The document analyzes why gold is different from other assets by empirically investigating the relationship between gold returns and macroeconomic variables. It finds that:
1) Returns on gold are not correlated with changes in GDP, inflation, and interest rates, unlike returns on stocks and bonds.
2) Changes in macroeconomic variables have a stronger impact on returns of other commodities like oil and zinc than on gold.
3) Returns on gold are less correlated with stock and bond returns than returns on other commodities.
These results support gold being an effective portfolio diversifier due to its returns being independent of the business cycle.