Biographer: Arnold’s belief that every offer of assistance on the part of his colleagues was a disguised attempt to make him look inadequate and that no expression of congratulations on his promotion should be taken at face value may seem irrational.
Conclusion
Arnold was right to think that his colleagues were lying / faking (duplicitous).
Evidence
At first blush, it might seem paranoid / irrational of Arnold to think 1. any time a colleague offers help, they're just trying to make him look inadequate 2. anyone who expressed congratulations at his promotion was just saying that for some ulterior motive (shouldn't be taken as face value)
But, his older sister really messed him up as a kid, making fun of his ambitions and achievements.
Evaluate
The psychological backstory of Arnold certainly explains why he's so paranoid about his coworkers' offering help or offering their (potentially sincere) congratulations.
But it doesn't mean that we suddenly start believing that Arnold is correct!
Goal
Look for an argument saying,