Zero sum bias
Zero sum bias is assuming that one person's gain must be another person's loss, even when both could benefit. Many situations, such as trade, friendship and learning, can create value for everyone involved.
In everyday life
A student refuses to share study notes, believing that classmates doing well will hurt their own grade, even though the class is not graded on a curve.
When a coworker is praised in a meeting, you feel as if some of your own standing just shrank, even though there is no limit on how many people can be recognized.
Why your mind does this
Our brains handle small, concrete quantities well but struggle with large numbers, fractions and odds. Rounding, grouping and labeling let us make fast decisions without doing the math. For most daily choices, a rough answer is good enough.
Its family, in 30 seconds: Simpler numbers
How to spot it
Ask whether there is truly a fixed amount to share here, or whether both sides could come out ahead.
How to counter it
Look for a deal or plan where both sides gain, and check whether the 'pie' is really fixed.
Same family: Simpler numbers
SourcesMeegan (2010)·Wikipedia