What do Norman Bates, Tony Soprano, and Miranda Priestly have in common? Aside from being entirely fictional characters dreamt up by writers and filmmakers, they all possess a characteristic called the "D-factor". The "D" here stands for "dark" but feel free to add in your own interpretation.

Psychologists from Germany and Denmark have defined "D" as "the general tendency to maximize one's individual utility – disregarding, accepting, or malevolently provoking disutility for others – accompanied by beliefs that serve as justification."

On an individual level, this may be expressed through narcissism, sadism, psychopathy, or any of the other six most commonly studied dark personality traits, including egoism, Machiavellianism, spitefulness, self-interest, moral disengagement, and physiological entitlement. Indeed, in a paper published in Psychological Review, the psychologists describe the D-factor as the "dark core of personality" and these traits as manifestations of that underlying disposition.

This means that if a person presents one dark personality trait (like, for example, Machiavellianism – "a manipulative, callous attitude and a belief that the ends justify the means"), they are more likely to exhibit one or more of the other traits (for example, egoism – "an excessive preoccupation with one's own advantage at the expense of others and the community" and psychopathy – "lack of empathy and self-control, combined with impulsive behavior"). 

The researchers came to this conclusion having surveyed more than 2,500 people over a series of studies, asking them to agree or disagree on a five-point scale with various statements to measure their propensity for dark thinking. The statements ranged from "I insist on getting the respect I deserve" to "I have hurt people because I could" to "even without any risk of getting caught, it is simply wrong to break the law". In some surveys, they also measured responses to ethically, morally, and socially dodgy behavior (for example, cheating in a test).

Then, they came up with the D-factor.

The inspiration for the D-factor comes from the g-factor, a far more positive spectrum thought up by English psychologist Charles Spearman more than 100 years ago. Spearman was able to show that those who were able to perform cognitively well in one area were more likely to do so in another because of something called general intelligence (or the g-factor). 

"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," Ingo Zettler, Professor of Psychology at the University of Copenhagen, explained in a statement.

"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. This is because the D-factor indicates how likely a person is to engage in behavior associated with one or more of these dark traits."

Want to know how dark you really are? Take the test for yourself here.