The study looked at how Japan's high government debt affects the country's well-being. By using a model that considers different income levels and risks, the researchers found that Japan's optimal debt level should actually be negative, at -50% of GDP. The current debt level is causing a welfare cost of 0.22% of consumption. This cost comes from factors like the debt level being too high and unequal, but there is a benefit from reducing uncertainty. Adjusting consumption taxes instead of income taxes could help reduce this welfare cost.