Incarceration in the United States (en-Wikipedia 05/07/11)
Violent and nonviolent crime (stats)
7.9% of sentenced prisoners in federal prisons on September 30, 2009 were in for violent crimes.  52.4% of sentenced prisoners in state prisons at year end 2008 were in for violent crimes.  21.6% of convicted inmates in jails in 2002 (latest available data by type of offense) were in for violent crimes. Among unconvicted inmates in jails in 2002, 34% had a violent offense as the most serious charge. 41% percent of convicted and unconvicted jail inmates in 2002 had a current or prior violent offense; 46% were nonviolent recidivists. 
From 2000 to 2008, the state prison population increased by 159,200 prisoners, and violent offenders accounted for 60% of this increase. The number of drug offenders in state prisons declined by 12,400 over this period. Furthermore, while the number of sentenced violent offenders in state prison increased from 2000 through 2008, the expected length of stays for these offenders declined slightly during this period. 
Violent crime was not responsible for the quadrupling of the incarcerated population in the United States from 1980 to 2003. Violent crime rates had been relatively constant or declining over those decades. The prison population was increased primarily by public policy changes causing more prison sentences and lengthening time served, e.g. through mandatory minimum sentencing, "three strikes" laws, and reductions in the availability of parole or early release. These policies were championed as protecting the public from serious and violent offenders, but instead yielded high rates of confinement for nonviolent offenders. Nearly three quarters of new admissions to state prison were convicted of nonviolent crimes. Only 49 percent of sentenced state inmates were held for violent offenses. Perhaps the single greatest force behind the growth of the prison population has been the national "war on drugs." The number of incarcerated drug offenders has increased twelvefold since 1980. In 2000, 22 percent of those in federal and state prisons were convicted on drug charges.