Recent data and credible, recent sources are what is most important. I
like numbers and figures as well as reasoning (but I especially like
numbers and figures, especially long-term trends).

(1) What damage do negative ads do on the political debate and on
voter participation/apathy?

(2) What percentage of expenditures/air time do negative ads occupy in
Congressional elections? If no data on Congress, then the past
Presidential election is fine.

(3) Do negative ads lead to more just political outcomes? What of the
interest groups who fund them?

That's quite a lot. This is my first Google question, so I'm
interested to see how this works! Question (1) and (2) are the most
important. Thanks!