Polygraphs are already used in other aspects of law
Polygraph tests have been successfully implemented in other parts of the legal process.
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Context
Lie detectors are already used in the construction of plea deals, monitoring convicted criminals released on probation, and within the governing bodies of several sports including deep-sea angling and natural bodybuilding and football.[1]
The Argument
Many organisations are already using polygraph testing as an effective tool to investigate potential misdeeds. In Singapore, the Football Association (FA) uses polygraph tests to combat match-fixing and has seen promising results.
Lie detectors are already used by the UK’s probation service. They use them to monitor sex offenders released on parole to ensure they are not breaching the terms of their release. Seven police department currently use them across England and Wales and maintain their effectiveness.[2]
Counter arguments
Just because some organisations are using polygraph tests to police their fields does not mean they should be used in the legal sphere. There are plenty of organisations that have publicly denounced polygraph testing and eradicated them from their governing processes.
There is also a big difference between the implications of a false lie detector result for those on probation, or from a sport's governing body, versus those in court for the first time.
Consider a false positive reading for a criminal that is on probation. The criminal has already been convicted of a crime. They have been released early on parole for good behaviour. The implication of a false positive on a lie detector test would see them return to prison to finish their original sentence.
While this is an injustice, it is a far less serious injustice than a law-abiding citizen being locked up for a crime they didn't commit based on a false positive lie detector result.
The Employee Polygraph Protection Act (EPPA) banned private sector employers from using polygraph tests on job candidates due to their wild inconsistencies. Why would we want to make something admissible as evidence in court that is not even accurate enough to use in the candidate vetting process for a job?[3]
Premises
[P1] Other law enforcement and governing bodies are using lie detectors.
[P2] They have found them to be successful in enforcing laws and rooting out offenders.
[P3] Therefore, they are likely to be effective in criminal investigations.
[P4] Therefore, they should be admissible in court.
Rejecting the premises
[Rejecting P2] The stakes are far lower when considering whether someone should be banned from a sport of returned to prison for breaching their parole than when considering someone's original guilt from innocence.
[Rejecting P4] When the stakes are so high, the outcome of a criminal trial should not be impacted in any way by the results of a polygraph test.