Stress symptoms are entirely normal in response to the usual problems and conflicts of life, and even in response to life's tragedies. Likewise, it's normal to go through times of being “obsessed” with some person or goal, and to follow small superstitious rituals. But sometimes these behaviors cross the line into disorders that affect a person's ability to function.

Insomnia, flashbacks, agitation, and other signs of distress are understandable reactions to going through a crisis or trauma, such as war, rape, torture, natural disasters, sudden bereavement, or terrorist attacks. But if the symptoms persist for 1 month or longer and begin to impair a person's functioning, the sufferer may have posttraumatic stress disorder (PTSD). Symptoms of PTSD include reliving the trauma in recurrent, intrusive thoughts, flashbacks, or nightmares; a sense of detachment from others and a loss of interest in familiar activities; and increased physiological arousal, reflected in insomnia, irritability, and impaired concentration. The video Memories We Don’t Want provides more information about PTSD.

For years after the 9/11 attacks on the World Trade Center and Pentagon, many people have suffered PTSD symptoms, especially if they lost loved ones, were first responders, or lived or worked nearby (C. Maslow et al., 2015; Neria, DiGrande, & Adams, 2011). Yet most people who live through a traumatic experience eventually recover without developing PTSD, and the epidemic of PTSD that many experts predicted after 9/11 never materialized (Bonanno et al., 2010). Why, then, if most people recover, do some continue to have PTSD symptoms for years, sometimes for decades?

One answer, again, involves a genetic predisposition. Behavioral-genetic studies of twins in the general population and of combat veterans have found that PTSD symptoms have a heritable component (Stein et al., 2002; Yehuda et al., 2009). PTSD has also been linked to certain personality and mental characteristics that have a heritable component (Almli et al., 2015). A prospective study that followed children from their early years to about age 17 found that people who develop PTSD after a traumatic experience often have a prior history of psychological problems, such as anxiety and impulsive aggression. And some lack the social, psychological, and neurological resources to avoid having preventable traumatic experiences in the first place or to cope with unavoidable ones. In this study, the children with above-average IQs were less likely to develop PTSD after a trauma than the average-IQ children were, apparently because the high-IQ kids had better cognitive coping skills (Breslau, Lucia, & Alvarado, 2006).

Interestingly, in many PTSD sufferers, the hippocampus is smaller than average (O'Doherty et al., 2015). The hippocampus is crucially involved in autobiographical memory. An abnormally small one may figure in the difficulty of some trauma survivors to react to their memories as events from their past, which may be why they keep reliving them in the present. An MRI study of identical twins, only one of whom in each pair had been in combat in Vietnam, showed that two things were necessary for a veteran to develop chronic PTSD: serving in combat and having a smaller hippocampus than normal. Twins who had smaller hippocampi but no military service did not develop PTSD, and neither did the twins who did experience combat but who had normal-sized hippocampi (Gilbertson et al., 2002).

In sum, many cases of long-lasting PTSD seem to be a result of impaired cognitive and neurological functioning that existed before the trauma took place, making it more likely that the trauma will trigger persistent, long-lasting symptoms.

Obsessive–compulsive disorder (OCD) is characterized by recurrent, persistent, unwished-for thoughts or images (obsessions) and by repetitive, ritualized behaviors that the person feels must be carried out to avoid disaster (compulsions). Of course, many people have trivial compulsions and practice superstitious rituals. Baseball players are famous for them; one won't change his socks and another insists on eating chicken every day while he is on a hitting streak. Obsessions and compulsions become a disorder when they become uncontrollable and interfere with a person's life.

People who have obsessive thoughts often find them frightening or repugnant: thoughts of killing a child, of becoming contaminated by a handshake, or of having unknowingly hurt someone in a traffic accident. Obsessive thoughts take many forms, but they are alike in reflecting impaired ways of reasoning and processing information.

As for compulsions, the most common are handwashing, counting, touching, and checking. A woman must check the furnace, lights, locks, and oven three times before she can sleep; a man must run up and down the stairs 60 times in 40 minutes or else start over from the beginning. OCD sufferers usually realize that their behavior is senseless, and they are often tormented by their rituals. But if they try to resist the compulsion, they feel mounting anxiety that is relieved only by giving in to it. However, not all OCD patients can specify a disaster that they believe their rituals prevent; some develop rituals to reduce general feelings of distress.

In many people with OCD, abnormalities in an area of the prefrontal cortex create a kind of cognitive rigidity, an inability to let go of intrusive thoughts, and behavioral rigidity, an inability to alter compulsive behavior after getting negative feedback (Snyder et al., 2015). Normally, after danger has passed or a person realizes that there is no cause for fear, the brain's alarm signal turns off. In people with OCD, however, false alarms keep clanging and the emotional networks keep sending out mistaken fear messages (Schwartz et al., 1996). The sufferer feels in a constant state of danger and tries repeatedly to reduce the resulting anxiety.

The DSM-5 includes hoarding disorder in the larger category of obsessive–compulsive disorders (Slyne & Tolin, 2014). Pathological hoarders fill their homes with newspapers, bags of old clothing, used tissue boxes—all kinds of junk. They are tormented by fears of throwing out something they may need later. A PET-scan study that compared obsessive hoarders with other people with obsessive symptoms found that hoarders had less activity in parts of the brain involved in decision making, problem solving, spatial orientation, and memory (Saxena et al., 2004). Perhaps these deficits help explain why hoarders keep things (their inability to decide what to throw away creates a constant worry) and why they often keep their papers and junk in the living room, kitchen, or even on the bed (they have trouble remembering where things are and thus feel the need to have them in sight).