Everyday life is full of examples of people acting ruthlessly or egotistically – just look at who's sitting in the Oval Office.
In psychology as well as in everyday language, we have diverse names for the various dark tendencies humans may have.
Most prominent among them are psychopathy (lack of empathy), narcissism (excessive self-absorption), and Machiavellianism (the belief that the ends justify the means), along with many others such as egoism, sadism, or spitefulness.
Although at first glance there appear to be noteworthy differences between these traits - and it may seem more "acceptable" to be an egoist than a psychopath - new research shows that all dark aspects of human personality are very closely linked and are based on the same tendency.
The common denominator of all dark traits, the so-called "D-factor", can be defined as the general tendency to maximise one's individual utility - disregarding, accepting, or malevolently provoking disutility for others - accompanied by beliefs that serve as justifications.
In other words, all dark traits can be traced back to the general tendency of placing one's own goals and interests over those of others even to the extent of taking pleasure in hurting others.
A study by German and Danish researchers shows that dark traits in general can be understood as instances of this common core - although they may differ in which aspects are predominant.
In a series of studies with more than 2500 people, the researchers asked to what extent people agreed or disagreed with statements such as "It is hard to get ahead without cutting corners here and there", "It is sometimes worth a little suffering on my part to see others receive the punishment they deserve," or "I know that I am special because everyone keeps telling me so".
Further, they studied other self-reported tendencies and behaviours such as aggression or impulsivity, and objective measures of selfish and unethical behaviour.
Their results were similar to century-old findings that showed how people who scored highly at one type of intelligence test typically also scored high on others.
"In the same way, the dark aspects of human personality also have a common denominator, which means that - similar to intelligence - one can say that they are all an expression of the same dispositional tendency," said study author Professor Ingo Zettler, of the University of Copenhagen.
"For example, in a given person, the D-factor can mostly manifest itself as narcissism, psychopathy or one of the other dark traits, or a combination of these.
"But with our mapping of the common denominator of the various dark personality traits, one can simply ascertain that the person has a high D-factor."
The politics of conspiracy theories
Conspiracy theories have been cooked up throughout history, but they're increasingly visible lately.
Given that any particular conspiracy theory is unlikely to be the subject of mainstream consensus, what draws people to them?
New research suggests that people with certain personality traits and cognitive styles are more likely to believe in conspiracy theories.
"These people tend to be more suspicious, untrusting, eccentric, needing to feel special, with a tendency to regard the world as an inherently dangerous place," said Josh Hart, an associate professor at Union College in the US.
"They are also more likely to detect meaningful patterns where they might not exist.
"People who are reluctant to believe in conspiracy theories tend to have the opposite qualities."
Hart and colleagues surveyed more than 1200 American adults.
Participants were asked a series of questions related to their personality traits, partisan bent and demographic background.
They were also asked whether they agreed with generic conspiratorial statements, such as: "The power held by heads of state is second to that of small unknown groups who really control world politics," and "groups of scientists manipulate, fabricate or suppress evidence in order to deceive the public".
Previous research had shown that people gravitate toward conspiracy theories that affirm or validate their political view: Republicans were vastly more likely than Democrats to believe the Obama "birther" theory or that climate change was a hoax.
Democrats were more likely to believe that Donald Trump's campaign colluded with the Russians, Hart said.
Some people were also habitual conspiracists who entertain a variety of generic theories.
For example, they believed that world politics was controlled by a cabal instead of governments or that scientists systematically deceive the public.
This indicated that personality or other individual differences might be at play.
Hart wanted to build on this research by testing how much each of several previously identified traits could explain generic conspiracy beliefs.
By examining multiple traits simultaneously, the pair could determine which ones were most important.
"Our results clearly showed that the strongest predictor of conspiracy belief was a constellation of personality characteristics collectively referred to as 'schizotypy'," Hart said.
The trait borrowed its name from schizophrenia, but it did not imply a clinical diagnosis.
Hart's study also showed that conspiracists had distinct cognitive tendencies: they were more likely than nonbelievers to judge nonsensical statements as profound - a tendency known as "BS receptivity".
In turn, they were more likely to say that nonhuman objects - triangle shapes moving around on a computer screen - were acting intentionally.
"In other words, they inferred meaning and motive where others did not," he said.
So what did this all mean?
"First, it helps to realise that conspiracy theories differ from other worldviews in that they are fundamentally gloomy," Hart said.
"This sets them apart from the typically uplifting messages conveyed by, say, religious and spiritual beliefs. At first blush this is a conundrum.
"However, if you are the type of person who looks out at the world and sees a chaotic, malevolent landscape full of senseless injustice and suffering, then perhaps there is a modicum of comfort to be found in the notion that there is someone, or some small group of people, responsible for it all.
"If 'there's something going on,' then at least there is something that could be done about it."
Hart hoped the research advances the understanding of why some people are more attracted to conspiracy theories than others.
But he said it was important to note that the study didn't address whether or not conspiracy theories are true.
"After Watergate, the American public learned that seemingly outlandish speculation about the machinations of powerful actors is sometimes right on the money," he said.
"And when a conspiracy is real, people with a conspiracist mindset may be among the first to pick up on it while others get duped.
"Either way, it is important to realise that when reality is ambiguous, our personalities and cognitive biases cause us to adopt the beliefs that we do.
"This knowledge can help us understand our own intuitions."