"The same threats and incentives that push the innocent to plead guilty also drive many suspects to testify against others. Deals with “co-operating witnesses”, once rare, have grown common. In federal cases an estimated 25-30% of defendants offer some form of co-operation, and around half of those receive some credit for it. The proportion is double that in drug cases. Most federal cases are resolved using the actual or anticipated testimony of co-operating defendants.
Co-operator testimony often sways juries because snitches are seen as having first-hand knowledge of the pattern of criminal activity. But snitches hoping to avoid draconian jail terms may sometimes be tempted to compose rather than merely to sing."