Memory is not like a library where you can check out a book whenever you want.

It can be hard to understand why some people can remember every aspect of a traumatic event while others lose the memory altogether, or for periods of time in their life. We all have a natural ability, a coping mechanism that allows us to ‘dissociate’ from unbearable events. It helps us to avoid conscious awareness of what is happening and sometimes for a little time after. The memories may come back after some time has passed.[See:  Sidran Institute:  Traumatic Stress Education and Advocacy]

Dissociation is often described as an “out of body” experience where someone feels detached from reality. It may be upsetting for someone to realize that they have dissociated, but it is a natural reaction to trauma.

Most professionals believe that dissociation exists on a spectrum. At one end of the spectrum is an experience like daydreaming. At the other end is chronic and complex dissociation which may make it difficult for a person to function in the "real" world.[See: RAINN: Dissociation]

The disturbance of memory is one of the signals of Post Traumatic Stress Disorder.  PTSD is a psychiatric diagnosis that is common among people who have survived highly traumatic events such as sexual assaults.  People with PTSD may remember every vivid detail (hypernesia) or, their memory forgets the event altogether (amnesia).

The memories in detail can be overwhelming. They intrude on and disrupt a person’s daily life. They often can’t get the “pictures” of the trauma out of their heads. They may aslo suffer from recurring nightmares, flashbacks, or they may even relive the trauma as if it was happening in present time.

The traumatic event may be triggered by a smell, sound or similar experience that is reminds them of the original event. This is why it is common for traumatized people to go to great lengths to avoid thoughts or feelings that may trigger them. Sometimes, the avoidance of reminders of the trauma may cause a person to have “dissociative amnesia,” or memory blanks for important aspects of the trauma.

Why do some trauma survivors have a continuous memory of the experience while others have amnesia for all or part of their experience?

There are different reasons. The nature and frequency of the traumatic events and the age of the victim play a role.

• Single-event traumas (assault, rape, witnessing a murder, etc.) are more likely to be remembered
• Repetitive traumas (repeated sexual violence or incest, repeated sexual abuse initiated when you were sleeping etc.) often result in memory disturbance.
• Extremely stressful experiences caused by natural or accidental disasters (earthquakes, plane crashes, violent weather, etc.) are more likely to be remembered
• Traumatic events deliberately caused by humans (i.e. incest, torture, war crimes) are less likely to be remembered
• People who are adults when they experience traumatic events are less likely to dissociate conscious memories of the events than children who experience trauma.
• Some research shows that the younger the child is at a time of the trauma, the less likely the event will be remembered.

If you have traumatic memory loss and then much later recover memories long after the event, it can be difficult for those around you to understand how you couldn’t remember what happened in the first place. They may even think you are making it up or imagining it. Understanding how traumatic memory works can help you feel confident about your own experience and can help educate the people you care about. 

See this infographic from the National Institute for the Clinical Application of Behavioral Medicine on how trauma impacts memory: