Asymmetric Risk–Return Dynamics of Sustainable Portfolios: A Regime-Switching Analysis on Borsa Istanbul

(1) Background: In integrated financial markets where traditional diversification often fails, analyzing sustainability-oriented investments under non-linear dynamics is critical to averting erroneous decisions. This study investigates whether corporate sustainability provides effective downside mitigation against volatility in emerging markets, using Borsa Istanbul as a case study. (2) Methods: The analysis employs US Dollar-denominated excess returns of an equal-weighted portfolio from the longest-tenured BIST Sustainability Index constituents versus the broader BIST 100 Index (2014–2025), utilizing Markov Regime Switching (MS-AR) and Regime-Switching CAPM methodologies to model non-linear dynamics. (3) Results: Empirical results reveal two distinct regimes, where market variance surges approximately 8.5-fold during crises. The sustainable portfolio exhibits a low systematic risk sensitivity (Beta: 0.76) in normal conditions, driven by its distinct structural composition without generating statistically significant Alpha. In crisis regimes, despite increased sensitivity (Beta: 0.90), the portfolio remains resilient with a beta strictly below 1.00. While BIST 100 investors suffered a massive 40.86% USD wealth erosion over the full period, the sustainability portfolio significantly mitigated this damage, limiting the total capital loss to 20.73% due to substantial compounding accumulated during normal regimes. (4) Conclusions: Consequently, sustainability proves to be not merely an ethical preference but a rational financial strategy offering diversification benefits in tranquility and acting as an effective partial hedge during turbulence in high-volatility markets.