Repressed memories are memories that have been unconsciously blocked due to the memory being associated with a high level of stress or trauma. The theory postulates that even though the individual cannot recall the memory, it may still be affecting them subconsciously,
 and that these memories can emerge later into the consciousness. Ideas 
on repressed memory hiding trauma from awareness were an important part 
of Sigmund Freud's early work on psychoanalysis. He later took a different view.
The existence of repressed memories is an extremely controversial
 topic in psychology; although some studies have concluded that it can 
occur in a varying but generally small percentage of victims of trauma, 
many other studies dispute its existence entirely.
 Some psychologists support the theory of repressed memories and claim 
that repressed memories can be recovered through therapy, but most 
psychologists argue that this is in fact rather a process through which 
false memories are created by blending actual memories and outside 
influences.
 One study concluded that repressed memories were a cultural symptom for
 want of written proof of their existence before the nineteenth century,
 but its results were disputed by some psychologists, and a work 
discussing a repressed memory from 1786 was eventually acknowledged, 
though the others stand by their hypothesis.
According to the American Psychological Association, it is not possible to distinguish repressed memories from false ones without corroborating evidence. The term repressed memory is sometimes compared to the term dissociative amnesia, which is defined in the DSM-V
 as an "inability to recall autobiographical information. This amnesia 
may be localized (i.e., an event or period of time), selective (i.e., a 
specific aspect of an event), or generalized (i.e., identity and life 
history)." 
According to the Mayo Clinic, amnesia
 refers to any instance in which memories stored in the long-term memory
 are completely or partially forgotten, usually due to brain injury.
According to proponents of the existence of repressed memories, such 
memories can be recovered years or decades after the event, most often 
spontaneously, triggered by a particular smell, taste, or other 
identifier related to the lost memory, or via suggestion during psychotherapy.
History
It was initially claimed that there was no documented writing about repressed memories or dissociative amnesia (as it is sometimes referred to), before the 1800s.
 This finding, by Harrison G. Pope, was based on a competition in which 
entrants could win $1000 if they could identify "a pre-1800 literary 
example of traumatic memory that has been repressed by an otherwise 
healthy individual, and then recovered."  Pope claimed that no entrant 
had satisfied the criteria. Ross Cheit, a political scientist at Brown 
University, cited Nina, a 1786 opera by the French composer Nicolas Dalayrac.
The concept of repressed memory originated with Sigmund Freud in his 1896 essay Zur Ätiologie der Hysterie ("On the etiology of hysteria").
One of the studies published in his essay involved a young woman by the name of Anna O.
 Among her many ailments, she suffered from stiff paralysis on the right
 side of her body. Freud stated her symptoms to be attached to 
psychological traumas. The painful memories had separated from her 
consciousness and brought harm to her body. Freud used hypnosis to treat
 Anna O. She is reported to have gained slight mobility on her right 
side. Freud's repressed memory theory joined his philosophy of psychoanalysis. Repressed memory has remained a heavily debated topic inside of Freud's psychoanalysis philosophy.
Research
Some research indicates that memories of child sexual abuse and other traumatic incidents may be forgotten. Evidence of the spontaneous recovery of traumatic memories has been shown, and recovered memories of traumatic childhood abuse have been corroborated.
 Forgetting trauma, however, does not necessarily imply that the trauma 
was repressed. It is also possible that trauma may be forgotten through 
normal cognitive processes. This theory is supported by evidence that 
forgetting trauma most often occurs when the trauma did not cause a 
strong emotional reaction in the moment it was experienced.
Van der Kolk and Fisler's research shows that traumatic memories 
are retrieved, at least at first, in the form of mental imprints that 
are dissociated. These imprints are of the affective and sensory 
elements of the traumatic experience. Clients have reported the slow 
emergence of a personal narrative that can be considered explicit 
(conscious) memory. The level of emotional significance of a memory 
correlates directly with the memory's veracity. Studies of subjective 
reports of memory show that memories of highly significant events are 
unusually accurate and stable over time.
