The book link: https://mitpress.mit.edu/books/how-history-gets-things-wrong

Well, the full criteria in the DSM-5 are copyrighted, and so I can't print them here, but the VA in the US has a convenient summary which I can copy-paste for your reference:

Criterion A (one required): The person was exposed to: death, threatened death, actual or threatened serious injury, or actual or threatened sexual violence, in the following way(s):

*Direct exposure

*Witnessing the trauma

*Learning that a relative or close friend was exposed to a trauma

*Indirect exposure to aversive details of the trauma, usually in the course of professional duties (e.g., first responders, medics)

Criterion B (one required): The traumatic event is persistently re-experienced, in the following way(s):

• Unwanted upsetting memories

• Nightmares

• Flashbacks

• Emotional distress after exposure to traumatic reminders

• Physical reactivity after exposure to traumatic reminders

Criterion C (one required): Avoidance of trauma-related stimuli after the trauma, in the following way(s):

*Trauma-related thoughts or feelings

• Trauma-related reminders

Criterion D (two required): Negative thoughts or feelings that began or worsened after the trauma, in the following way(s):

*Inability to recall key features of the trauma

*Overly negative thoughts and assumptions about oneself or the world

*Exaggerated blame of self or others for causing the trauma

*Negative affect

*Decreased interest in activities

*Feeling isolated

*Difficulty experiencing positive affect

Criterion E (two required): Trauma-related arousal and reactivity that began or worsened after the trauma, in the following way(s):

*Irritability or aggression

*Risky or destructive behavior

*Hypervigilance

*Heightened startle reaction

*Difficulty concentrating

*Difficulty sleeping

Criterion F (required): Symptoms last for more than 1 month.

Criterion G (required): Symptoms create distress or functional impairment (e.g., social, occupational).

Criterion H (required): Symptoms are not due to medication, substance use, or other illness.

What do psychiatrists and psychologists think cause PTSD?

With the proviso that the research in this area is very much unfinished, it's important to note that not every modern person who goes to war - or experiences other traumatic events - gets PTSD. Research does seem to suggest that some people are more prone to developing PTSD than others. There might be some genetic basis to it; after all, in a very real way, PTSD is a disorder which manifests both psychologically and physiologically, and is a disorder which is clearly related to the body's infrastructure for dealing with stress (some of which is biochemical).

So, did ancient soldiers fit these criteria?

One important problem here is that they're no longer around to ask. We almost certainly do not have certain evidence that anyone from antiquity meets all of these criteria. There are certainly some suggestive tales which look familiar to people familiar with PTSD, but Homer and Herodotus and the various other historians simply weren't modern psychiatrists. They didn't do an interview session with the person in question, asking questions designed to see whether they fit all of these criteria, because, like I said - not modern psychs. It's also difficult to know whether symptoms were due to other illness; after all, the ancient Greeks did not have our ability to diagnose other illnesses either.

To reiterate: diagnosis is usually done in privacy, with psychs who know what they're looking for asking detailed questions about it. It's partially for this reason that psychiatrists and psychologists are reluctant to diagnose people in public (and that there was a big controversy in 2016 about whether psychiatrists and psychologists were allowed to publicly diagnose a certain American political candidate with a certain manifestation of a personality disorder, despite having never met him.) But, well, unless psychs suddenly find a TARDIS, no Ancient Greek soldier has ever been diagnosed with PTSD.

Additionally, it's clear from the history of psychiatry that disorders are at the very least culturally situated to some extent. In Freud's Introductory Lectures On Psychoanalysis, he discusses cases of a psychiatric disorder called hysteria at length, essentially assuming of his readers that they already know what hysteria looks like, in the same way that a psychologist today might start discussing depression without first defining it. Hysteria was common, one of the disorders that a general psychiatric theory like Freud's would have to cover to be taken seriously. Hysteria is still in the DSM-5, under the name of 'functional neurological symptom disorder', but was until recently also called 'conversion disorder'. However, you've probably never had a friend diagnosed with conversion disorder; it's not anywhere as common a diagnosis as it used to be a century ago.

