While offering donors a gift may improve a campaign’s success, the study found the language project creators used to express the reward made the difference. For example, the phrases “also receive two,” “has pledged” and “project will be” strongly foretell that a project will reach funding status, while phrases such as “dressed up,” “not been able” and “trusting” are attached to unfunded projects.
An interesting look at an interesting question. (Here’s the actual paper.)