The ordeal often includes a huge emotional burden. In the film, Lamson reaches out to fellow sole survivors, who open up about their complex feelings related to survivorship.
Disasters that take the lives of everyone involved except for one may leave a survivor feeling guilty for receiving the gift of continued life when no one else did. They might not believe they deserved it, Raison said.
The pain of living
Survivor guilt can be an immediate response to a tragedy, and the extent of the feelings depends on the individual, Russell Jones, a psychologist at Virginia Tech who helped counsel victims at the school after a shooting rampage in 2007, has said.
Being the sole survivor of a tragedy is likely even more difficult than being one of several, Raison said.
In working with patients who fought in the Vietnam War, Raison observed that their survivor guilt was linked to their perceptions of their own actions at the time when others died. They felt worse if they regretted their actions, or didn't feel they had done enough to help others.
"If they had lost their nerve at a key moment, or if they had done something they felt ashamed of, it ate them alive for the rest of their lives," he said.
He added, "If you survive something while heroically trying to save everybody else, you're going to have a different feeling about it than if you took the last seat on the lifeboat, so to put it."
What is PTSD?
Survivor guilt can be an expression of grief and loss, according to Melissa Brymer, director of terrorism and disaster programs at the UCLA Neuropsychiatric Institute in Los Angeles.
Sitting back and taking time to process the event can help survivors cope, Jones said. Seeking support from friends, family and community or faith leaders can help an individual work through difficult feelings.
But if there's lingering guilt and anxiety, Jones recommended consulting a mental health professional. A survivor may experience symptoms of post-traumatic stress disorder and depression that requires formal treatment.
Classic PTSD symptoms include intrusive thoughts, flashbacks and nightmares. People may avoid anything that reminds them of the event and have difficulty paying attention. It may be paired with depression and suicidal feelings.
These symptoms can lead to full-blown PTSD over time, especially if a person has a history of mental illness. Previous trauma, stress, loss of financial stability, and grieving the loss of family and friends are also risk factors, Jones said.
About 8% to 15% of people who experience trauma develop severe PTSD symptoms that persist and require professional help, Jones said.
A 2002 study in the British Journal of Psychiatry
interviewed survivors of the 1988 Piper Alpha disaster, in which an oil rig in the North Sea exploded, killing 167 people. More than 10 years after the event, seven out of 33 people met PTSD criteria.
"Features such as physical injury, personal experience and survivor guilt were associated with significantly higher levels of post-traumatic symptoms," the study authors wrote.
With support, people generally do well, Jones said.
When to seek help
For some people, survivor guilt is just part of working through complex feelings after experiencing a traumatic event involving deaths and a way of mourning. But it may become all-consuming and impede functioning.
"If they're having changes within their behaviors or in their emotions or with their relationships with others, then we want to make sure that people get help," Brymer said.
Unfortunately, people who need help often don't seek it, Jones said.
"So often, there's stigma attached to getting help from a mental health professional," Jones said. "Therefore, they go without seeking treatment, oftentimes until they reach a breaking point."
There are several treatment strategies that have been shown to help people with survivor guilt and PTSD symptoms. There's no one-size-fits-all approach, Raison said.
One strategy involves exposure, Jones said. Getting people to talk about the event and assimilating the event into everyday living are key, Jones said. Therapists may also take the individual back to the setting where the tragedy occurred and allow the person to express his or her thoughts, feelings and emotions.
But this technique may make some individuals with PTSD more traumatized, Raison said. Instead, he recommends that therapists try to reframe tragedy as an event that can be empowering.
Some people may benefit from viewing their survivorship as a positive gift, one that has a meaning in the context of a life story, Raison said. This may especially be helpful for people of faith, who may believe there is a divine purpose in their survival.
But others may reject the idea that they survived "for a reason," Raison said. A cognitive-behavioral approach would help the person examine his or her own beliefs that underlie their haunting feelings of guilt, and challenge irrational thought processes, he said.
"If they're still really suffering survivor guilt, it's sort of helping them reframe it, where they recognize that it's not their fault they survived, it's not their blessing they survived, it just happened," he said.
Secular survivors may also come to view the traumatic experience as something that gives them a deeper reason for living.
One coping strategy is to do something meaningful for someone else, Brymer said. Organizations that help people who have survived violence or natural disasters, or advocacy groups, may be good options for people who are trying to move on from a tragedy.
"If someone has died, is there something that you can do that's meaningful and representative of that person?" she asked. "Is there something you can do so you're unstuck, so you can do something powerful and meaningful (for) someone else?"
Ky Dickens, director of "Sole Survivor," told CNN affiliate KARE
that there is a misperception that a survivor is lucky, and that he or she is a "miracle" with a life purpose -- an attitude that puts undue pressure on them, she said.
Survivors, she said, are victims too.
If you are experiencing suicidal thoughts, call the National Suicide Prevention Lifeline at 1-800-273-8255.