when assets that are often negatively correlated move in the same direction this is an example of systematic risk systematic risk can not be diversified away it will exist in financial markets and is the inherent risk present in investing though asset classes may traditionally be negatively correlated macroeconomic conditions may result in asset classes acting similarly due to broader impacts to the market 
example of negative correlationexamples of negative correlation are common in the investment world a well known example is the negative correlation between crude oil prices and airline stock prices jet fuel which is derived from crude oil is a large cost input for airlines and has a significant impact on their profitability and earnings if the price of crude oil spikes up it could have a negative impact on airlines earnings and hence on the price of their stocks but if the price of crude oil trends lower this should boost airline profits and therefore their stock prices here s how the existence of this phenomenon can help in the construction of a diversified portfolio as the energy sector has a substantial weight in most equity indices many investors have significant exposure to crude oil prices which are typically quite volatile as the energy sector for obvious reasons has a positive correlation with crude oil prices investing part of one s portfolio in airline stocks would provide a hedge against a decline in oil prices 