For most of the past two decades, psychologists studying workplace toxicity have worked trait by trait. One body of research described narcissism. Another described psychopathy. A third described Machiavellianism. A fourth, newer, described everyday sadism. Each came with its own questionnaire, its own case studies, and its own claim to explain the difficult person in the next office. What none of them explained was why these “different” traits kept turning up in the same people, correlating with each other far more than chance would predict.
In 2018, three European psychologists proposed an answer. Morten Moshagen, Benjamin Hilbig and Ingo Zettler argued that, rather than nine separate dark traits, the field was looking at nine flavoured expressions of one underlying disposition, which they named the Dark Factor of Personality, or D. In their formulation, D is the general tendency to maximise your own utility while disregarding, accepting, or malevolently provoking disutility for others, sustained by beliefs that justify the behaviour to yourself. A psychopath, on this account, is someone high in D who mostly doesn’t notice the harm they cause. A Machiavellian is someone high in D who notices and doesn’t care. A sadist is someone high in D who actively enjoys it.
This article follows that claim from its origin to the point where the evidence becomes more contested. It examines each of the nine traits the original research programme built its case on (egoism, Machiavellianism, narcissism, moral disengagement, psychological entitlement, psychopathy, sadism, self-interest and spitefulness), tracing each to the specific measure used to represent it, because the strength of the unifying claim rests on the quality of what it is unifying. It then turns to how D is actually measured, using item sets refined across samples of more than 165,000 people. It also turns to the four-year longitudinal study, which is the strongest single piece of evidence for treating D as a genuine, stable disposition rather than a statistical artefact of overlapping questionnaires.
The article does not stop at the favourable evidence. A live, published debate asks whether D is doing anything that low Honesty-Humility or low Agreeableness didn’t already do, and a separate, older debate asks whether the “Dark Triad” it partly absorbs was ever three traits or mostly just psychopathy wearing different hats. Both are treated as open questions, not settled ones.
Finally, the article turns to the workplace, where a newly published 2026 study links D directly to vocational interests using both self-reports and government job registries across three countries. That is evidence more current than almost anything else in this series. It closes with a case against the most tempting practical use of this research: screening job candidates for D. The reasons for that caution are as evidence-based as the model itself.