Lead researcher Karen Melrose of the University of Warwick and colleagues performed two experiments and found people's judgments of whether they were depressed or anxious were not mainly predicted by their symptoms' objective severity -- but by where they ranked that severity compared with their perception of others' symptoms.
The study showed participants' beliefs about the distribution of symptoms in the wider population varied greatly.
For example, 10 percent of participants thought half the population felt depressed on at least 15 days a month, and 10 percent thought they felt depressed on two days or fewer a month.
"It is the patient that initiates most doctor consultations about depression and anxiety, so that personal decision to see a doctor is a vital factor in determining a diagnosis," Melrose said in a statement. "Given that fact, our study may explain why there are such high rates of under and over-detection of depression and anxiety."
The findings were published in the Journal of Behavioral Decision Making.