Depressed people have a peculiar view of the past - rather than glorifying the 'good old days', they project their generally bleak outlook on to past events, according to new research.
It is known depression makes sufferers see the present and the future as sad, but this is the first time research has shown it also casts a long shadow over people's memories of the past.
Psychologists at Germany's Heinrich Heine Universität Düsseldorf and at the UK's University of Portsmouth published their research in Clinical Psychological Science.
It establishes the first clear link between depression and hindsight bias, or a distorted view of the past.
Dr Hartmut Blank, in the University of Portsmouth's Department of Psychology, is one of the authors.
He said: "Depression is not only associated with a negative view of the world, the self and the future, but we now know with a negative view of the past."
Hindsight bias includes three core elements: exaggerated perceptions of foreseeability - we think we knew all along how events would turn out; inevitability - something 'had' to happen; and memory bias - misremembering what we once thought when we know the outcome of something.
Hindsight bias has been studied in various settings, including sports events, political elections, medical diagnoses or bankers' investment strategies. Until now, it hasn't been used to study depression.
Dr Blank said: "Everyone is susceptible to hindsight bias, but it takes on a very specific form in depression. While non-depressed people tend to show hindsight bias for positive events but not negative events, people with depression show the reverse pattern.
"Making things worse, depressed people also see negative event outcomes as both foreseeable and inevitable - a toxic combination, reinforcing feelings of helplessness and lack of control that already characterise the experience of people with depression.
"Everyone experiences disappointment and regret from time to time and doing so helps us adapt and grow and to make better decisions. But people with depression struggle to control negative feelings and hindsight bias appears to set up a cycle of misery.
"We have shown hindsight bias in people who are depressed is a further burden on their shoulders, 'helping' to sustain the condition in terms of learning the wrong lessons from the past."
The researchers tested over 100 university students, about half of whom suffered from mild to severe depression. They were asked to imagine themselves in a variety of everyday scenarios with positive or negative outcomes (from different domains of everyday life, e.g. work, performance, family, leisure, social, romantic). For each scenario, the researchers then collected measures of hindsight bias (foreseeability, inevitability and distorted memory for initial expectations).
The results showed that with increasing severity of depression, a specific hindsight bias pattern emerged - exaggerated foreseeability and inevitability of negative (but not positive) event outcomes, as well as a tendency to misremember initial expectations in line with negative outcomes. Characteristically, this 'depressive hindsight bias' was strongly related to clinical measures of depressive thinking, suggesting that it is part of a general negative worldview in depression.
Dr Blank said: "This is only a first study to explore the crucial role of hindsight bias in depression; more work needs to be done in different experimental and real-life settings, and also using clinical samples, to further examine and establish the link between hindsight bias and depression."