Fannie Mae and Freddie Mac support the mortgage market in two critical ways. They provide deep and broad-based liquidity for mortgages, which translates into lower rates for borrowers throughout the country and through the economic cycle. And they generate a cross-subsidy among those who benefit from that support, to provide still deeper help for those borrowers and markets that need it most. The authors here focus on this second means of support, explaining how the cross-subsidy works and how it can be improved to better serve the GSEs’ affordability mission.