The study looked at how income is distributed among high earners in Japan and calculated the optimal highest tax rate based on this distribution. They found that high income in Japan is concentrated among a few thousand to tens of thousands of top earners. Recent research suggests that Japan's highest tax rate should be higher than it currently is, to ensure fairness and support social welfare. The way taxes are collected and deductions are applied can also affect how much high earners are willing to pay in taxes.