Reactive devaluation
Reactive devaluation is valuing a proposal less because it came from an opponent or someone you see as on the other side. The same offer seems more appealing when it comes from a neutral or friendly source.
In everyday life
In a labor dispute, a union rejects a pay offer from management, though it would likely have welcomed the identical terms if a mediator had suggested them.
Your ex suggests a way of splitting holiday time with the kids that is actually quite fair, and you immediately feel it must be some kind of trick.
Why your mind does this
Trusting people and things we know is usually safe, because familiarity means we have dealt with them before without harm. Favoring our own group helped our ancestors cooperate with the people they depended on. Liking what is close and known also saves the effort of judging everything from scratch.
Its family, in 30 seconds: Familiar is better
How to spot it
Ask how you would rate this offer if a friend or a neutral party had proposed it.
How to counter it
Restate the proposal in your own words with the source removed, and judge it on its terms.