 The imprints of traumatic experiences appear to be qualitatively 
different from those of nontraumatic events. Traumatic memories may be 
coded differently from ordinary event memories, possibly because of 
alterations in attentional focusing or the fact that extreme emotional 
arousal interferes with the memory functions of the hippocampus.
Another possibility is that traumatic events are pushed out of 
consciousness until a later events elicits or triggers a psychological 
response. A high percentage of female psychiatric in-patients, and outpatients have reported experiencing histories of childhood sexual abuse. Other 
clinical studies have concluded that patients who experienced incestuous
 abuse reported higher suicide attempts and negative identity formation as well as more disturbances in interpersonal relationships.
There has also been significant questioning of the reality of 
repressed memories. There is considerable evidence that rather than 
being pushed out of consciousness, the difficulty with traumatic 
memories for most people are their intrusiveness and inability to 
forget.  One case that is held up as definitive proof of the reality of repressed memories, recorded by David Corwin has been criticized by Elizabeth Loftus
 and Melvin Guyer for ignoring the context of the original complaint and
 falsely presenting the sexual abuse as unequivocal and true when in 
reality there was no definitive proof.
Retrospective studies (studying the extent to which participants 
can recall past events) depend critically on the ability of informants 
to recall accurate memories.
 The issue of reliability in participants’ introspective abilities has 
been questioned by modern psychologists. In other words, a participant 
accurately recalling and remembering their own past memories is highly 
criticized, because memories are undoubtedly influenced by external, 
environmental factors.
Psychologists Elizabeth Loftus and Katherine Ketcham are authors of the seminal work on the fallacy of repressed memory, The Myth of Repressed Memory (St. Martin's Press, 1994).
Cause
It is hypothesised that repression
 may be one method used by individuals to cope with traumatic memories, 
by pushing them out of awareness (perhaps as an adaptation via psychogenic amnesia) to allow a child to maintain attachment to a person on whom they are dependent for survival. Researchers have proposed that repression can operate on a social level as well.
Other theoretical causes of forgotten memories have stemmed from the idea of Retrieval-Influenced Forgetting,
 which states that “false” memories will be more accurately recalled 
when rehearsed more, than when actual memories get rehearsed. In this 
scenario, the action of rehearsing a falsified memory can actually take 
precedence over the actual memory that a person experiences. Anderson et
 al.
 discovered that rehearsal of novel information exhibits inhibitive 
processes on one’s ability to remember or recall the prior (real) 
memory. This conclusion indicates that past memories can be easily 
forgotten, simply by attending to “real”, novel memories that are 
brought into awareness.
Authenticity
Memories can be accurate, but they are not always accurate. For example, eyewitness testimony even of relatively recent dramatic events is notoriously unreliable.
 Memories of events are a mix of fact overlaid with emotions, mingled 
with interpretation and "filled in" with imaginings. Skepticism 
regarding the validity of a memory as factual detail is warranted.
 For example, one study where victims of documented child abuse were 
reinterviewed many years later as adults, 38% of the women denied any 
memory of the abuse.
Arguments against the existence of "traumatic amnesia" note that 
 various manipulations can be used to implant false memories (sometimes 
called "pseudomemories"). These can be quite compelling for those who 
develop them, and can include details that make them seem credible to 
others. A classic experiment in  memory research, conducted by Elizabeth Loftus,
 became widely known as "Lost in the Mall"; in this, subjects were given
 a booklet containing three accounts of real childhood events written by
 family members and a fourth account of a wholly fictitious event of 
being lost in a shopping mall. A quarter of the subjects reported 
remembering the fictitious event, and elaborated on it with extensive 
circumstantial detail.
 This experiment inspired many others, and in one of these, Porter et 
al. could convince about half of his subjects that they had survived a 
vicious animal attack in childhood.
