The Dark Triad is a group of three personality traits: narcissism, Machiavellianism and psychopathy, all of which are interpersonally aversive. The Dark Triad refers to three theoretically distinct but empirically overlapping personality constructs. The term reflects the perception that these three diagnostic categories have at least some common underlying factors:
- The narcissistic personality (in the clinical sense) is characterized by a grandiose self-view, a sense of entitlement, lack of empathy, and egotism. Some theories, such as those of Heinz Kohut, associate it with the protection of a radically weak, shamed, or damaged self.
- The Machiavellian personality is characterized by manipulation and exploitation of others, with a cynical disregard for morality and a focus on self-interest and deception.
- The psychopathic personality is characterized by impulsive thrill-seeking, and in its "primary" form by selfishness, callousness, lack of personal affect, superficial charm, and remorselessness.
All three characters involve a callous-manipulative interpersonal style, and are considered aversive. Jakobwitz and Egan carried out a factor analysis and found agreeableness strongly dissociated with all dark triad personality types, but other factors (neuroticism, lack of conscientiousness) were associated only with some members of the triad. However, more recent evidence suggests that agreeableness has nothing to do with the core of the Dark Triad. Instead the common variance is accounted for by callousness and manipulation, otherwise known as "Factor 1" of psychopathy according to Hare. In other words, once callousness and manipulation are accounted for, the Dark Triad characteristics are unrelated to each other.
Research on the Dark Triad traits began in isolation from one another. Narcissism is debatably the oldest, dating back to Sigmund Freud; psychopathy and Machiavellianism are more recent. Psychopathy, like narcissism, is a concept rooted in clinical and abnormal psychology. By contrast, Machiavellianism was first proposed as a personality trait.
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 the famous Paulhus and Williams (2002) research on the "Dark Triad". Paulhus and Williams found enough behavioral, 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.
Personality traits and subclinical dimensions vs. disorders 
Two of the Dark Triad traits, narcissism and psychopathy, were first proposed as explanations for observed clinical disorders. Narcissism is debatably the oldest, tracing back to the writings of Sigmund Freud. Psychopathy as an observed syndrome was addressed in the early writings of Hervey Cleckley in 1941 with the publication of The Mask of Sanity.
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.
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. Although some argue that the term "subclinical" simply means that the characteristic is a less severe version of the syndrome, others have argued that "subclinical" means that the characteristic does not interfere with day-to-day life in a way that would warrant diagnosis or intervention, e.g. treatment or imprisonment.
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.
Evolutionary perspectives 
In general, most researchers treat these traits as pathological, something that needs to be treated, and inherently undesirable, e.g. socially sanctioned 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.
As mating strategy 
The time-course of the benefits of these traits is an important consideration. 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, those high on the Dark Triad tend to 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.
However, the research pertaining to the Dark Triad as a mating strategy is based almost exclusively on college students, which limits generalizability, although it does have the benefit of selecting a group at what would evolutionarily be the prime mating ages. Further, there is research to suggest empirical inconsistency. For example, using established measures of the Dark Triad, these traits are not universally short-term-oriented  nor are they all impulsive. In addition, 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. Readers are especially cautioned against drawing conclusions based on research that: (a) is solely represented by student samples while claiming to study evolutionary or mating research, and (b) rely only on self-report data when analyzing personality traits often characterized by interpersonal deception.
See also 
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- Paulhus, D. L., Williams, K. M. (2002). "The Dark Triad of personality: narcissism, Machiavellianism, and psychopathy". Journal of Research in Personality 36 (6): 556–563. doi:10.1016/S0092-6566(02)00505-6.
- Leonard M. Horowitz; Stephen Strack, Ph.D. (14 October 2010). Handbook of Interpersonal Psychology: Theory, Research, Assessment and Therapeutic Interventions. John Wiley & Sons. pp. 252–255. ISBN 978-0-470-88103-3. Retrieved 15 February 2012.
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- The Hare Psychopathy Checklist-Revised. New York: Multi-Health Systems.
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- Jonason, P.K., & Kavanagh, P. (2010). The dark side of love: The Dark Triad and love styles. Personality and Individual Differences, 49, 606-610.
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- Vernon P. A., Villani V. C., Vickers L. C., Harris J. A. (2008). "A behavioral genetic investigation of the Dark Triad and the Big Five". Personality and Individual Differences 44 (2): 445–452. doi:10.1016/j.paid.2007.09.007.
- O'Boyle E. H., Forsyth D. R., Banks G. C., McDaniel M. A. (2012). "A meta-analysis of the Dark Triad and work behavior: A social exchange perspective". Journal of Applied Psychology 97 (3): 557–579. doi:10.1037/a0025679. PMID 22023075.
- Peter K. Jonason and Gregory D. Webster have published a brief measure of the dark triad traits called "The Dirty Dozen". An interactive online version can be found here.
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. This measure is in the process of publication.