The portfolio effect principle is adding more assets to a portfolio to make the total investment less risky. Generally, this is true, as long as you have more profitable investments than losing investments and you diversify them properly.
The portfolio effect shows the value of diversifying your investments. If you invest in two direct competitors, their stock value will rise and fall depending on many factors: politics, weather, market conditions and simple market share. However, investing across a variety of industries lessens your risk, since economic conditions affect industries differently.
Conservation officials use the portfolio effect when breeding salmon. Basically, adding several different populations to breeding stocks help stabilize the population growth. This means there are fewer population booms and busts.