Such experimental studies have been criticized in particular about whether the findings are really relevant to trauma memories and psychotherapeutic situations. Nevertheless, these studies prompted public and professional concern about recovered memory therapy for past sexual abuse.
 When memories are "recovered" after long periods of amnesia, 
particularly when extraordinary means were used to secure the recovery 
of memory, it is now widely (but not universally) accepted that the 
memories are quite likely to be false, i.e. of incidents that had not 
occurred.
 It is thus recognised by professional organizations that a risk of 
implanting false memories is associated with some similar types of 
therapy. The American Psychiatric Association advises: "...most 
leaders in the field agree that although it is a rare occurrence, a 
memory of early childhood abuse that has been forgotten can be 
remembered later. However, these leaders also agree that it is possible 
to construct convincing pseudomemories for events that never occurred.
Nevertheless, many therapists believe in the authenticity of the 
recovered memories that they hear from their clients. In a non-random 
study by Loftus and Herzog (1991) with 16 clinicians, 13 (81%) said that
 they invariably believed their clients. The most common basis for this 
belief was the patient’s symptomology (low self-esteem, sexual 
dysfunction, self-destructive behaviour) or body memories (voice frozen 
etc.).
The mechanism(s) by which both of these phenomena happen are not 
well understood and, at this point it is impossible, without other 
corroborative evidence, to distinguish a true memory from a false one."
 Sheflin and Brown state that a total of 25 studies on amnesia for child
 sexual abuse exist and that they demonstrate amnesia in their study 
subpopulations. However, an editorial in the British Medical Journal states on the Sheflin and Brown study that "on critical examination, the scientific evidence for repression crumbles."
Obviously, not all therapists agree that false memories are a 
major risk of psychotherapy and they argue that this idea overstates the
 data and is untested. Several studies have reported high percentages of the corroboration of recovered memories,
 and some authors have claimed that the false memory movement has tended
 to conceal or omit evidence of (the) corroboration" of recovered 
memories.
Both true and false "memories" can be recovered using memory work
 techniques, but there is no evidence that reliable discriminations can 
be made between them. Some believe that memories "recovered" under hypnosis are particularly likely to be false.
According to The Council on Scientific Affairs for the American Medical 
Association, recollections obtained during hypnosis can involve 
confabulations and pseudomemories and appear to be less reliable than 
nonhypnotic recall.
Brown et al. estimate that 3 to 5% of laboratory subjects are vulnerable
 to post-event misinformation suggestions. They state that 5–8% of the 
general population is the range of high-hypnotizability. Twenty-five 
percent of those in this range are vulnerable to suggestion of 
pseudomemories for peripheral details, which can rise to 80% with a 
combination of other social influence factors. They conclude that the 
rates of memory errors run 0–5% in adult studies, 3–5% in children's 
studies and that the rates of false allegations of child abuse 
allegations run 4–8% in the general population.
Neurological basis of memory
The neuroscientist Donald Hebb (1904–1985) was the first to distinguish between short-term memory and long-term memory.
 According to current theories in neuroscience, things that we "notice" 
are stored in short-term memory for up to a few minutes; this memory 
depends on "reverberating" electrical activity in neuronal circuits, and
 is very easily destroyed by interruption or interference. Memories 
stored for longer than this are stored in "long-term memory". Whether 
information is stored in long-term memory depends on its "importance"; 
for any animal, memories of traumatic events are potentially important 
for the adaptive value that they have for future avoidance behavior, and
 hormones
 that are released during stress have a role in determining what 
memories are preserved. In humans, traumatic stress is associated with 
acute secretion of epinephrine and norepinephrine (adrenaline and noradrenaline) from the adrenal medulla and cortisol from the adrenal cortex.
 Increases in these facilitate memory, but chronic stress associated 
with prolonged hypersecretion of cortisol may have the opposite effect. 
