Excerpts from The Evidence for Dissociative Amnesia :A review of 100 Years Research
by Stephanie Dallam RN, MS, FNP

A traumatic memory is a memory of personally traumatic event. For the purpose of diagnosing PTSD, a personally traumatic event  is defined as:

“The person experienced, witnessed or was confronted with an event or events that involved actual or threatened death or serious injury, or a threat to the physical integrity of self or others....The person’s experience involved intense fear, helplessness, or horror.” (DSM-IV, APA 1994, p. 427-31).

One of the most effective ways people cope with overwhelming trauma is called "dissociation." Dissociation is a complex mental process during which there is a change in a person's consciousness which disturbs the normally connected functions of identity, memory, thoughts, feelings and experiences.

Research has shown that traumatized individuals respond by using a variety of psychological mechanisms. One of the most common means of dealing with the pain is to try and push it out of awareness. Some label the phenomenon of the process whereby the mind avoids conscious acknowledgment of traumatic experiences as dissociative amnesia.  Others use terms such as repression, dissociative state, traumatic amnesia, psychogenic shock, or motivated forgetting.  Semantics aside, there is near-universal scientific acceptance of the fact that the mind is capable of avoiding conscious recall of traumatic experiences.

Overview of Historical Research

One of the first reports in the scientific literature was by Myers, C. S. (1916, January 8). Contributions to the study of Shell Shock. The Lancet, 65- 69.

Myers noted, “After nine months of special work in France and Belgium upon these disorders I have not the slightest hesitation in maintaining the genuineness of the cases above described.” Regarding “shell shock”, he noted, “…it will be seen that certain disturbances are common to all, e.g., amnesia, varying from complete obliterations of the past to obliviscence of the scenes occasioning and following the shock, severe headache, and a mental condition varying from slight depression to severe stupor.”

Accoring to Myers, hypnosis was very effective in reversing the amnesia.

“…in all these cases whenever the memories dissociated from the normal personality were revived they were accompanied by an outburst of emotion, sometimes of frenzy, but generally of fear.”

“…there was frequently a strong disinclination to talk of the forgotten periods, as if they were being actively inhibited or “suppressed” rather than as if they had been passively “dissociated.”

Rivers, W.H R. (1918). The repression of war experiences. Lancet, 194, 173-7.

Rivers regarded repression as a process of active, conscious exclusion of traumatic memories from awareness, -- suppression, stating, “It is natural to thrust aside painful memories just as it is natural to avoid dangerous or horrible scenes in actuality, and this natural tendency to banish the distressing is especially pronounced in those whose powers of resistance have been lowered by the long-continued strains of trench life, the shock of explosion, or other catastrophes of warfare.…”

He also recognized that there was a social component – that people were motivated to forget bad things and other people reinforced this tendency. “[T]he natural tendency to repress beingin my experience almost universally fostered by their relatives and friends, as well as by their medical advisers.” (p. 173).

Thom, DA & Fenton, N. (1920). Amnesias in war cases. American Journal of Insanity, 76, 437-448.

Thom related the amnesia to dissociation. “There was no question of these soldiers being psychotic or neurotic prior to their loss of memory. All were considered desirable for military service.” Further, Thom & Fenton recognized that the etiology of this amnesia was primarily psychic in origin – a finding confirmed by the fact that in two of these cases the memory was completely restored by the use of psychotherapy.

Sargent, W., & Slater, E. (1941). Amnesic syndromes in war. Proceedings of the Royal Society of Medicine, 34, 757-764.

In 1941, we see the first large scale study. Sargent and Slater examined the incidence of amnesia in 1,000 serial admissions to a military hospital.

TABLE 1. ---
FREQUENCY OF CASES SHOWING AN AMNESIC SYNDROME IN A THOUSAND SERIAL ADMISSIONS.
 

	Cases admitted	Amnesia present	Frequency
Severe Stress	251	87	35%
Moderate Stress	155	20	13%
Triffling Stress	594	37	6%
Total	1,000	144	14%

  
“Severe stress” means prolonged marching and fighting under heavy enemy action (e.g., in the battle of Dunkirk through which the great majority of these cases passed); “moderate stress” includes experiences like periodic dive-bombing at home bases and aerodromes; “trifling stress” means the normal life of a training camp or depot, involving no greater strain than an unfamiliar life and separation from home.
 
