"The potency of incarceration is further diminished by three other forces, researchers have found. The first, sometimes referred to as the 'replacement effect,' applies largely to crimes that occur as part of a market, such as fencing stolen property or, most notably, drug transactions. Once incarcerated, drug dealers tend to be quickly replaced by new dealers and, as during the crack epidemic, the new recruits can be younger and more prone to violence than their predecessors.57 Thus while drug dealers no doubt deserve punishment, most leading researchers, and many law enforcement officials, now agree that incarcerating the foot soldiers in drug gangs, not to mention drug users, has a negligible impact on crime.58 Moreover, by creating job openings in drug-dealing organizations, it draws more people into criminal lifestyles and may in certain cases exacerbate crime.59"

Source

Pew Center on the States, "One in 31: The Long Reach of American Corrections" (Washington, DC: The Pew Charitable Trusts, March 2009), p. 19.