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Diversification reduces risk, but does this base on the assumption that expected return of each asset is always in proportion to its risk?
Diversifying your portfolio between asset A and asset B only reduces the portfolio risk if asset A and asset B are not correlated. If they have either a low correlation or a negative correlation to each other, then you benefit from combining them in a portfolio in terms of risk reduction. The standard deviation of returns will be lower in a portfolio of low or uncorrelated assets. If on the other hand you combine two correlated assets into a portfolio you are doubling down on the same assumption, which means you are not reducing your risk. You are also wasting capital because now you have allocated capital to 2 separate trades / investments yet they have shown a high tendency of moving together. Here is an article that discusses this further: Why Diversify your Stock Portfolio