Carlson, E., & Rosser-Hogan, R. (April, 1993). Mental health status of Cambodian refugees ten years after leaving their homes. American Journal of Orthopsychiatry, 63(2), 223-231.

Dissociation is also a frequent finding in survivors extreme terror. Between 1975 and 1979, an estimated one to three million of a population of seven million Cambodians were killed or died of starvation. Carlson, E., & Rosser-Hogan selected 50 subjects at random from a list of all refugees (~500) resettled by nonprofit organization between 1983 and 1985. None had any formal education and had lived in the US for a mean of 5 years. 86% met the criteria for PTSD. The mean number of traumatic experiences the refugees endorsed was 14 and "90% reported amnesia for upsetting events." Mean DES score was 37.1 – which is extremely high compared to the mean score for the general population samples in the US – 8.6.

Krell, R. (1993). Child survivors of the Holocaust: Strategies of adaptation. Canadian Journal of Psychiatry, 38, 384-389.

Krell reported on 22 Holocaust survivors who, as children, hid from the Nazis.

"As children they were encouraged not to tell, but to lead normal lives and forget the past....”

“The most pervasive preoccupation of child survivors is the continuing struggle with memory, whether there is too much or too little....”

“For a child survivor today, an even more vexing problem is the intrusion of fragments of memory - most are emotionally powerful and painful but make no sense. They seem to become more frequent with time and are triggered by thousands of subtle or not so subtle events....”

Marks, J. (1995). The hidden children: The secret survivors of the Holocaust. Toronto: Bantam Books.

One holocaust survivor, Ava Landy, describes her amnesia:

“So much of my childhood between the ages of four and nine is blank....It's almost as if my life was smashed into little pieces... The trouble is, when I try to remember, I come up with so little. This ability to forget was probably my way of surviving emotionally as a child. Even now, whenever anything unpleasant happens to me, I have a mental garbage can in which I can put all the bad stuff and forget it.... I'm still afraid of being hungry....I never leave my house without some food....Again, I don't remember being hungry. I asked my sister and she said that we were hungry. So I must have been! I just don't remember." (p. 188). 

Summary of Research Examining the Prevalence of Full or Partial Dissociative Amnesia for Traumatic Events

A review of 50 studies revealed that amnesia rates tend to increase with severity of trauma and is particularly high in victims of sex crimes.

What factors increase likelihood that the trauma will be forgotten?

There are several factors which influence whether a traumatic experience is remembered or dissociated.

The nature and frequency of the traumatic events and the age of the victim seem to be the most important.
Single-event traumas (assault, rape, witnessing a murder, etc.) are more likely to be remembered, but repetitive traumas (repeated domestic violence or incest, political torture, prolonged front-line combat, etc.) often result in memory disturbance.
Stressful experiences caused by natural or accidental disasters (earthquakes, plane crashes, violent weather, etc.) are more likely to be remembered than traumatic events deliberately caused by humans (i.e. incest, torture, war crimes).
People who are adults when they experience traumatic events are less likely to dissociate conscious memories of the events than children who experience trauma.
Clinical evidence indicates that the population most likely to develop amnesia for traumatic experiences consists of child victims coerced into silence about repetitive, deliberately caused trauma such as incest or extra-familial physical, emotional, or sexual abuse.
Another factor that contributes to memory disturbances is the double-bind felt by children trying to make sense of living in abusive relationships on which they depend for nurturance.
The double-bind that children are placed in when a child suffers abuse at the hands of a caretaker, is discussed by Jennifer Freyd in her book on Betrayal Trauma. Freyd's theory explains how a blockage of information is functional, allowing a child who is abused by a parent to be able to ignore information that would otherwise interfere with their ability to function within an essential relationship.

Freyd, Jennifer. (1996). Betrayal Trauma: The Logic of Forgetting Childhood Sexual Abuse. Cambridge, MA: Harvard University Press.

Seven factors predicting amnesia:

Abuse by caregiver
Explicit threats demanding silence
Alternative realities in environment (abuse context different from nonabuse context)
Isolation during abuse
Young at age of abuse
Alternative reality-defining statements by caregiver
Lack of discussion of abuse
What is the relation to memory recovery to treatment?


