Modern Portfolio Theory makes a preposition that diff assets move in different (some say opposing) directions particularly when markets experience periods of high volatility.
However during extreme periods of vol we will often see all assets move down in unison -e.g post 2008 crisis. However, once markets get over the shock of the event, the dynamics of MPT can then reassert it’s self. It is for this reason that I have been recommending, to my clients, to allocate to Gold, within their portfolios. Recently we have seen: Euro Down,Markets down,Gold Up. Looks like our friend MPT may be back!