[hutzlibu]

Not in my understanding: it is

group A believes in X

group B believes in Y

What the parent poster seemed to imply was

group A believes in not Y.

Or a more concrete example of today, many people today define themself by being anti green, anti progressive, anti woke, antifashist etc. but often struggle to define what they are standing for.

[drc500free]

The theory would be that given A believes X and B believes Y, A is more likely to incorporate ~X and B is more likely to incorporate ~Y.

And because beliefs tend to be on a bit of a continuum, what starts with A believes Z <= 4 and B believes Z >=5 evolves to A believes Z <=1 and B believes Z >=9, and both believe anyone who believes 3<=Z<=7 is not part of their group.