A 'false memory' is a memory that did not actually occur. The severity of the falsity and the amount of distortion will vary case by case. Some people have memories that are entirely false, and some, just slightly altered. The distortion of the memory does not develop instantly. It can take months. Interestingly, false memories can occur in people without any mental illnesses. [1, 2]
Some interesting examples...
1. Child Sexual Abuse
In the American legal case Ramona v Isabella, two psychologists induced a false memory in their patient, Holly Ramona. She accused her father of sexual abuse and she claimed that her mind had repressed the memory. This turned out to be wrong, so much so, that she wrote a book about the experience. 
In the case State of New Jersey v Margaret Kelly Michaels, it was found that if a child is repeatedly questioned, they may recall a false memory in order to satisfy the questioning. 
In America, false memories are screened in cases with claims of child sexual abuse.
2. Voluntary Confessions of Murder
A 17-year-old man had voluntarily walked into a police station to try to ascertain in his own mind whether or not he had committed a particularly brutal murder.
The young man had been interviewed by police in routine enquiries, and he spent the following evening preoccupied with ‘visions’ of the victim’s face, and he determined to go to the police station the next day to make up his mind whether or not he had committed the murder.
During the course of the next 24 hours, the young man's statements progressed from ‘it might have been me' and "I don’t know if l killed her or not. I keep seeing her’, through ‘I must have done it because l can see a picture of her", to “I am sure killed her… I know I did it.
'Ultimately, this resulted in a written ‘confession.’ In this case, the man continued to equivocate, and he went to trial, unsure what he would plead, but wanting to see the evidence to help him make up his mind regarding his own guilt.
He was convicted and served 25 years in prison before his conviction was quashed on the basis of fresh psychological and psychiatric evidence but during the intervening years he made at least two unequivocal ‘confessions.’ 
3. The Pathological Liar
In addition to being a pathological liar, the following man had a low IQ.
[a researcher] once saw a young man, not very bright and originally from a family of travellers, who talked endlessly about being a pop star, about other stars he knew and about his show business activities despite the fact that it soon became clearly apparent that he led a fairly mundane life in South London and he had come to attention after being charged with a minor offence.
Nevertheless, he appeared to believe his own stories, including his statement that he was ‘world-wide famous in Streatham.' [The researcher] claims that some such subjects "enjoy dressing up in uniforms and posing as ofﬁcers in the armed forces while others pass themselves off as secret service agents." 
4. Alien Abductions
Will believes that he was abducted by aliens, and he has vivid memories of having been experimented on medically, psychologically, and sexually for at least ten years. In fact, he says, his alien guide became pregnant by him, producing twin boys, now eight years old, whom, he says sadly, he will never see but who play a large emotional role in his life. 
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 The Science of False Memory: An Integrative Approach by C.J. Brainerd & V.F. Reyna
 Mistake Were Made (But Not By Me): How We Justify Foolish Beliefs, Bad Decisions and Hurtful Acts by Carol Tavris & Elliot Aronson
 The Cognitive Neuropsychology of False Memories by Daniel L. Schacter
Pierre Janet, a French psychologist, philosopher, & psychotherapist
Sigmund Freud, an Austrian neurologist
Categorisation: Psychology → memory
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