This concept of diversification is one of the main tenets of modern portfolio theory – volatility is
reduced through the addition of more assets to a portfolio. It should be noted, however, that the
rate of volatility reduction from adding assets decreases as the number of assets in the portfolio
increases. As the chart below demonstrates for one potential scenario (20% volatility on each
asset and zero covariance between assets), the general rule of thumb is that a portfolio containing
30 or more assets is considered well-diversified.