Okay, this is more scientific than literary theorizing, so I'm not entirely sure it belongs here. If not, let me know.
The more I see The Big Damn movie, the more fascinated I become by the Reavers. I started to ponder a few questions about the mechanics of Reaverness, and the following essay is the result.
We know that the original batch of Reavers comprised one tenth of one percent of the population of Miranda, made insane by the G-32 paxilon hydrochlorate. Also, if you look closely, you'll notice that all the Reavers we've ever seen are male. Interestingly enough, one tenth of one percent is also the exact same percentage of the population that has a double-Y chromosome. Biologists believe that the double-Y is responsible for increased aggression and minor mental retardation. Also, obviously all those who are double-Y are male. I therefore theorize that it's the double-Y that caused the anomalous reaction to the pax in the first place; I think the pax simply drastically increased the effect of the double-Y.
Reavers are capable of modifying and flying spaceships. They can also, when pressed, use firearms, grappling apparatuses, and dart guns. This definitely signifies that there’s more going on in their heads than rabid bloodlust; there must be at least a modicum of planning and mechanical knowledge in there. We therefore must assume that they have periods of relative lucidity when they are not actually in the grip of hunting frenzy. This goes partway to explaining why they don’t wipe each other out, and also how they might be capable of intentionally maintaining their ranks.
Reavers all modify their ships to run without reactor containment. According to our sane characters (and all medical sense), this is definite suicide. Since it is a deliberate choice and not simply carelessness or indifference, there must be a reason for this preference for radiation. We must assume that the amount of radiation is not imminently lethal, but rather more of a long-term problem. Long-term effects of chronic low-level radiation may take quite some time to cause death; I believe decades in some cases. Most interestingly, the brain and endocrine (hormone producing) systems are likely to be affected. Neurological symptoms may include: decreased intellect, memory impairment, confusion, personality changes, atrophy, leukoencephalopathy, radiation necrosis, neurological deterioration and dementia. Radiation necrosis and diffuse cerebral atrophy are considered long-term complications that occur from months to decades after radiation exposure. Of course there are other effects, but I theorize that it’s the effects on the mind that interest the Reavers. Why do I think the Reavers would deliberately be making themselves more crazy? Because they probably don’t have access to more pax. While the damage to the brains of the initial Reaver population is apparently permanent, they have no way of replicating this exact effect on new recruits. Therefore, they have to improvise a mixture of torture, trauma, radiation-madness, and what I suspect to be a riff on the Helsinki syndrome.
The number of Reavers is also something of a puzzle. We're given 30 million as the population of Miranda. A quick bit of math reveals that our original Reaver population should have numbered about 30,000. Why don't they just eat each other, and how are we not running out of Reavers by this time? I suspect that they do prey on each other, but, as we see on several occasions (and as various examples of predator behavior in the wild support), they are always more interested in frightened, fleeing targets. We know from "Serenity" (the pilot, not the movie) that Reavers always chase ships that run, but might leave those that don't alone. Also, on Lilac, the Reavers instantly lose interest in the man whom Mal shoots. This would cut down on attrition by cannibalism, but there should still be a significant decrease in their numbers due to radiation poisoning from running without containment alone. I theorize that the Reavers can sense other double-Y chromosome men, possibly by pheromonal scent cues. When they come across these men while raiding, as in “Bushwhacked”, the Reavers leave them alive for the type of conditioning I mentioned above.
Why did the Reavers leave the man in "Bushwhacked" if they wanted to add him to their ranks? I think that starvation, isolation, and the fear of slow death sounds like an integral part of the process of Reaverization. I think they left him there to finish his transition. If he succeeded, he'd be meant to start his career in sadistic cannibalism with his rescuers, should any happen along. Also, as River said about the cows: "They weren't cows inside. Now that they see sky, they remember what they are." Our budding Reaver does not complete his transformation until he encounters normal people. Once he found prey, he knew what he was. Mal said that the Reavers are known to leave some when they raid, so I figure they don't have to really worry about all the ones they leave making the successful transition; it'd be a bit like planting seeds and waiting to see which ones made it. Those who survived the process would be the strongest and the craziest by the time they made their way out to Reaver territory. Also, I figure that they don't have to make more of themselves very often, just enough to keep their ships running and their raiding parties relatively strong. Seeing as there were 30,000 to begin with, they probably wouldn't have to make too many---at least, not until after so many of them died in Serenity. It'd be interesting to see if they tried to replenish themselves en masse after that.