That [sharply different levels of increased inequality] is significant because it means that new technologies and the ability of top talent to work on a global scale cannot explain the diverging fortunes of the top 1 percent and those below, since the Japanese have access to the same technologies and global markets as Americans. The answer must lie elsewhere. The authors point to government policy.As the paper shows, the income share of the top 1% in the U.S. declined from a high of around 24% just before the Great Depression to a low of about 9% in the late 1970s. Since then, it has soared all the way back to about 23% just before the Great Recession, but falling back to 20% in 2010. Other English-speaking countries have had similar "U shaped" patterns, as the authors describe them (i.e., reaching Great Depression levels again), but the share of the 1% is much less in other countries. For example, in Australia, even though the 1% share is close to what it was in the 1920s, it is still only 10% of total income, compared to 20% in the U.S. This difference is part of the reason that median wealth is so much higher in Australia.
The paper gives examples of other countries where the 1% share is permanently below its 1920s level, such as Germany, Japan, France, and Sweden. In all four cases, that share is only about 10%. As Johnston emphasizes, these countries are all essentially equal to the U.S. technologically (remember back in the 1980s when so many people thought Japan was poised to eclipse the U.S. in technology?), so their substantially lower levels of inequality stand in direct contradiction to frequent economists' claims that technology is the problem (Richard Freeman has a balanced analysis).
It is also important to point out, as Johnston does, that lower tax rates on the 1% have an impact on this. One suggestion the paper makes is that lower tax rates give CEOs and other top managers more incentive to bargain for higher income, so the effect even shows up in pre-tax income. Obviously, lower tax rates make post-tax income even more unequally distributed.