“…The time-varying nature of asset correlations has long been of much interest to financial practitioners, with short-term increases in the co-movement of financial returns widely documented around periods of market stress (Preis et al, 2012;Packham & Woebbeking, 2019;Campbell et al, 2002;Cappiello et al, 2006;Billio et al, 2010). From a portfolio construction perspective, shortterm increases in asset correlations have frequently been linked to spikes in market volatility (Campbell et al, 2002) -with explanatory factors ranging from impactful news announcements (At-Sahalia & Xiu, 2016) to "riskon risk-off" effects (Dungey et al, 2018) -indicating that diversification can breakdown precisely when it is needed the most. In terms of systemic risk, increased co-movement between market returns is viewed as a sign of market fragility (Kritzman et al, 2011), as the tight coupling between markets can deepen drawdowns B. Lim, S. Zohren and S. Roberts are with the Department of Engineering Science and the Oxford-Man Institute of Quantitative Finance, University of Oxford, Oxford, United Kingdom (email:{blim, zohren, sjrob}@robots.ox.ac.uk). when they occur, resulting in financial contagion.…”