The Congressional Research Service is the research arm of Congress. It is very respected and I have found their reports in the past to be nonpartisan and very accurate. This CRS report was based on statistics starting in 1945. It is report R42729 so check it out yourself.
I realize that everyone with a different point of view will dismiss the report even if it were signed by God himself. Our Democracy today is based on ideology and opinion, not facts. It is all about what you want to believe and not what the reality is.