Albach et al. studied 97 adult victims of extreme sexual abuse and a control group of 65 women, matched for age and education who reported on their memories of “ordinary unpleasant childhood experiences.”  The abuse survivors were broken into two groups.  One group had participated in psychotherapy while the other group had not. There was no significant differences in amnesia, memory recovery, or other memory phenomena between the survivors who participated in psychotherapy and those who did not.

Wilsnack S. C., et al. (2002).  Self-reports of forgetting and remembering childhood sexual abuse in a nationally representative sample of US women. Child Abuse & Neglect, 26, 139-147.

Wilsnack et al. studied a national probability sample of 711 women, aged 26 years to 54 years. 22% of respondents reported having sexually coercive experiences while growing up; of these, 69.0% indicated that they felt they had been sexually abused. More than one-fourth of respondents who felt sexually abused reported that they had forgotten the abuse for some period of time but later remembered it on their own. Only 1.8% of women self-described as sexually abused reported remembering the abuse with the help of a therapist or other professional person. The vast majority of memories were recovered in other contexts. 

How Accurate are Recovered Memories?

Williams, L. M. (1995). Recovered memories of abuse in women with documented child sexual victimization histories. Journal of Traumatic Stress, 8(4), 649-673.

129 with documented histories of sexual victimization in childhood were interviewed 17 years later and asked about abuse history. 80 of the women recalled the victimization. However, 16% of those who recalled the abuse reported that at some time in the past they had forgotten about the abuse.

There were similar numbers of discrepancies in reports of women who had recovered memories and those who had always remembered compared to the reports from the early 1970s. Despite limited discrepancies, the women themselves were very often unsure about their memories and said things such as 'What I remember is mostly a dream.' Or, 'I'm really not too sure about this.'  Level of uncertainty about recovered memories was not associated with more discrepancies in her account.

Orr, S. P., Lasko, N. B., Metzger, L. J., Berry, N. J., Ahern, C. E., & Pitman, R. K. (1997). Psychophysiologic assessment of PTSD in adult females sexually abused during childhood. In R. Yehuda & A.C. McFarlane (Eds.), Psychobiology of Posttraumatic Stress Disorder. Annuals of the New York Academy of Sciences, Volume 821. NY: The New York Academy of Sciences, pp. 491-3.

This study examined abused women’s physiologic responses during imagery of personal childhood sexual abuse experiences.  The sample consisted of 71 women with a history of two or more episodes of sexual abuse prior to age 13. The investigators found that magnitudes of physiologic responses (i.e., heart rate, skin conductance, and electromyograms) during personal abuse imagery did not differ between those who recovered memories and those who had continuous memories. In addition, PTSD associated with continuous and recovered memories was found to be comparable.

Dalenberg, C. J. (1996). Accuracy, timing and circumstances of disclosure in therapy of recovered and continuous memories of abuse. Journal of Psychiatry & Law,24(2), 229-75.

Accuracy for Continuous Versus Recovered Memories

In depth family research and interviews revealed that the amount of evidence supporting continuous versus recovered memories were similar.

What causes the memory to resurface?

Albach, F., Moormann, P. & Bermond, B. (1996). Memory Recovery of Childhood Sexual Abuse. Dissociation,9(4), 261-273.

Events that triggered recall of abuse:

Discovering their own daughter had been abused
After another trauma occurred
When confronted with specific sensory triggers, especially during a time when they were “physically and emotionally exhausted.”
Triggers mentioned included:

Being touched on the back
Smell of alcohol or tobacco on a man's breath; male sweat; specific soap or after-shave; semen
Hearing panting noises or footsteps on the stairs
Seeing things such as an erect penis, dirty hands, specific masculine pajamas, a handkerchief, and white liquids.
Reading articles or seeing television programs about incest Verbal cues were not as significant as olfactory, sensorimotor, auditory, and visual cues.
Conclusion

Scientific evidence shows that it is not rare for traumatized people to experience amnesia or delayed recall for the trauma. Amnesia has been reported in combat, for crimes, and for concentration camp experiences and torture.
The more severe the trauma, the more likely it is to be forgotten.
Overall, a recovered memory is just as likely to be accurate as a continuously remembered one. However, recovered memories have a prominence of emotional and sensory-perceptual elements vs. declarative (verbal) elements. They are often fragmentary and incomplete and thus hard to make into coherent story.