People who agree with you all the time may be lying - but the way their brain is wired makes disagreeing much too stressful.
In a study, MRI scans of people who tend to agree showed areas of the brain associated with anxiety lighting up at statements they disagreed with.
Many agreed just to avoid conflict, even if they knew they were getting bad advice.
This means they give in to peer pressure too easily and lets others take advantage of them, say scientists at Monash University in Australia. Study chief Dr Juan Dominguez said: "It has to do with autonomy and agreeing for the sake of agreeing.
"In a relationship you might find yourself agreeing more often with your partner when making decisions, even if you hold a different opinion.
"This might be good for the relationship in the short term, but not necessarily good for you or the relationship long-term."
Subjects had to agree or not with true and false statements. Samples (both true) included "Orchid flowers have the most species" and "The first public library was opened in England. Those who mostly agreed felt 'a heightened state of mental stress and discomfort".
The report in Frontiers of Neuroscience, warns: "Having a lot of trouble disagreeing due to heightened mental stress may be indicative of an array of emotional, attitudinal or social issues. This can potentially lead to poor decision-making, anxiety, or difficulties in interpersonal relationships."
Dr Dominguez advises people who find themselves agreeing too often to "exercise and engage your critical faculty more".
- Daily Mail