An ankle monitor (also known as a tether, or ankle bracelet) is a device that individuals under house arrest or parole are often required to wear. At timed intervals, the ankle monitor sends a radio frequency signal containing location and other information to a receiver. If an offender moves outside of an allowed range, the police will be notified. Ankle monitors are designed to be tamper-resistant and can alert authorities to removal attempts, such as cutting the conductive band causing a circuit break.
The most common configuration is a radio-frequency transmitter unit that sends a signal to a fixed location receiving unit in the offender's residence. The residence unit uses either a land line or a cellular network to relay information to a service center computer. If the offender is not at the residence at times stipulated, an alert message is sent to the service center, and then relayed to the supervising probation or parole officer. GPS units are similar in design, but the offender also carries a GPS cell phone unit that receives a signal from the ankle unit, or both functions may be combined into one ankle unit.
Electronic monitoring was originally developed by a small group of researchers at Harvard University in the 1960s, headed by R. Kirkland Schwitzgebel and his twin brother, Robert Schwitzgebel (family name shortened to "Gable" in 1983). In 1983, Judge Jack Love in Albuquerque, New Mexico, inspired by a Spider-Man comic strip, initiated the first judicially sanctioned program using monitoring devices. These were produced by Michael T. Goss, a former Honeywell computer sales representative. Shortly thereafter, programs began in Florida using a cuff invented by Thomas Moody.
Within six years, at least 16 manufacturers were listed in the Journal of Offender Monitoring. In 2007, an estimated 130,000 units were deployed daily in the United States. They also gained popularity in the United Kingdom, but adoption in the rest of the EU was a little slower. A collection of early equipment and a written summary, with photographs, of the history of commercial devices in the United States  is housed at the Archives of the History of American Psychology, University of Akron, Akron, Ohio, USA.
The effectiveness of monitoring in reducing crime is uncertain, especially as most people who violate parole are not committing crimes. It is thought that the monitoring may serve as a deterrent to criminal behaviour; however, a thorough and comprehensive review of research literature has indicated that, over a period of three years, monitoring did not reduce crime more than other prison diversion programs. The inventors, Kirkland and Robert Gable, who are now emeritus Professors of Psychology at California Lutheran University and the Claremont Graduate University, have been strongly advocating the use of positive incentives in monitoring programs.
A SCRAM bracelet ("Secure Continuous Remote Alcohol Monitoring") – waterproof and worn on either leg as a condition of probation or parole – can detect any alcohol consumption via transdermal measurements throughout the day. Anti-tamper features are in place to thwart attempts such as placing a flat item between the leg and the device to block skin contact. They typically also function as a normal tether, i.e. providing 24-hour location-based data on the person's whereabouts, and calling in any violations of either protocol. Both cannot be removed by the user without it being reported to the base station. The National Highway Traffic Safety Administration produced a favorable report on the use of electronic monitoring of DUI offenders, with lower recidivism rates and less cost compared to jail.
- Cronin, Brian. "Comic Book Urban Legends Revealed #38!" Comic Book Resources, 16 February 2006.
- Robert S. Gable, An Informal History of the Beginning of Electronic Monitoring of Offenders, Dec. 8, 2009
- See Kathy S. Padgett, et al., Under Surveillance: An Empirical Test of the Effectiveness and Consequences of Electronic Monitoring, Criminology and Public Policy,2006, vol. 5, pp. 61-92
- See Marc Renzema & Evan Mayo-Wilson, Can Electronic Monitoring Reduce Crime for Moderate to High-risk Offenders? Journal of Experimental Criminology, 2005, vol 1, pp. 1-23.
- Ralph Kirkland Gable & Robert S. Gable, Electronic Monitoring: Positive Intervention Strategies, Federal Probation, 2005, vol 69, pp. 21-25, at http://www.uscourts.gov/uscourts/FederalCourts/PPS/Fedprob/2005-06/intervention.html
- Marques, Paul R.; A. Scott McKnight (November 2007). "Evaluating Transdermal Alcohol Measuring Devices". National Highway Traffic Safety Administration. Retrieved 30 March 2012.
- http://www.nhtsa.dot.gov/people/injury/enforce/PromisingSentence/pages/PSP6.htm NHTSA article