U.S. jails now hold nearly 700,000 inmates on any given day, up from 157,000 in 1970, and the Vera Institute of Justice found that smaller counties now hold 44 percent of the overall total, up from just 28 percent in 1978. Jail populations in mid-sized counties with populations of 250,000 to 1 million residents grew by four times and small-sized counties with 250,000 residents or less grew by nearly seven times, Vera's analysis shows. In that time large county jail populations grew by only about three times. Exactly what's behind that trend is not clear but experts say a range of factors likely contribute, from law enforcement's increased use of summonses and traffic tickets to the closing of state mental hospitals in that time. Unlike state prisons that hold inmates doing lengthy terms, local jails and county lockups are generally used to house pretrial detainees or those who have been sentenced to serve stints of a year or less for relatively minor crimes. Jail use continues to rise though crime rates have declined since peaking in 1991, the analysis shows. Blacks are jailed at nearly four times the rate of whites and the number of women locked up in jails has grown 14-fold since 1970, according to the Vera report. The number of jails with 1,000 beds or more has soared from 21 in 1970 to 145 in 2014, and the average number of days people stay locked up in jail has grown from nine in 1978 to 23 in 2014.