- Narcissism is characterized by grandiosity, pride, egotism, and a lack of empathy.
- Machiavellianism is characterized by manipulation and exploitation of others; a cynical disregard for morality, and a focus on self-interest and deception.
- Psychopathy is characterized by enduring antisocial behavior, impulsivity, selfishness, callousness, and remorselessness.
All three traits have been associated with a callous-manipulative interpersonal style. A factor analysis carried out at the Glasgow Caledonian University found that among the big five personality traits, the trait of agreeableness is strongly absent in regards to the dark triad, while other traits such as neuroticism and a lack of conscientiousness were associated with some.
Research on each of the traits represented in the dark triad began in isolation from one another. The term "Machiavellianism" dates back to the 16th century, and "narcissism" and "psychopathy" to the 19th century.
In 1998, McHoskey, Worzel, and Szyarto provoked a controversy by claiming that narcissism, Machiavellianism, and psychopathy are more or less interchangeable in normal samples. Delroy L. Paulhus and McHoskey debated these perspectives at a subsequent American Psychological Association (APA) conference, inspiring a body of research that continues to grow in the published literature. Paulhus and Williams found enough behavioral, personality, and cognitive differences between the traits to suggest that they were distinct constructs; however, they concluded that further research was needed to elucidate how and why they overlap.
Subclinical dimensions vs. disorders
Two of the Dark Triad traits, narcissism and psychopathy, were traditionally framed as personality disorders. Narcissism was discussed in the writings of Sigmund Freud, and psychopathy as a clinical diagnosis was addressed in the early writings of Hervey Cleckley in 1941 with the publication of The Mask of Sanity. Given the dimensional model of narcissism and psychopathy, complemented by self-report assessments that are appropriate for the general population, these traits can now be studied at the subclinical level.
With respect to empirical research, psychopathy was not formally studied until the 1970s with the pioneering efforts of Robert Hare, his Psychopathy Checklist (PCL), and its revision (PCL-R). Hare notes in his book, Without Conscience  that asking psychopaths to self-report on psychologically important matters does not necessarily provide accurate or unbiased data. However, recent efforts have been made to study psychopathy in the dimensional realm using self-reported instruments, as with the Levenson Primary and Secondary Psychopathy Scales, The Psychopathic Personality Inventory, and the Self-Report Psychopathy Scale.
Similarly, assessment of narcissism required clinical interviews, until the popular "Narcissistic Personality Inventory" was created by Raskin and Hall in 1979. Since the NPI, several other measures have emerged which attempt to provide self-report alternatives for personality disorder assessment. In addition, new instruments have been developed to study "pathological" narcissism  as opposed to "grandiose" narcissism, which is what many argue the NPI measures.
Machiavellianism has never been referenced in any version of the Diagnostic and Statistical Manual (DSM) for psychological disorders. It has been treated as strictly a personality construct. The original published version of the Mach-IV  is still the most widely used in empirical research.
As a disorder
In general, clinicians treat these traits as pathological, something that needs to be treated, and inherently undesirable, e.g. socially condemned or personally counter-productive. However, others argue that adaptive qualities may accompany the maladaptive ones. The evolutionary perspective (below) considers the Dark Triad to represent different mating strategies. Their frequency in the gene pool requires at least some local adaptation.
The everyday versions of these traits appear in student and community samples, where even high levels can be observed among individuals who manage to get along in daily life. Even in these samples, research indicates correlations with aggression, racism, and bullying among other forms of social aversiveness.
In the workplace
Oliver James identifies each of the three dark triadic personality traits as typically being prevalent in the workplace (see also Machiavellianism in the workplace, narcissism in the workplace and psychopathy in the workplace).
Recent studies have found that people who are identified as trolls tend to have dark personality traits and show signs of sadism, antisocial behavior, psychopathy, and Machiavellianism. The 2013 case study suggested that there are a number of similarities between anti-social and flame trolling activities and the 2014 survey indicated that trolling is an Internet manifestation of everyday sadism. Both studies suggest that these trolling may be linked to bullying in both adolescents and adults.
As mating strategy
It has been suggested that the dark triad traits appear to predispose individuals to short-term rewards and relationships over long-term rewards and benefits. Although advanced societies attempt to promote long-term thinking (environmental protection, saving money for retirement), there are reproductive benefits for the individual for thinking and acting on a shorter time-course. Also, men exhibiting these traits appear to be more successful at generating sexual attraction in women.
Consistent with this perspective, studies have suggested that on average, those who exhibit the dark triad of personality traits have an accelerated mating strategy, reporting more sex partners, more favorable attitudes towards casual sex, lowered standards in their short-term mates, a tendency to steal or poach mates from others, more risk-taking in the form of substance abuse, a tendency to prefer immediate but smaller amounts of money over delayed but larger amounts of money, limited self-control and greater incidence of ADHD symptoms and a pragmatic and game-playing love style. These traits have been identified as part of a fast life strategy that appears to be enacted by an exploitative, opportunistic, and protean approach to life in general and at work.
The evidence is mixed regarding the exact link between the dark triad and reproductive success. For example, there is a lack of empirical evidence for reproductive success in the case of psychopathy. Additionally, these traits are not universally short-term-oriented nor are they all impulsive. Furthermore, much of the research reported pertaining to the dark triad cited in the above paragraph is based on statistical procedures that assume the dark triad are a single construct, in spite of genetic and meta-analytic evidence to the contrary.
Vulnerable dark triad
The vulnerable dark triad (VDT) comprises three related and similar constructs: vulnerable narcissism, factor 2 psychopathy, and borderline personality disorder. A study found that vulnerable narcissism, factor 2 psychopathy, and borderline personality disorder are significantly related to one another and manifest similar nomological networks. Although the VDT members are related to negative emotionality and antagonistic interpersonal styles, they are also related to introversion and disinhibition.
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- Peter K. Jonason and Gregory D. Webster have published a brief measure of the dark triad traits called "The Dirty Dozen".
- Daniel N. Jones and Delroy L. Paulhus have also created a brief measure of the dark triad called the SD3 or Short Dark Triad, which can be found in Jones and Paulhus (2014).
- Mooney, Chris (February 14, 2014). "Internet Trolls Really Are Horrible People: Narcissistic, Machiavellian, psychopathic, and sadistic". Slate.