So why did hysteria more or less disappear? Well - hysteria was famously something that, predominantly, women experienced. And there are perhaps obvious reasons why women today might experience less hysteria; we live in a post-feminist world, where women have a great deal more freedom within society to follow their desires (whether they be social, career, emotional, sexual) than they had cooped up in Vienna, where their lives were dominated by the family, and within the family, dominated by a patriarch. But maybe, also, the fact that everybody knew what hysteria was played a role in the way that their symptoms were interpreted, and perhaps even in the symptoms they had, given that we're talking about disorders of the mind here, and that the mind with the disorder is the same mind that knows what hysteria is. It might be that hysteria was the socially recognised way of dealing with particular mental and social problems, or that doctors saw hysteria everywhere, even where it wasn't actually present. There was certainly a movement in the 1960s - writers like Foucault, Szasz and Laing - who argued that society plays a much bigger role in mental illness than previously appreciated. Some of their arguments, at the philosophical level, are hard to argue against.

PTSD may be similar to hysteria in this way. It might be that there is a feedback loop between knowledge of PTSD and the experience of PTSD, that people who have experienced traumatic events in a society that recognises PTSD can express their minds as such.

What do psychologists see as the aetiology of PTSD?

Aetiology is simply the study of causes. Broadly speaking, there is no clear agreed-upon single cause for PTSD, judging by recent research. Sripada, Rauch & Liberzon (2016) argue that four key factors play a role in the occurence and maintenance of PTSD after a traumatic event: a) an avoidance of emotional engagement with the event, b) a failure of fear extinction, meaning that fear responses related to the event are not inhibited as well, c) poorer ability to define the narrower context in which a stress response is justified in civilian life vs a military situation, d) less ability to tolerate the feeling of distress - perhaps something like being a bit less resilient, and e) 'negative posttraumatic cognitions' - not exactly being sunny in disposition or how you interpret events. Kline et al., (2018) found that with sexual assault survivors, the levels of self-blame immediately after the assault seemed to correlate with the extent to which PTSD was experienced. Zuj et al. (2016) focus on fear extinction as a specific mechanism by which genetic and biochemical factors which correlate with fear extinction might be expressed. There's also a body of research suggesting that concussion, and the way that it disorients and causes cognitive deficits, plays a larger role in PTSD than previously suspected.

These factors are likely not to be the be-all and end-all, it should be said - it's a complicated issue and research is still in its infancy. But nonetheless, you can see many ways in which culture and environment might effect these factors, including the genetic ones. Broadly speaking, some societies are more inclined towards emotional engagement with war events than others - Ancient Greece was heavily militarised in ways that most Anglophone countries in 2018 are not. Some upbringings probably lead to more resilience than others, and depending on the norms of a society, those upbringings might be more concentrated in those societies. The way that people around you interpret your 'negative posttraumatic cognitions' is going to be different depending on the culture you grow up in. Some societies may be structured in such a way that fear extinction is more likely to occur.

So in this context, what do Crowley and Tritle actually argue?

Broadly speaking, what I argued in the last paragraph is the kind of thing that Crowley's paper in Combat Trauma and the Ancient Greeks argues. There are much more severe injunctions against killing in modern American society than Ancient Greek society, which was not Christian and thus didn't have Christianity's ideals of the sacredness of life - instead, in many Ancient Greek societies, war was considered something that was fucking glorious, and societies were fundamentally structured around the likelihood of war in ways that modern America very much is not.

Additionally, in Ancient Greek society, war was a communal effort, done next to people you knew before the war in civilian life and continued to know after the war; in contrast, in modern war situations, where recruits are found within a diverse population of millions, there is a constantly rotating group of people in a combat division who may not have strong ties. Additionally, with the rise of combat that revolves around explosive devices and guns, fighting has changed, and Crowley argued, made people more susceptible to PTSD; these days, if soldiers are in a tense, traumatic situation, it is better for them to be spread out so as to limit the damage when under attack. This, Crowley argues, leads to many more feelings of self-blame and helplessness - the kind of thing that might lead to negative posttraumatic cognitions - because blame for events is not spread out amongst a group in quite the same way.

In contrast, Tritle points to a lot of evidence from ancient sources of people seeming to be traumatised in various ways after battles, ways which do strike veterans with PTSD as being of a piece with their experiences:

...Young’s claim that there is no such thing as “traumatic memory” might well astound readers of Homer’s Odyssey. On hearing the “Song of Troy” sung by the bard Demodocus at the Phaeacian court, Odysseus dissolves into tears and covers his head so others do not notice (8. 322). 11 Such a response to a memory should seem to qualify as a “traumatic” one, but Young would evidently reject Odysseus’ tears as “traumatic” and other critics are no less coldly analytic.

