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Rejali makes a case for there being 3 motives behind torture in democracies: interrogation, confession, and maintenance of societal order. Interrogation provokes torture in no small part when those hired by the peoples representatives to carry out work requiring expertise run short of information. Confession principally arises when the instruments of the judicial system favor confessions over circumstantial evidence or witnesses, and is used to ensure the safety of the society (law & order). Maintenance of the social order manifests itself in a variety of ways, by distinguishing a dehumanized class from a citizen class. Note that this third motive is ascribed by others (e.g. Gray) to mass or administrative torture, but Rejali has examples where intimidation is not the motive.
It might be argued that torture in non-democratic societies would differ, however (and I haven't yet finished the book), the examples he gives of torture by the Gestapo and the Soviet NKVD don't differ much in motive.
The individual torturers he details do probably include some people with abnormal psychological tendencies going into the mix, however, he does document that states which torture tend to avoid such people. The invention of torture methods is likewise (at least as far as I have read) not a task that has been done by people with abnormal psychological tendencies either, rather usually an adaptation of existing technology by a technician class of people to the goals of the task at hand.
Note that you can get all sorts of abnormalcy if you move from torture, which is about state actors, to just plain criminal abuse. Torture, you will find if you look closely, is almost always defined by people who study it as something carried out, as the convention says, "under color of law". Many of the tactics originate not in the militaries and in war, but in police interrogations, obtaining confessions, and maintaining the social status quo.
Source for all of the above, Rejali, Torture and Democracy, pp. 1-150, or so, with the motives given in the first chapter.
As for sexuality, the current use in American "enhanced interrogations" is not the norm, as far as I can ascertain from reading him and some others, and was derived from a belief, at the top of the food chain and perhaps elsewhere, that the "Arab mind" was especially sensitive to it, as it supposedly was to dogs. Exactly how Pakistanis and Afghans fit the "Arab mind" I'm not really sure, but the U.S. government we currently have frequently substitutes "Arab" for "Muslim" and vice versa. A case for childhood trauma or sexual problems could probably be made for some of this philosophy, as I had said, but it is also abnormal for technique to flow from top down according to Rejali [Note that this does not mean that the techniques aren't directed at sexual organs because of their sensitivity].
Source for the last para is my own observations from studying several texts, D.Rejali, P.Sands, B.Olshansky, P.Zimbardo, J.Mayer, E. Saar, plus papers, reports, and essays by others, including D.Rejali, G.Gray, M.Benjamin, B.Harrison, S.Grassian, HRW, PHR, AI, ACLU, ICRC, Errol Morris, other rights groups, etc. you can probably find others in the links on Humanity Against Crimes. I have them all saved, but can't remember them all off hand.