The number of low-income audits also nearly doubled after 1990, when the IRS began categorizing nonfilers about whom it lacked information in the $25,000-and-under category.
The increase in audits focused on the top 1% of economic scale. 
mnli hypothesis: The increase in audits focused on the top 1% of economic scale.  premise: The number of low-income audits also nearly doubled after 1990, when the IRS began categorizing nonfilers about whom it lacked information in the $25,000-and-under category.