Feels like an impostor despite results
A strong performer tells you, half joking, that they're waiting to be found out, and overprepares for every meeting.
Rule out first
A real gap: a new role or domain beyond their experience. Being the only person like them in the room. A boss who gives no feedback.
What could explain it
Peter principle: The doubt is accurate. They were promoted on skills that don't transfer, and there's specific evidence of struggle.
Impostor phenomenon: Results and feedback are good; the fear survives all evidence and moves to the next goal.
Perfectionism: The bar is the problem: anything short of flawless counts as failure, and the fear is mostly of mistakes.
Conditions of worth (Rogers): They feel worth something only while performing; holidays and gaps between projects feel dangerous.
Stereotype threat: The doubt rises in settings where their group is scarce or stereotyped and fades elsewhere.
A next move
Set their self-assessment beside an objective measure. If they match, it's a real gap to close. If not, ask what would count as proof they belong.
If it’s you
What would I need to see to believe I'm good enough here, and have I already seen it?