The prison population here in the United States grew again last year at a rate of about nine hundred prisoners per week. A new government study released Sunday shows that 2.1 million people are in the nation’s prisons and jails. According to the Justice Policy Institute, the United States has the highest rate of incarceration in the world. One in every 138 U.S. residents is now in jail. African-American men make up a disproportionate number of prisoners. Nearly 13 percent of all African-Americans in their late 20s were in jail or prison last year. This compares to less than 2 percent of white men in the same age group.