from the article:
“These observations can be ascribed to self determination theory (Deci & Ryan, 1995; Deci, Koestner, & Ryan, 1999). In particular, tangible incentives imply external interventions. As a consequence, individuals feel their autonomy is compromised and hence their intrinsic needs are not fulfilled. Similarly, the tangible incentives might also imply the person is not competent or motivated to engage in the task otherwise; their need to feel competent is not satisfied. Finally, the tangible incentives do not enable the individual to demonstrate their behavior is altruistic; their capacity to fulfill their need to develop relationships is also challenged.
“These considerations imply that tangible incentives will curb intrinsic motivation, but only in specific circumstances. If the incentives are perceived as supportive–reinforcing the natural preferences of individuals, for example–these problems dissipate.
“Kivetz (2005) uncovered some evidence of this proposition. This study showed that individuals who reviewed books preferred to be rewarded with more books, whereas individuals who reviewed songs preferred to be rewarded with music CDs . Rewards that match the task imply the activity is indeed inherently interesting, eliciting intrinsic motivation, and potentially improving performance.”
(read more here: http://www.psych-it.com.au/Psychlopedia/article.asp?id=389)