as significant behavioural determinants shaping M&A outcomes, thereby contributing to the ongoing discourse on hubris and hubris syndrome in M&A, with a suggestion that narcissism may be more prevalent than hubris. Additionally, the study identifies the joint tenure period of the Chairman and Chief Executive Officer (CEO) in the acquiring firm as a statistically significant factor contributing to acquiring firm shareholder value in M&A activities. Notably, a significantly negative correlation between CEO remuneration change and M&A outcomes is found. The paper recommends enhanced transparency for remuneration and nominations committees to address corporate governance issues raised by the study.