The limbic system
 is involved in memory storage and retrieval as well as giving emotional
 significance to sensory inputs. Within the limbic system, the hippocampus is important for explicit memory, and for memory consolidation; it is also sensitive to stress hormones, and has a role in recording the emotions of a stressful event. The amygdala may be particularly important in assigning emotional values to sensory inputs.
Although memory distortion occurs in everyday life, the brain mechanisms involved are not easy to study in the laboratory, but neuroimaging
 techniques have recently been applied to this subject. In particular, 
there have recently been studies of false recognition, where individuals
 incorrectly claim to have encountered a novel object or event, and the 
results suggest that the hippocampus and several cortical regions may 
contribute to such false recognition, while the prefrontal cortex may be
 involved in retrieval monitoring that can limit the rate of false 
recognition.
Amnesia
Amnesia
 is partial or complete loss of memory that goes beyond mere forgetting.
 Often it is temporary and involves only part of a person's experience. 
Amnesia is often caused by an injury to the brain, for instance after a 
blow to the head, and sometimes by psychological trauma. Anterograde amnesia is a failure to remember new experiences that occur after damage to the brain; retrograde amnesia
 is the loss of memories of events that occurred before a trauma or 
injury. For a memory to become permanent (consolidated), there must be a
 persistent change in the strength of connections between particular 
neurons in the brain. Anterograde amnesia can occur because this 
consolidation process is disrupted; retrograde amnesia can result either
 from damage to the site of memory storage or from a disruption in the 
mechanisms by which memories can be retrieved from their stores. Many 
specific types of amnesia are recognized, including:
- Childhood amnesia is the normal inability to recall memories from the first three years of life. Sigmund Freud observed that not only do humans not remember anything from birth to three years, but they also have “spotty” recollection of anything occurring from three to seven years of age. There are various theories as to why this occurs: some believe that language development is important for efficient storage of long-term memories; others believe that early memories do not persist because the brain is still developing.
 - A fugue state, formally dissociative fugue, is a rare condition precipitated by a stressful episode. It is characterized by episode(s) of traveling away from home and creating a new identity.
 
The form of amnesia that is linked with recovered memories is 
dissociative amnesia (formerly known as psychogenic amnesia). This 
results from a psychological cause, not by direct damage to the brain, 
and is a loss of memory of significant personal information, usually 
about traumatic or extremely stressful events. Usually this is seen as a
 gap or gaps in recall for aspects of someone's life history, but with 
severe acute trauma, such as during wartime, there can be a sudden acute
 onset of symptoms.
Effects of trauma on memory
"Betrayal
 Trauma Theory" proposes that in cases of  childhood abuse, dissociative
 amnesia is an adaptive response, and that “victims may need to remain 
unaware of the trauma not to reduce suffering but rather to promote 
survival.”
When stress interferes with memory,
 it is possible that some of the memory is kept by a system that records
 emotional experience, but there is no symbolic placement of it in time 
or space.
Traumatic memories are retrieved, at least at first, in the form of 
dissociated mental imprints of the affective and sensory elements of the
 traumatic experience. Clients have reported the slow emergence of a 
personal narrative that can be considered explicit (conscious) memory. 
Psychiatrist Bessel van der Kolk divided the effects of traumas on memory functions into four sets:
- Traumatic amnesia; this involves the loss of memories of traumatic experiences. The younger the subject and the longer the traumatic event is, the greater the chance of significant amnesia. He stated that subsequent retrieval of memories after traumatic amnesia is well documented in the literature, with documented examples following natural disasters and accidents, in combat soldiers, in victims of kidnapping, torture and concentration camp experiences, in victims of physical and sexual abuse, and in people who have committed murder.
 - Global memory impairment; this makes it difficult for subjects to construct an accurate account of their present and past history. "The combination of lack of autobiographical memory, continued dissociation and of meaning schemes that include victimization, helplessness and betrayal, is likely to make these individuals vulnerable to suggestion and to the construction of explanations for their trauma-related affects that may bear little relationship to the actual realities of their lives"
 - Dissociative processes; this refers to memories being stored as fragments and not as unitary wholes.
