Psychopaths comprise a small percentile of the population at large (commonly estimated at 1%). These individuals differ from the rest of the population because, research has shown, their brains have a dysfunction.
A small part of the brain called the amygdala is generally-understood to be responsible for forming empathy with others: a person who lacks empathy lacks the emotional understanding of how others feel, and therefore lacks the knowledge of how to react to it properly. It is this "empathy circuit" that makes people behave in a caring and humane manner towards others: if that circuit is "broken" - as it is in psychopaths - then an individual has no compunction (or necessity) to act in a way sympathetic to other people. On the contrary, they will do whatever they need to do to achieve their goals, regardless of the consequences for others.
An individual crime wave?
Psychopathy is a collection of traits and behaviours that are a result of lack of empathy. Much of the research into psychopathy was originally carried out in prisons, mostly because this was the only place where it was possible to find a "stable" population of psychopaths. Research has shown that psychopaths make up a highly-disproportionate percentage of the prison population; this disparity increases even further when you look at violent offenders and serial sex offenders. In other words, psychopaths (who make up only 1% of the population) are responsible for a large part of violent and sexual offences. Furthermore, psychopaths, by the nature of their "disorder" are incapable of learning from their mistakes, and not surprisingly, make up a disproportionate percentage of those that are repeat offenders. So we can see that psychopaths bear a great responsibility for much of the crime that exists in society. And this doesn't even factor in other, more discreet - yet socially extremely-damaging - types of crime (more on that later).
However, most psychopaths are not even in prison. It can be said that only the "unsuccessful" psychopaths are the ones that we actually see in prison. Most serial killers are not psychopaths; however, a disproportionate number of serial killers are psychopaths. What makes the difference between a psychopath being a serial killer (which is in fact, very rare), a violent gang member, a rapist, a serial adulterer, a fraudster, or a boardroom CEO? Circumstance.
Choose your weapon
Robert Hare, the foremost expert on psychopathy, has said that psychopaths exist in all sections of society, and thus bad parenting or an abusive childhood cannot be said to be solely to blame, as some others have argued. Hare suggests that psychopathy is, at least partially, a biological (i.e. genetic) condition. The environment of a "psychopathic child" can play a part in aggravating the onset of the condition, but there are equally - as Hare claims - too many examples of psychopaths from well-adjusted families to say for sure that environment plays a decisive role. While environment is a factor, Hare's thinking suggests it is largely a biological condition.
Regardless of the origins of how a person becomes a psychopath, what is clear that apart from being a disproportionate part of the prison population, in the general population, we know that people ranking high on Hare's "psychopathy checklist" also tend to feature disproportionately in some fields of work.
Psychopaths have low anxiety levels and high risk-taking tendencies, amongst other behaviours, such as manipulation and superficial charm. This ruthlessness and ability to make quick decisions gives them a natural advantage in professions such as business, banking and law, but also other fields such as medicine and even the military and the police - and yes, politics, too. Psychopaths have been shown to have a heightened sense of alertness, and an ability to focus in on a person's strengths and weaknesses. Thus, they are "hard-wired" to be adaptive survivors, and - depending on their individual circumstances - know how to make the most of their "attributes".
This explains why there are a disproportionate numbers of violent and sexual offenders - but very few fraudsters - in prison.
Those psychopaths that have been born at the wrong end of society are most likely to be destined to become petty "career" criminals, gang members or father a string of illegitimate children. However, equally, a disproportionate number of these "low end" psychopaths may be able, through force of circumstances, be able to become highly-successful entrepreneurs: what is often called a "rags-to-riches" millionaire who has "the gift of the gab". These types of individuals were able to use their cunning and ruthlessness in more "pro-active" ways to make themselves financially well-off using legal means, whereas a psychopathic gang member was not.
And then we have what might be called "mid-level" psychopaths. These are usually "socially-adapted" psychopaths that have found some middling way to find an outlet for channeling their attributes: born into an "average" family, they may be successful salesmen or con men (more on these types of psychopaths, and the sales industry in general here). Equally, they may well go into the police force or even the military. In short, they have chosen professions that fit the glove of their personality, without making their lack of empathy seem too obvious or intrusive on their way of life. In some ways, these types of psychopaths are perhaps the most "socially-useful" to society, because they contribute something positive to society, in spite of their other negatives. This is not to say that these people are "well-adjusted" - they may still be capable of violence, ruthlessness, manipulation, and sometimes behave appallingly - but it may be well-concealed so to be less obviously-apparent.
Lastly (and most ominously) are the "high-end" psychopaths. Many of these are people either born into the higher end of society, or by their own high intelligence and cunning, are able to quickly work their way into it. While "low-level" psychopaths may be the ones most likely you'll encounter in prison and responsible for a large degree of "everyday" crime, the "high-end" psychopaths are the ones most likely to defraud businesses, bankrupt corporations, lay off entire workforces, cause environmental disasters, or worse. These are the people who have "the system" wrapped around their finger, so that any amoral acts they do are either ignored, excused, blamed on others (or underlings), or have been already made legal to neutralise the threat to their position. These people aren't in prison because they already have the political and legal system in their pockets.
The most infamous recent example is the financial crisis of 2008, where the elite of the banking system had devised a fraudulent banking model that caused the entire system to collapse. The banking cartel was then able to effectively blackmail governments into "bailing out" the banks without seriously forcing them to change their systems of operation, let alone sending many of them to prison for mass fraud on an epic scale. The modern economic system, which first came to into wide use thirty years ago, is effectively propping-up a system of economic extraction by an elite at the expense of the conditions of the regular workforce. These are conditions where the elite are leeching off the lower half of society in the same manner that psychopaths leech off their targeted victims. With the justification of Ayn Rand's philosophy, neo-liberalism sees social injustice as a "necessary evil"; even a good thing. This is precisely the same logic that psychopaths use to justify their acts. Is that a coincidence?
It is this reason why the gap between the highest and lowest earners has expanded by many times in the last thirty years. In this sense, it is difficult to separate the effects that psychopathy has on crime and on how our economy is governed overall: if the modern "Capitalist" system is ran on the principle of amoral exploitation, then how is it different, morally-speaking, from psychopathy?
When "high-end" psychopaths are in positions of authority the results are far more socially-devastating than a "low-end" psychopath. A low-end psychopath in a town can harm individuals and families through, for example, petty crime and casual violence. A high-end psychopath, by closing down a town's factory, can bring about the social conditions to breed more crime and social deprivation in the whole town.
This is what makes psychopathy a cause of so much of humanity's social problems, from poverty and inequality to crime. Psychopaths are nature's predators, amorally seeking to exploit others for their own advantage. When these individuals gain entry into the field of politics, the result can be truly devastating. History has seen many examples; and in the modern-day, we see many countries around the world that are ran as little more than modern-day feudal states. These nations are ruled by elites that see their populations as little more than resources to exploit, and are the main cause of poverty in the world today: by definition, these elites must lack empathy for those they rule in order to justify how they "govern", breeding further generations "little dictators" for their elites to continue their hereditary exploitation of their populations.
North Korea is a prime example of this, though there are many others.