Volatility can be effectively reduced without significant cost by diversifying, so it makes sense
that investors should not be compensated for that portion of volatility which is merely stock
specific and has no impact on a well diversified portfolio. This type of volatility is called
unsystematic risk in the finance literature because it does not covary with the market as a whole,
but is merely the additional random “noise” present in that specific asset’s returns. Since this
random noise has an expected return of zero, it can be diversified away by adding more
securities to the portfolio. Its mean will be zero, and its standard deviation will be reduced as
more assets are added.