 - Traumatic memories’ sensorimotor organization. Not being able to integrate traumatic memories seems to be linked to posttraumatic stress disorder (PTSD).
 
According to van der Kolk, memories of highly significant events are 
usually accurate and stable over time; aspects of traumatic experiences 
appear to get stuck in the mind, unaltered by time passing or 
experiences that may follow. The imprints of traumatic experiences 
appear to be different from those of nontraumatic events, perhaps 
because of alterations in attentional focusing or the fact that extreme 
emotional arousal interferes with  memory.
 van der Kolk and Fisler's hypothesis is that under extreme stress, the 
memory categorization system based in the hippocampus fails, with these 
memories kept as emotional and sensory states.  When these traces are 
remembered and put into a personal narrative, they are subject to being 
condensed, contaminated and embellished upon. 
When there is inadequate recovery time between stressful situations, alterations may occur to the stress response
 system, some of which may be irreversible, and cause pathological 
responses, which may include memory loss, learning deficits and other 
maladaptive symptoms. In animal studies, high levels of cortisol
 can cause hippocampal damage, which may cause short-term memory 
deficits; in humans, MRI studies have shown reduced hippocampal volumes 
in combat veterans with PTSD, adults with posttraumatic symptoms and 
survivors of repeated childhood sexual or physical abuse. Trauma may 
also interfere with implicit memory, where periods of avoidance may be 
interrupted by intrusive emotional occurrences with no story to guide 
them. A difficult issue is whether those presumably abused accurately 
recall their experiences.
Criticism
The existence of repressed memory recovery has not been accepted by mainstream psychology,
 nor unequivocally proven to exist, and some experts in the field of 
human memory feel that no credible scientific support exists for the 
notions of repressed/recovered memories.
 A survey revealed that whilst memory and cognition experts tend to be 
skeptical of repressed memory, clinicians are much more apt to believe 
that traumatic memory is often repressed.
 One  research report states that a distinction should be made between 
spontaneously recovered memories and memories recovered during 
suggestions in therapy.
 A criticism from Loftus is that recovered memories can be  tainted by 
the process of recovery, the suggestions used in that process, or even 
cultural and environmental influences.
The Working Group on Investigation of Memories of Child Abuse of 
the American Psychological Association presented findings mirroring 
those of the other professional organizations. The Working Group made 
five key conclusions:
- Controversies regarding adult recollections should not be allowed to obscure the fact that child sexual abuse is a complex and pervasive problem in America that has historically gone unacknowledged;
 - Most people who were sexually abused as children remember all or part of what happened to them;
 - It is possible for memories of abuse that have been forgotten for a long time to be remembered;
 - It is also possible to construct convincing pseudo-memories for events that never occurred; and
 - There are gaps in our knowledge about the processes that lead to accurate and inaccurate recollections of childhood abuse.
 
Many critics believe that memories may be distorted and false. Psychologist Elizabeth Loftus
 questions the concept of repressed memories and the possibility of them
 being accurate. Loftus focuses on techniques that therapists use in 
order to help the patients recover their memory. Such techniques include
 age regression, guided visualization, trance writing, dream work, body 
work, and hypnosis.
Loftus' research indicates that repressed memory faces problems, such as
 memory alteration. In one case a teenage boy was able to “conjure a 
memory of an event that never occurred.” According to Loftus, if a 
stable person could be influenced to remember an event that never 
occurred, an emotionally stressed person would be even more susceptible.
Writer Mark Pendergrast has denounced the theory of repressed memories and its applications in sex abuse cases, including in particular the Jerry Sandusky case.
Medico-legal issues
Serious
 issues arise when recovered but false memories result in public 
allegations; false complaints carry serious consequences for the 
accused. Many of those who make false claims sincerely believe the truth
 of what they report. A special type of false allegation, the false memory syndrome,
 arises typically within therapy, when people report the "recovery" of 
childhood memories of previously unknown abuse. The influence of 
practitioners' beliefs and practices in the eliciting of false 
"memories" and of false complaints has come under particular criticism.
