The dark triad is a group of three personality traits: narcissism, Machiavellianism and psychopathy. The use of the term "dark" reflects the perception that these traits have interpersonally aversive qualities:
- Narcissism is characterised by grandiosity, pride, egotism, and a lack of empathy.
- Machiavellianism is characterised by manipulation and exploitation of others, a cynical disregard for morality, and a focus on self-interest and deception.
- Psychopathy is characterised by enduring antisocial behaviour, impulsivity, selfishness, callousness, and remorselessness.
All three traits have been associated with a callous-manipulative interpersonal style. Jakobwitz and Egan carried out a factor analysis and found agreeableness strongly dissociated with these traits, and other factors, such as neuroticism and a lack of conscientiousness, associated with some traits. However, it has been suggested that agreeableness is not relevant to the core of the dark triad, and that instead the common variance is accounted for by callousness and manipulation.
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 claimed that Machiavellianism is nothing more than a mild form of psychopathy: aside from their relative severity, there is no difference between them. Delroy L. Paulhus and McHoskey debated these perspectives at a subsequent American Psychological Association (APA) conference, inspiring research by Paulhus and Kevin M. Williams on the dark triad. Paulhus and Williams found enough behavioural, personality, and cognitive differences between the traits to suggest that they were different from each other; however, they concluded that further research was needed to elucidate how and why they overlap.
Subclinical dimensions vs. disorders
Two dark triad traits, narcissism and psychopathy, were initially proposed as explanations for observed clinical 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 1960s and 1970s with the pioneering efforts of Robert Hare, in 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, narcissism lacked a means of assessing the disorder outside of 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 versions of the Mach-IV and Mach-V scales  are still the most widely used in empirical research.
As a disorder
In general, researchers treat these traits as pathological, something that needs to be treated, and inherently undesirable, e.g. socially condemned or personally counter-productive. However, some argue that this view of the traits is the result of researchers having an implicit group selection orientation towards personality traits.
In the workplace
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. 
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.
Contrarily, there is a lack of empirical evidence for a correlation between psychopathy and reproductive success. Additionally, using established measures of the dark triad, 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 erroneously assume the dark triad are a single construct, in spite of genetic and meta-analytic evidence to the contrary.
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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 here.
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