PTSD is an internalising disorder or mixed? I am a bit confused. In an open access article (Caspi, et al. 2020), is the following — under Methods: Assessing Psychopathology (emphasis mine):

[...] externalizing disorders (ie, attention-deficit/hyperactivity disorder, conduct disorder, alcohol
dependence, cannabis dependence, other drug dependence, and tobacco dependence),
internalizing disorders (ie, depression, generalized anxiety disorder, fears [including social phobia, simple phobia, agoraphobia, and panic disorder], posttraumatic stress disorder, and eating disorders
[including bulimia and anorexia]), and thought disorders (ie, obsessive-compulsive disorder, mania,
and schizophrenia)

I can understand that PTSD will be caused by internalisation of the trauma experience(s), but from my experience, elements of PTSD often involve externalising (hypervigilence for example) and sometimes unhelpful thoughts/behaviours (for example, fear of all people of a certain religion, culture, background and/or gender; and to the extreme, violent tendencies towards those people to &quot;prevent&quot; further traumatising events).
So is PTSD just an internalising disorder, or can it be a mixture of an internalising, externalising and thought/behaviour disorder?
References
Caspi, A., Houts, R. M., Ambler, A., Danese, A., Elliott, M. L., Hariri, A., ... &amp; Rasmussen, L. J. H. (2020). Longitudinal Assessment of Mental Health Disorders and Comorbidities Across 4 Decades Among Participants in the Dunedin Birth Cohort Study. JAMA Network Open, 3(4), e203221-e203221. https://doi.org/10.1001/jamanetworkopen.2020.3221
My understanding is that the internalizing/externalizing dichotomy refers to the way symptoms are expressed, not the source/cause of the disorder. Internalizing disorders are those that mostly have an internal impact and tend to lead sufferers to withdraw or hide their affliction. Externalizing disorders tend to impact others more directly with outward-focused behaviors. Therefore, I don't think hypervigilance would be considered an &quot;externalizing&quot; feature even though it is directed at the outside world, because all that is directed outward is attention; the primary impact on the individual is the internal mental stress that maintaining a hypervigilant state creates. Outbursts of violence clearly would be &quot;externalizing&quot;, however.
The DSM-5 doesn't categorize by internalizing/externalizing, and puts PTSD in the category of &quot;Trauma- and Stressor-Related Disorders&quot;. The DSM-5 mentions specifically the combination of internalizing and externalizing symptoms in these disorders:

In some cases, symptoms can be well understood within an anxiety- or fear-based context. It is clear, however, that many individuals who have been exposed to a traumatic or stressful event exhibit a phenotype in which, rather than anxiety- or fear-based symptoms, the most prominent clinical characteristics are anhedonic and dysphoric symptoms, externalizing angry and aggressive symptoms, or dissociative symptoms.


In the paper you refer to, the authors did group PTSD with the internalizing disorders. They've done this in reference to a previous paper which identified three factors, labeled Internalizing, Externalizing, and Thought Disorder, that together explained well the spectrum of conditions. However, PTSD is not really involved in this paper.
It's unclear why they chose to include it as &quot;internalizing&quot; in the Caspi 2020 paper, except that their approach meant that if they included it at all it had to be placed in one of those three categories. Given the other items on the list, I think it's quite reasonable to group PTSD along with anxiety, phobias, and depression, rather than among substance dependence or schizophrenia, but I don't think this should be taken as anything but a choice of designation within the scope of this particular paper.