It is generally accepted that people sometimes are unable to recall traumatic experiences. An old version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), published by the American Psychiatric Association,
 states that "Dissociative amnesia is characterized by an inability to 
recall important personal information, usually of a traumatic or 
stressful nature, that is too extensive to be explained by ordinary 
forgetfulness."
The term "recovered memory", however, is not listed in DSM-IV or used by any mainstream formal psychotherapy modality.
Legal state
Some
 criminal cases have been based on a witness's testimony of recovered 
repressed memories, often of alleged childhood sexual abuse. In some 
jurisdictions, the statute of limitations
 for child abuse cases has been extended to accommodate the phenomena of
 repressed memories as well as other factors.  The repressed memory 
concept came into wider public awareness in the 1980s and 1990s followed
 by a reduction of public attention after a series of scandals, 
lawsuits, and license revocations.
A U.S. District Court accepted repressed memories as admissible evidence in a specific case. Dalenberg argues that the evidence shows that recovered memory cases should be allowed to be prosecuted in court.
The apparent willingness of courts to credit the recovered 
memories of complainants but not the absence of memories by defendants 
has been commented on: "It seems apparent that the courts need better 
guidelines around the issue of dissociative amnesia in both 
populations."
In 1995, the Ninth Circuit Court of Appeals ruled, in Franklin v. Duncan and Franklin v. Fox, Murray et al. (312 F3d. 423, see also 884 FSupp 1435, N.D. Calif.),
 that repressed memory is not admissible as evidence in a legal action 
because of its unreliability, inconsistency, unscientific nature, 
tendency to be therapeutically induced evidence, and subject to 
influence by hearsay and suggestibility. The court overturned the 
conviction of a man accused of murdering a nine-year-old girl purely 
based upon the evidence of a 21-year-old repressed memory by a lone 
witness, who also held a complex personal grudge against the defendant.
In a 1996 ruling, a U.S. District Court allowed repressed memories entered into evidence in court cases.
 Jennifer Freyd writes that Ross Cheit's case of suddenly remembered 
sexual abuse is one of the most well-documented cases available for the 
public to see. Cheit prevailed in two lawsuits, located five additional 
victims and tape-recorded a confession.
On December 16, 2005, the Irish Court of Criminal Appeal issued a
 certificate confirming a Miscarriage of Justice to a former nun, Nora Wall whose 1999 conviction for child rape was partly based on repressed-memory evidence. The judgement stated that:
There was no scientific evidence of any sort adduced to explain the phenomenon of "flashbacks" and/or "retrieved memory", nor was the applicant in any position to meet such a case in the absence of prior notification thereof.
On August 16, 2010 the United States Second Circuit Court of Appeals 
in a case reversed the conviction that relied on claimed victim memories
 of childhood abuse stating that "The record here suggests a "reasonable
 likelihood" that Jesse Friedman was wrongfully convicted.  The "new and
 material evidence” in this case is the post-conviction consensus within
 the social science community that suggestive memory recovery tactics 
can create false memories" (pg 27 FRIEDMAN v. REHAL Docket No. 08-0297).
 The ruling goes on to order all previous convictions and plea bargains 
relying in repressed memories using common memory recovered techniques 
be reviewed.
Clinical relevance
Recovered memory therapy
Recovered memory therapy is a range of psychotherapy methods based on recalling memories of abuse that had previously been forgotten by the patient. The term "recovered memory therapy" is not listed in DSM-IV or used by mainstream formal psychotherapy modality.  Opponents of the therapy advance the explanation that therapy can create false memories through suggestion techniques; this has not been corroborated, though some research has shown supportive evidence. Nevertheless, the evidence is questioned by some researchers. It is possible for patients who retract their claims—after deciding their recovered memories are false—to suffer post-traumatic stress disorder due to the trauma of illusory memories.