The article explains a new way to build investment portfolios based on the mean-variance criterion. It shows that portfolios can be simplified into two main parts that help manage risks over time. By studying data from 1963 to 2012, the researchers found that the traditional approach may not fully capture investors' risk aversion, especially for short-term investments. They suggest that the mean-variance model works better for medium to long-term investors with moderate to high risk tolerance.