Tritle - a veteran himself - clearly wishes to see his experiences as being contiguous with those of ancient soldiers. And there is actually something of an industry in putting together reading groups where veterans with PTSD read accounts of warriors from the classics. The books Achilles In Vietnam and Odysseus In America by the psychiatrist Jonathan Shay explicitly make this link, and it does seem to be useful for many veterans to make this comparison, to view a society where war and warriors are more of a integral part of society than they are in modern America (notwithstanding the fad for saying something about 'respecting your service'). For Tritle, there's something offensive in the way that critics like Crowley dismiss the idea that there was PTSD in Ancient Greece because of their being too 'coldly analytic'. Tritle also emphasises the physical structure and pathways of the brain:

A vast body of ongoing medical and scientific research demonstrates that traumatic stressors —especially the biochemical reactions of adrenaline and other hormones (called catecholamines that include epinephrine, norephinephrine, and dopamine)—hyperstimulate the brain’s hippocampus, amygdala, and frontal lobes and obstruct bodily homeostasis, producing symptoms consistent with combat-stress reactions. In association with these, the glucocorticoids further enhance the impact of adrenaline and the catecholamines.

But while I'm happy as a psychologist for veterans to learn about ancient warriors if evidence suggests that it helps them contextualise their experiences, as a historian I am personally more on Crowley's side than Tritle's here. The mind is fundamentally an interaction between the brain and the environment around us - we can't be conscious without being conscious of stuff, and all the chemicals and structures in the brain fundamentally serve that purpose of helping us get around in the environment. And history does tell us that, as much as people are people, the world around us, and the societies we make in that world, can vary very considerably. It may well be that PTSD is to some extent a result of modernity and the way we interact with modern environments. This is not to say that people in the past didn't have (to use Tritle's impressive neurojargon) adrenaline and other hormones that hyperstimulate the brain's hippocampus, amygdala, and frontal lobes. Human neuroanatomy and biochemistry doesn't change that much, however modern our context. But so many of the things that lead to these brain chemistry changes, that trigger PTSD as an ongoing disorder beyond the heat of battle - or even those which increase the trauma of the heat of battle - seem to be contextual, situational.

Edit for a new bit at the end for clarity and conclusiveness

I am in no way saying that the people with PTSD have something that's not really real. PTSD as a set of symptoms - whatever its cause, however socially bound it is - causes a whole lot of genuine suffering in people who have already been through a lot. Those people are not faking, or unduly influenced by society. They are simply normal people dealing with a set of circumstances that might not have existed in the same way before the 20th century. I am also not saying that people in the ancient world didn't experience psychological trauma of various sorts after traumatic events - clearly they did; I'm just saying that the specific symptomology of PTSD is enough of a product of its times that we should distinguish between it and the very small amount that we know of the trauma experienced by ancient warriors (or others). And finally, PTSD can be treated successfully by psychologists - if you are suffering from it and you have the means to do so, I do encourage you to make steps in that treatment.

Other related /r/AskHistorians answers of mine you might find interesting:

• Did Vietnam war vets come home with PTSD?
• The first copy of the DSM was published in 1952. How did society accept the DSM? Was there any backlash about it?
• How did mental illnesses in World War II soldiers eventually lead to the diagnosis of PTSD?
• Was depression and mental illness as prominent as it is now, a hundred or a thousand years ago?
• Is the concept of mental illness a modern phenomenon?
• Many people who suffer from paranoid schizophrenia have this fear of an overarching government conspiracy to spy on them and hide cameras and such. How would a medieval peasant with this condition be affected since they didn't have much of the technology at the time that we have now, to worry about?
• Is there any evidence that the soldier in this photo has shell shock?
• What behaviour prompted psychologists across the country to say that Barry Goldwater was unfit to be President?

References:

Kline, N. K., Berke, D. S., Rhodes, C. A., Steenkamp, M. M., & Litz, B. T. (2018). Self-Blame and PTSD Following Sexual Assault: A Longitudinal Analysis. Journal of Interpersonal Violence, 088626051877065. doi:10.1177/0886260518770652

Meineck, P., & Kontan, D. (2014). Combat Trauma and the Ancient Greeks. New York: Palgrave.

Sripada, R. K., Rauch, S. A. M., & Liberzon, I. (2016). Psychological Mechanisms of PTSD and Its Treatment. Current Psychiatry Reports, 18(11). doi:10.1007/s11920-016-0735-9

Zuj, D. V., Palmer, M. A., Lommen, M. J. J., & Felmingham, K. L. (2016). The centrality of fear extinction in linking risk factors to PTSD: A narrative review. Neuroscience & Biobehavioral Reviews, 69, 15–35. doi:10.1016/j.neubiorev.2016.07.014