These are the bare facts, but a more exact calculation of the extent of psychopathy in the general population is near impossible, so even these facts are disputable. The vast majority of known (i.e. medically diagnosed) psychopaths are convicted criminals, since diagnosis usually only possible after a person has been convicted of a crime, and shows some signs of psychopathic "behaviour". As no "self-aware" but formally non-diagnosed psychopath would ever voluntary submit himself to clinical testing, this is what makes it so difficult to measure psychopathy's extent across humanity.
Not only is there the problem of "psychopaths in hiding"; there is also the problem that many psychopaths are able to successfully mask their "true nature" from the rest of society. Furthermore, psychopathy is tested by a scale - the generally-used "Hare Checklist" is the most reliable method to date; it is more a question to what extent a person is psychopathic, not whether they are or are not.
Hunters versus farmers
In this way, psychopathy has probably existed in humanity since at least prehistoric times. Psychopaths are humanity's "predators", so were therefore likely to have prospered in the nomadic hunter-gatherer societies that existed before farming and agriculture brought about settlements and "civilisation" in the formal sense - city life. Not only that, but psychopaths are by nature somewhat restless and always looking for new challenges, so would have felt at home in the "adventure" of nomadic hunter-gatherer society.
Thinking back to a concrete example, the wars between the ancient Germanic tribes and the Roman Empire can be called "clash of civilisations" - albeit, where Rome was a city-dwelling civilisation, and the ancient Germans were more faithful to the "hunter-gatherer" society that more closely aligns with the traits of a psychopath. The tribe of Atilla the Hun also fit into the nomad, hunter-gatherer society, as did the migrating Turkic tribes just over a thousand years ago, and most famously, that of Genghis Khan and his Mongol successors. For the Huns, the Turks and the Mongols, the horse was an integral part of their society, symbolising their nomadic instincts, even if their cultures became integrated into those absorbed into their larger empires. The rituals of the hunt and the ethic of total warfare (that the Mongols were most famous for) was what made these three tribes equally feared and respected by their more "civilised" rivals. Before the "Pax Americana" of the modern day, its closest equivalent of a "world empire" was the "Pax Mongolica", which spread across most of Eurasia like a human plague, decimating any populations that resisted.
But for all its seemingly-barbaric (psychopathic?) flaws, the "Pax Mongolica" was an extremely efficient political structure; in ways that a modern-day multinational company would probably recognise. In spite of its enormous scope, communications across the Eurasian landmass became more efficient than they had ever been until the modern era. Peace and trade flourished within the empire itself. The "empire" was divided up into spheres of influence, and ruled accordingly, in much the same way that a modern multinational has regional branches. In other words, the "Pax Mongolica" was brutally-efficient: it was unflinchingly brutal when it was necessary to be; but it was also very efficient at leadership and organisation.
The psychopath as Capitalist predator
The (psychopathic) example of the "Pax Mongolica" is a demonstration of medieval power politics. But the success of the "hunter-gatherer" model of the Mongols was unusual compared to most other medieval societies, which were almost always ruled as agriculture-based societies ruled through a system of serfdom; a poor environment for psychopaths. Unless a psychopath found a niche to use his talents or a way to gain access to power (e.g. through military glory or ecclesiastical advancement), he was doomed to a (short) life of frustration. Most medieval societies tended to be socially and economically static, where movement (and therefore trade and socio-economic independence and entrepreneurial thought) was very difficult.
These deeply fixed and hierarchical societies were socially-geared to suppress the environmental factors that are understood to encourage psychopathy today. Instead, those at the apex of the social hierarchy may well have tended more towards aspects of psychopathic behaviour than otherwise due to its dysfunctional and self-enclosed nature. A modern-day comparison would be the regime of North Korea.
Capitalism in its rudimentary form began to flourish in the mercantile empire of Venice, which reached its zenith at around the same time as the Mongols. What Capitalism shares with the mentality of the hunter-gatherer is the instinct for self-advancement and self-preservation, and a natural curiosity for the next challenge or opportunity. Both Capitalism and the hunter-gather society are inherently anarchic, and reward behaviour that would elsewhere be seen as cold-blooded and selfish, which fits the psychology of the psychopath. The brutal efficiency of the Mongol Empire also had the upside that it served as a conduit for Far Eastern innovations and trade to Europe, and vice versa. So the Capitalist embryo that was formed from the likes of the mercantile Venetian Republic, also provided the ideal environment for psychopaths to flourish.
I've spoken before about the links between psychopathy and the mindset of Capitalism. But it is also evident that many of the aspects of the renaissance were at least an indirect result of factors such as trade, exchanging ideas, and independent thought and creativity. And psychopaths in such a social situation are hard-wired to excel, having all the natural attributes necessary. In other words, while in hunter-gatherer societies, psychopaths would become the natural leaders, in proto-Capitalist societies like the Venetian Republic and its neighbours, psychopaths would have the natural attributes to become the most successful entrepreneurs and merchants. In other words, Capitalism replaced the function of hunting in human society, allowing psychopaths to successfully adapt to the situation.
As psychopaths are hard-wired with all the attributes that are an advantage in hunting and war-like societies, this also gives them some added social attributes in regard to the opposite sex. Psychopathy may be considered more of a genetic aberration, but natural selection and the role in society of the hunter-Capitalist (predator) therefore makes psychopathy more likely to spread over generations. This explains the potentially massive under-measuring of the true scale of the psychopathic condition in society. It also explains a lot about why globalisation works the way it does.
When people talk about "Social Darwinism", the reality is that this model of Capitalism as envisaged by modern-day Neo-liberals is ideally-suited for psychopaths. In an almost literal dog-eat-dog society, because psychopaths are amoral, sexually-promiscuous, adaptable, self-confident and guiltless, they are capable of almost anything; unfortunately, this also means whenever psychopaths gain positions of power, the result is chaos for everyone else.
Then again, there are some cultures for which psychopathy becomes almost a prerequisite for survival.