We absolutely do know that. Even without any aggressive tax avoidance schemes, if you have a significant investment income, you probably pay an effective ~15% tax rate [1] [2]. I can pretty much guarantee you that most of the people named are in this bucket.
If you earn a salary that puts you in the literal 1%, you probably pay an effective tax rate of 30-50%. I don't know how low your income needs to get to reach a 15% tax rate on income, but it's really low.
1. http://www.nytimes.com/2011/08/15/opinion/stop-coddling-the-...
2. http://www.csmonitor.com/USA/Elections/President/2012/0119/U...