Reverse speech is a phenomenon where people allegedly embed statements backward in spoken communications. Most people regard the idea as pseudoscience, arguing that it is difficult to demonstrate reverse speech in controlled and repeatable conditions. The man who claimed to discover it, David John Oates, an Australian who practices hypnotherapy, was also one of its primary promoters.
The underlying ideas behind the theories Oates proposes are reasonably sound. The subconscious can have an impact on how people frame spoken communications, and it is possible for people to unwittingly express subconscious thought when they speak aloud. The reverse speech theory, however, is rather dubious. According to Oates, it is possible to play back selected spoken statements made by a person to cover the hidden interjections from the subconscious, which may contradict or add to the original statement.
The primary issue with spoken speech is a documented phenomenon known as pareidolia, the tendency to hear meaningful strings of sound in static and other purposeless noises. This trick of the mind can play a role in auditory perception and is often pointed at as the real culprit when people claim to hear things like disembodied voices. When people hear speech played backward, the brain struggles to make sense of the sound, and may apply meaning to it. Two different people may hear the same statement played back at the same speed and come up with different interpretations.
Proponents of reverse speech claim to hear words and phrases when they play sentences backward, but it is difficult to independently verify them. Backward speech can sound meaningful, but people may disagree on the meaning. Speech-language pathologists and others who study speech production believe it is unlikely that people could be speaking backward at the same time they are constructing spoken phrases, and warn against applying reverse speech to activities like criminal investigations.
The idea of hidden messages in spoken and written communications is a very old one, as numerous urban legends about mysterious phrases embedded in music recordings can testify. The subconscious often expresses itself in a variety of ways, and believers in reverse speech may use it for everything from psychotherapy to analysis of speeches made by public figures. Other cues can provide far more information about what is happening on a subconscious level, and they are supported by research; things like facial expression, body movement, and word choice can be revealing when people analyze them to get more information about what someone is thinking or feeling.