In the past three decades, upper echelon studies have generated inconclusive findings regarding the direct link between top management team (TMT) characteristics and various organisational outcomes. The heterogeneous results have hinted at a missing link between the "who", describing the characteristics of decision-makers, and organisational outcomes, labelled as a "black box" problem. It is the missing theoretical constructs to explain "relationships between demographic variables and organisational outcomes" (Lawrence, 1997: 1). The process by which a strategic decision is made by a TMT, the strategic decision-making process (SDMP), could be a critical explanation for the "black box" problem. So far, SDMP studies primarily see strategic decisions as the outcome of rational processes but overlook intuitive elements. Therefore, the current research draws upon the dual-process theory to investigate a TMT's SDMP from a cognitive perspective. However, TMT's SDMP is not free of constraints, highlighting the relevance of contingency theory. By combining dual-process theory and contingency theory, this study develops a double-layered contextual model of SDMP to understand the upstream development and downstream application of TMT's SDMP, which contributes to a richer understanding of the effects of upper echelons on organisational performance.Mergers and Acquisitions (M&A), as rare and complex strategic decisions, have been selected as the research context. The conceptual model was examined empirically across a sample of 109 M&A transactions. The data was collected via cross-sectional primary data research from British firms that made acquisition decisions between 2014 to 2018 and analysed with partial least square (PLS) structural equation modelling (SEM). The findings indicate the duality of TMT's SDMPs (i.e. procedural rationality and collective intuition). This research also finds that most of TMT social-psychological characteristics (i.e. cohesion, behavioural integration and transactive memory system) affect the development of SDMP only if considering the organisational contexts (i.e. organisational structure and board strategic involvement). Similarly, the effect of SDMP on M&A performance is largely contextual and depends on decision-making contexts (i.e. environmental dynamism and the importance of strategic decision). In light of those findings, this research identifies important implications for SDMP and M&A research and practices. 
Keywords: Upper Echelon Theory, Top Management Team, Strategic Decision-making Process, Dual-Process Theory, Contingency Theory, Mergers and Acquisitions (M&A)