War, even for us Clausewitzians (well, actually, especially for us Clausewitzians) is a Hegelian dialectic. There is a thesis, an antithesis, and the interaction of the two result in a synthesis. It is so obvious that it should scream out, that post-synthesis, in an unconventional environment, the byproducts will return to their origins, and their experiences will have secondary effects in their countries of origin. War changes all of us. Noted. But my bibliography dealt only with the US, UK, and France, and in each case dealt also with the states which had sent those men to war with the authority of the nation-state.
I like having friends that make me think.
For starters the question about post-combat behaviors of returning jihadists is a complex one on a couple of different planes. As I understand the literature (and this is a fusion observation), there are at least four independent variables which apply to anyone who has seen combat/trauma of any sort. These variables apply to me (an Ohioan), my friend who hails from a state to the South, Oxford boys like J.R.R. Tolkien (who was at the Somme), and Frenchmen from Provence who found Verdun decidedly inhospitable. But it also applies to Saudis, and Syrians, Egyptians and Yemenese. (Extrapolate, as you like, from there.) It is within their separate cultures that the variants emerge. I leave it to others to hypothesize further. This is what I got:
A. Nature of your combat experience: The most significant here seems to be duration and frequency, but also type. In other words, episodic combat, even when horrific, seems to have produced less trauma. Up through WWI you could say this characterized most soldiers' experience, which makes PTSD a problematic diagnosis in, say, the War of 1812. The type of combat also obtains: If you were subjected to shelling, particularly intense shelling by 8 inch howitzers, that is worse than most firefights. (The slaughter of a WWI frontal assault must be taken out of that equation, because that sort of thing didn't really count as a "firefight" in the modern context.) Apparently the feeling of intense and complete helplessness one has when being shelled, especially accurately shelled, is worse than most firefights. IEDs, therefore, have a similar effect, though I believe it is less than were it comparable to indirect fire. (e.g. Few people have been hit by 300+ IEDs, let alone 300 IEDs in 5 minutes, eight or 20 times over the course of a tour. This would be considered "light experience" for some regiments of the British Expeditionary Force, circa 1917, and moderate for most line unit veterans of WWII.) Flipping that on its head: For the jihadist airpower just does not do this in the same way. WWII airpower, probably, but not today's version. The completely unsuspected and instantaneous bomb from an unknown, unseen, unheard aircraft flying 20K overhead does not invoke sustained fear. It creates fatalism. If you've experienced periodic, but pretty nearly random indirect fire (as many in Iraq and Afghanistan have) you understand this. You're scared at first, but then you devolve into, "eh, whatever."
B. Social network IN the environment: Were you serving with men from your hometown (or your regular unit) or were you an individual replacement/augmentee? The former is obviously better than the latter, and quite a lot has been written on this. Most jihadists, I should think, generally went solo. This makes their experiences generally worse than they would be otherwise. You have to factor in culture in this one though, and quite heavily. So the next factors probably mitigate.
C. Social mores of your originating culture: This is where we really get into the history side. Dean's book on PTSD in the Civil War which I cited in the bibliography fails in this regard, because a lot of what he described was not PTSD, but it was actual cracking up IN combat, which is different. Social standards in your dominant culture determine what is an appropriate and/or acceptable response to extremes of experience. Thus, in the mid-19th century it just wasn't done for men to cry, or hug, or any of that namby-pamby shit. (See my essay on hugging in the Pentagon.) Over time, this changed. One can make a pretty solid argument (as Shephard does) that an individual's reaction to things one only sees in combat is as much about what you learn from your culture in the 20 years preceding your combat experience, as what happens to you in combat. To put it in academic terms, it is about appropriate or normed behaviors condoned by the culture. Hence in WWI, some men reacted to obscene levels of death and dismemberment around them with what would have been diagnosed as "hysteria" ten years earlier (catatonic, twitching, loss of motor control or eyesight, etc.). Indeed, the lack of physical evidence for which is what led to the original diagnosis of "Shell Shock" (the physicians literally thought that what was happening was a physiological response to overpressure from shell explosions...until they realized that a good number of their patients hadn't been shelled...). So, does your originating culture require what we'd now call "suppression" (and which I always learned was just "being a man", cue Dennis Leary) of things, or does it suggest that one should "open up and let your feelings out." Note, this is not an either/or, but for each is an individual spot on the continuum. A man raised in 1850's Wisconsin would likely have an extremely different reaction than, say, a man raised in 1970's Wisconsin, to the same exact event, all other things being equal. This also applies across cultures. A Japanese (or Korean) man of the 1940s or 1950s has a different reaction than a man from the same culture 50 years later. And one can make pretty good arguments that a Japanese man today will have significantly different reactions than a Dutch man today. Follow? Related to this is the next point.
D. Social network upon return: How close are you to the dominant culture within your home culture? Are you from a higher prestige family? Etc. This, surprisingly, is not something which has been explored yet to my knowledge, probably because of fears of elitism bias and charges of class in the egalitarian West. But my hunch is that if you are from a higher prestige family/group, and you reintegrate, you are less likely to show the signs of PTSD unless you're really messed up bad. In other words, I think that some PTSD and affiliated behaviors can get WORSE when you get home, if you are originally from (and return to) a lower socioeconomic strata where such behavior might be expected or condoned. Let us call this the "Springer Effect." There is a reason why there are never any rich (or even middle-class) people on the Jerry Springer show, and it's not the money. Socially such behaviors as you see there (confessionals, exhibitionism, extreme reactions, etc) would be de facto culturally forbidden to the middle-and-upper classes. But at lower and lower levels of socio-economic status, it becomes more and more acceptable to go on the show and talk about how you impregnated four women, but don't know WHICH four, etc. This same may possibly apply to PTSD/Reactions to Combat. Does your status (rank/prestige) in your hometown mean that you display or conceal?
OK, so there is the basic construct. Apply this to what you know about the Jihadists, and their cultures (no need for me to lecture any here on SWJ on those things). But a couple of serious historical caveats to this whole line of thought must be applied in the interest of intellectual integrity:
First: It is entirely possible that there was a lot more PTSD in the wake of, say, the American Civil War, or WWI, or WWII, than we know. This is because the victims (that is to say "all the people around a PTSD-displaying person) would be socially less inclined to talk about it. Rather like teen pregnancy, or rape, in the 1950s...it happened, but as one moved up the social scale, it was more and more likely to get hidden by those around the pregnant girl or the woman who was raped. Historically, this skews the data. The problem is that we cannot know to what degree this skews any potential historical assessment. The same might apply to Middle Eastern cultures. For example: Abuse in the home (a signal of PTSD here in the West) is more often concealed, or not considered anomalous, in some segments of the population. Excessive drinking might be the same. Etc.
Second: Reception status. A returning jihadist, if he is lauded, is likely to subconsciously incorporate that into his conceptualization of personal honor (which is a whole 'nuther essay, which you must take into account, sorry) and this will modify his behavior. Rather like the contrast in the reception of WWII veterans and Korean War veterans (since those two wars were so close together), the reception of the veteran/jihadist by his home culture will affect his behavior. Positive and public reactions would seem to mitigate against PTSD.