PTSD is caused by exposure to a traumatic event, but whether it develops depends on more than the event itself: the nature and severity of the trauma, what came before, and, crucially, the support and stress that follow. Underneath, it reflects a fear memory that has not been processed and filed as past, so it keeps firing as if the danger were still present.
This page works through three linked questions: what kinds of event can cause PTSD, why only some people who face them develop it (the risk and protective factors), and what appears to be happening in the brain. It closes by clearing up two damaging myths.
The kinds of event that can cause PTSD
PTSD follows exposure to actual or threatened death, serious injury, or violation, whether experienced directly, witnessed, or learned about happening to someone close. That covers a wide range of events, and no single type has a monopoly on causing it. Broadly, the events fall into a few families, kept general here rather than described in detail.
Sudden, dangerous events
Serious accidents, natural disasters, fires, and other events that pose a real threat to life and safety, often experienced or witnessed without warning.
Interpersonal harm
Violence, assault, and abuse, where the danger comes from another person. Trauma deliberately inflicted by others tends to carry a higher risk of PTSD than impersonal events.
Prolonged or repeated trauma
Sustained situations that are hard to escape, such as long-term abuse or captivity, which are also linked to complex PTSD.
Loss, illness, and duty
Sudden bereavement, life-threatening illness, and the repeated exposure faced by some frontline and emergency workers, whose risk accumulates over many incidents.
Why only some people develop PTSD
The same event can leave one person shaken but recovering and another with lasting PTSD. Decades of research, including meta-analyses by Brewin and colleagues and by Ozer and colleagues, have mapped the factors that tilt the odds. They divide naturally into influences that raise risk and influences that protect, and one of the clearest findings is how much the period after the trauma matters.
Factors that raise risk
Greater severity and closeness to the event; trauma that is prolonged, repeated, or deliberately inflicted by another person; earlier trauma or adversity, especially in childhood; a lack of support afterwards; and high levels of additional stress in the aftermath. Feeling intense helplessness or fear during the event also plays a part.
Factors that protect
Strong social support after the event is one of the most consistent protective factors. Others include being able to talk about what happened when ready, stable circumstances, access to help, and healthy coping. None of these guarantees recovery, but together they meaningfully improve the odds.
The single most striking finding: across the research, the factors that operate after the trauma, the strength of a person's support and the amount of further stress they face, are among the strongest predictors of who develops PTSD. This is genuinely hopeful, because unlike the event itself, the aftermath is something that support, care, and treatment can influence.
Key terms
A few terms recur in any account of why PTSD develops. Defining them briefly makes the brain science that follows easier to read.
- Fear conditioning
- The rapid learning that links a cue to danger, so that a sound or sight tied to the trauma later triggers alarm on its own. It is a normal survival mechanism that, in PTSD, becomes overactive and hard to unlearn.
- Memory processing
- The mind's work of organising an experience and filing it as something that happened in the past. In PTSD this processing is incomplete, so the memory stays raw and present rather than settled.
- Contextualisation
- Placing a memory in its proper time and place, so the brain knows the danger was then and there, not here and now. When this fails, reminders can trigger the full alarm as if the threat were current.
- Resilience
- The common human capacity to recover from adversity over time. Resilience is the norm after trauma, not a rare trait, and PTSD represents an interruption of it rather than its absence.
The brain and memory science
At its core, PTSD can be understood as a problem of a memory that was never properly processed. Normally, a frightening experience is gradually filed away as a memory of something that happened in the past. In PTSD, that filing does not complete, and the memory stays raw, vivid, and bound to a sense of present danger, so that reminders set off the full alarm as though the event were happening now.
Three brain regions are usually named in this account, and it helps to think of them as a team that has fallen out of balance.
The amygdala flags the threat
The amygdala is the brain's alarm, quick to detect danger and trigger the fear response. In PTSD it appears to be overactive, firing readily at cues linked to the trauma and keeping the body primed for threat.
The hippocampus struggles to place the memory in time
The hippocampus helps put memories in context and mark them as past. When it functions less effectively, the trauma memory is poorly contextualised, so it is not clearly tagged as over and can feel present when triggered.
The prefrontal cortex cannot calm the alarm
The prefrontal cortex normally helps regulate the amygdala and tell it the danger has passed. In PTSD this calming influence appears weaker, so the alarm is harder to switch off once it fires.
This is a simplified picture of an area still being researched, and it should not be read as a complete or fixed explanation. But it captures the useful core: PTSD is less a sign of damage than of a memory that has not been allowed to settle, which is exactly what trauma-focused treatment is designed to help along.
Two myths worth clearing up
Two beliefs cause real harm, because they add shame to injury and can stop people seeking help. Neither survives contact with the evidence.
PTSD only affects soldiers.
PTSD was first widely recognised in combat veterans, and it remains important there, but it can follow accidents, assault, abuse, disaster, bereavement, serious illness, and many other traumas. It affects people of every background, age, and walk of life.
Developing PTSD means you are weak.
PTSD is not a failure of character or willpower. Who develops it is shaped by the severity of the trauma, prior experiences, and the support available afterwards, not by strength or weakness. It is an injury to the mind's threat and memory systems, and, like other injuries, it can be treated.
Where to go next
Understanding the causes points naturally toward what helps. If the problem is a memory that has not been processed, the solution is to help it process safely, which is what treatment does. See the symptoms for the pattern these causes produce, and treatment for how the fear memory can be unwound.
Sources
- Brewin CR, Andrews B, Valentine JD. Meta-analysis of risk factors for posttraumatic stress disorder in trauma-exposed adults. Journal of Consulting and Clinical Psychology. 2000;68(5):748-766.
- Ozer EJ, Best SR, Lipsey TL, Weiss DS. Predictors of posttraumatic stress disorder and symptoms in adults: a meta-analysis. Psychological Bulletin. 2003;129(1):52-73.
- American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR). 2022.
This page is educational and is not medical advice. It does not diagnose any condition. PTSD can only be diagnosed by a qualified healthcare professional. If you think you may have PTSD, please speak with a doctor or mental health professional. If you are in crisis or thinking about harming yourself, contact your local emergency services or a crisis line now: in the UK and Ireland call Samaritans on 116 123, in the US call or text 988, or find your nearest helpline at findahelpline.com.