Are there any studies or statistics out there that justify bidding on a search term when your listing is already #1 organically?

I've heard that they reinforce eachother and result in more incremental sales, however I haven't seen any "proof" of that statement.

I too have heard they reinforce each other. First off, there might be some keywords you don't rank for and can through PPCs. Another element is that the people that click on a paid listing aren't necesarily going to click on your organic listing. I haven't seen any formal studies of it. However, if you test it out, it would be nice if you post the results here.

Where was I just reading about 80% of traffic goes towards organic? But I guess it depends on your search term, competitors and target market really. It would vary a great deal.

Personally, if a search term is in the natural results I will not have it in the adwords. I turn it off and go for other search terms where ranking may be lacking.

What I have seen when I have the overlap of both is that top ten ranking wins by far.