You stay late perfecting a deck for an important client pitch, proud of one particular insight that feels fresh and promising. The next day in the meeting, a colleague presents that idea as if it were their own. Leadership nods approvingly. You sit frozen, wondering if you’re imagining things. Maybe you didn’t explain it clearly enough. Maybe it wasn’t really your idea at all. By the end of the meeting, you’re no longer sure. Instead of calling out the behavior, you question your memory, your presence, even your value.

The truth? You were a victim of a toxic personality.

Psychologists call the most destructive of these the Dark Tetrad. These are four personality types that consistently undermine trust, collaboration, and performance. Unlike healthy conflict, which can make teams smarter, Dark Tetrad personalities erode Team Intelligence (the collective genius that emerges when groups work well together).

1. The Psychopath

In movies, psychopaths are killers. In the office, they show up as bold risk-takers. They’re confident in presentations, quick to volunteer for stretch projects, and often the first to suggest bold pivots.

But behind that charm is something colder: a lack of empathy. Imagine a product launch where they push through a risky strategy despite clear evidence it will burn out the team or alienate customers. When layoffs come, they sleep just fine at night.

Their charisma wins them promotions, but their disregard for others leaves wreckage that weakens a team’s ability to think collectively. They destroy Team Intelligence by valuing bold moves over sustainable collaboration.

Put simply: They are bold, lack remorse, and seem charmingly normal.

2. The Narcissist

Narcissists are obsessed with recognition. They light up when they get credit for a win, but when things go wrong, they are quick to point fingers elsewhere.

Picture the teammate who swoops into a project at the last minute, contributes little, but positions themselves as the hero in front of leadership. Or the manager who can’t stop reminding you about their alma mater, their awards, or how many people report to them.

Narcissists also use a tactic psychologists call DARVO: Deny, Attack, and Reverse Victim and Offender. Confront them about stealing credit, and they’ll deny it, attack your competence, and cast themselves as the victim of your “unfair accusations.” It’s a dizzying maneuver that leaves you questioning your own judgment while they walk away unscathed.

The result: they hoard credit, choke off collaboration, and turn teams into a supporting cast for their ego. And because Team Intelligence relies on trust and openness, narcissists cut it off at the root.

Put simply: It’s all about them.

3. The Machiavellian

Machiavellians aren’t as brash as narcissists or as obvious as sadists. Instead, they see the workplace as a chessboard and colleagues as pieces to move.

Think of the coworker who always knows a rumor before you do. They quietly pit colleagues against one another, whisper in the boss’s ear, and smile as chaos unfolds.Or the manager who dangles a promotion just out of reach—”Keep working late, keep proving yourself, and it’ll be yours”—all while enjoying the benefits of your burnout. Quarter after quarter, the promotion never comes.

They thrive in environments with low transparency and high politics. But every scheme, every rumor, every betrayal chips away at trust. When a Machiavellian is at play, Team Intelligence collapses into paranoia.

Put simply: you are just a tool.

4. The Sadist

The most chilling of the four is the sadist. Unlike the others, who manipulate for gain, sadists manipulate for sport. They take pleasure in other people’s suffering.

You may have worked for a boss like this: the one who assigns impossible deadlines just to watch people sweat. Or the colleague who ridicules others in meetings, not to prove a point, but to humiliate.

Sadists enjoy seeing people squirm. They may appear to be “toughening the team up,” but in reality, they poison morale. Over time, their presence doesn’t just demotivate individuals—it dismantles Team Intelligence by making teammates fearful of contributing or speaking up.

Put simply: they enjoy your suffering.

Why They’re So Dangerous

Research shows that the negative impact of a toxic personality is far greater than the positive contributions of a supportive one. One Dark Tetrad employee can undo the goodwill, productivity, and morale of several healthy teammates.

And here’s the real kicker: they’re often rewarded. The psychopath’s boldness looks like confidence. The narcissist’s self-promotion gets mistaken for leadership. The Machiavellian’s scheming looks like strategy. The sadist’s cruelty gets passed off as “tough love.”

This is why cultivating Team Intelligence requires not only rewarding glue players, but also identifying and containing toxic ones. If left unchecked, they prevent teams from becoming smarter, more creative, or more resilient together.

How to Protect Yourself

You can’t always avoid Dark Tetrad personalities, but you can reduce their impact.

• Document everything. Toxic colleagues lie, manipulate, and rewrite history.
• Limit engagement. Minimize one-on-one interactions if possible.
• Create transparency. Open sharing of responsibilities and decisions makes manipulation harder.
• Build allies. Trusted teammates can help you reality-check situations.
• Don’t call them out directly. They are more practiced at manipulation than you are at defense.

The Bigger Picture

It’s tempting to believe that brilliance excuses toxicity. But for most organizations, tolerating Dark Tetrad behavior erodes trust, breaks collaboration, and destroys Team Intelligence.

Healthy teams don’t just need skill; they need psychological safety. They need glue players who make others better. They need leaders who reward collaboration instead of cruelty.