emotional condition that sometimes follows a traumatic event, particularly an event that involves actual or threatened death or serious bodily injury to oneself or others and that creates intense feelings of fear, helplessness, or horror. The symptoms of post-traumatic stress disorder include the reexperiencing of the trauma either through upsetting thoughts or memories or, in extreme cases, through a flashback in which the trauma is relived at full emotional intensity. People with PTSD often report a general feeling of emotional numbness, experience increased anxiety and vigilance, and avoid reminders of the trauma, such as specific situations, thoughts, and feelings. It is normal to experience such reactions to some extent following trauma, and they are not considered symptoms of PTSD unless they last for at least one month or have a delayed onset. People with PTSD can also suffer from other psychological problems, particularly depression, anxiety, and drug abuse.

The experience of traumatic stress is very common, and an estimated 10 percent of women and 5 percent of men experience PTSD at some point in their life. The risk for developing PTSD varies greatly with different kinds of trauma. Women are especially likely to develop PTSD following rape or other forms of sexual assault. Combat exposure has been found to be the most common cause of PTSD in men in the United States. A major disaster or traumatic event can cause PTSD on a large scale. For example, in the immediate aftermath of the September 11 attacks on the World Trade Center, 7.5 percent of New Yorkers who lived in Manhattan below 110th Street—that is, in the general area of the World Trade Center—were found to be suffering from PTSD. The disorder is most likely to develop among people who suffer the greatest exposure to the trauma, who have the least social support, and who fail to allow themselves to experience their difficult feelings and find a new way of eventually understanding their experience.

Studies employing positron emission tomography (PET) and functional magnetic resonance imaging (magnetic resonance) (MRI) have shown that people with symptoms of PTSD have altered activity in the brain, primarily in the regions of the medial prefrontal cortex, thalamus, and anterior cingulate gyrus. This altered activity may facilitate and reinforce the brain's ability to recall specific traumatic memories, thereby making it difficult for people with PTSD to break the pattern of negative memory recall.

About 12.5 percent of people with PTSD have increased levels of a kinase (a type of regulatory enzyme) called CDK5 (cyclin-dependent kinase 5). Normally, CDK5 works with other proteins in nerve cells to regulate brain development, and its absence has been shown to facilitate the elimination of memories associated with fear. In people with PTSD, the elevated levels of CDK5 may interfere with and prevent fear-extinction and delay the ability to control emotional states and reactions when a traumatic memory is recalled.

Some professionals believe that PTSD following a traumatic event can be reduced by early psychological interventions that encourage a sharing of emotional experiences concerning the event. However, scientific research has shown that these interventions offer little help and may even exacerbate the disorder. Once an individual has developed PTSD, the two most effective treatments are antidepressant medication and trauma reexposure. Trauma reexposure is a form of directive psychotherapy that involves encouraging the victim to recount the trauma and, through gradual reexposure to the trauma in memory, change his or her emotional reactions in an effort to come to a new understanding of the experience.