This paper studies the economic effects of endorsements. In the
publishing sector, endorsements are found to be a business stealing
form of advertising that raises title level sales without expanding
the market size. If anything, the endorsements decrease aggregate
adult fiction sales. This might be a result of the endorsed books
being more difficult than those that otherwise would have been purchased. Economically meaningful sales increases are also found
for nonendorsed titles written by endorsed authors. This spillover
demand demonstrates the broad range of benefits of advertising for
firms operating in a multiproduct brand setting.
Garthwaite, Craig L.
"Demand Spillovers, Combative Advertising, and Celebrity Endorsements."
American Economic Journal: Applied Economics,