Evolutionary basis of posttraumatic stress disorder

1. Evolutionary basis of
Posttraumatic Stress Disorder
A student seminar - Bshr Nammouz - 3’rd year medical school - SSST

2. What is Posttraumatic Stress Disorder?

3. Posttraumatic stress disorder (PTSD)
Is defined as a pathological anxiety that
usually occurs after an individual
experiences or witnesses severe trauma
that constitutes a threat to the physical
integrity or life of the individual or of
another person.

4. The main symptoms of PTSD include:
● Persistent reexperiencing of a traumatic event
● Resultant numbness, avoidance, and hyperarousal
● Avoidance
● Negative thoughts and mood or feelings

5. Diagnosis criteria:
The first DSM criterion has 4 components,
as follows:
● Directly experiencing the traumatic
event(s).
● Witnessing, in person, the event(s) as it occurred to others.
● Learning that the traumatic event(s) occurred to a close family member or friend.
● Experiencing repeated or extreme exposure to aversive details of the traumatic event(s);
this does not apply to exposure through media such as television, movies, or pictures.

6. The second criterion involves the persistent reexperiencing of the event in 1 of several ways:
● Thoughts or perception
● Images
● Dreams
● Illusions or hallucinations
● Dissociative flashback episodes
● Intense psychological distress or reactivity to cues that symbolize some aspect of the
event

7. Evolutionary basis of PTSD

8. How memory fails us sometimes
persistence describes the way that we often suffer
recollections of unpleasant, embarrassing or traumatic
events. The most serious form is PTSD that can seriously
affect the sufferer’s ability to live a normal life.
Schacter argues that one of the many functions of memory is
to learn from negative events as well as positive ones and the
majority of negative memories, although unpleasant, would
not be so severe as to result in PTSD. Memory, by making
negative events prominent, is warning you not to make the
same mistake again.

9. Randolph Nesse and George Williams suggested three reasons why we have
psychiatric problems:
● Genes that predispose people to psychiatric disorders may also have inclusive
fitness benefits.
● Environmental factors that cause psychiatric disorders may be novel when
compared to our ancestral past.
● Some psychiatric disorders may be due to design compromises rather than
genetic flaws.

10. ● Genes that predispose people to psychiatric disorders may also have inclusive
fitness benefits:
Many genes have more than one phenotypic effect, so the negative effects of a gene may be
maintained in a gene pool because the positive ones outweigh them. A variation of this
argument is that the harmful genes provide a benefit when combined with other genes, but
that sometimes such genes are found in bodies that lack the necessary ‘good’ genes.

11. ● Environmental factors that cause psychiatric disorders may be novel when
compared to our ancestral past:
Humans have developed a lifestyle that did not exist in our ancestral past too rapidly for
selection pressures to have led to appropriate changes (also known as the mismatch
hypothesis).

12. ● Some psychiatric disorders may be due to design compromises rather than
genetic flaws:
Selection pressures act on inclusive fitness, not on perfecting psychological (and physical)
devices.

13. We might add: the trait variation argument:
This is based on the notion of a normal distribution curve.
Within a given population, individuals may be plotted along various dimensions for
psychological traits such as level of intelligence and personality factors.

14. A large number of factors determine an individual’s
position in the normal distribution of each
characteristic measured. These include the
combination of genes they inherit from their
parents (sexual reproduction leads to unavoidable
variation) and their life experiences.
When such measurements are plotted for a
population they generally produce a bell-shaped
graph with most people in the middle of the range
and progressively fewer individuals as we move to
the extremes.

15. Thanks for your attention
References:
1. Risser HJ, Hetzel-Riggin MD, Thomsen CJ, McCanne TR. PTSD as a mediator of sexual revictimization: the role of
reexperiencing, avoidance, and arousal symptoms. J Trauma Stress. 2006 Oct. 19(5):687-98.
2. American Psychiatric Association. American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders,
Fifth Edition. Arlington, VA: 2013.
3. Workman, Lance. & Reader, Will. Evolutionary Psychology. Cambridge: Cambridge University Printing House, 2014.
4. Stearns, Stephen C. & Medzhitov, Ruslan. Evolutionary Medicine. Yale: Sinauer Associates, 2015.