Don't be too optimistic - it'll depress you and lead to more stressful situations, a study tracking couples through their first four years of marriage has found.

The study, published this week, followed 338 people and compared their mental states against how much they sugarcoated their experiences.

The authors asked the question: "What is the best way for people to protect their mental health when challenges arise?"

Mental health experts have traditionally said that it is best to confront reality without bias, but more recent studies have demonstrated some benefits to false optimism.

The new analysis, in the Journal of Personality and Social Psychology, found that thinking positively about bad experiences helped out people with only minor problems, but it promoted more depression and bad experiences among spouses with serious issues.

The study contacted newlywed couples every six months for four years, asking about their stressful experiences.

Each time, the participants individually rated how severe they considered their experiences to be, and the researchers compared these against their own ratings of the same experiences.

On average, the participants rated their experiences more positively than the researchers, but there was a spread from the more realistic to the highly optimistic.

Over four years, how realistic or optimistic the participants were predicted not only how depressed they became but also how bad their situations would actually become in reality.

The trend diverged between two groups within the participants - the "More-minor negative experiences" and the "More-severe negative experiences".

In simple terms, realistic people with little problems grew more depressed as the study went on, while realistic people with big problems became happier.

Meanwhile, optimistic people without much to worry about grew happier and those staying positive in the face of major stresses became more depressed and entangled in worse situations.

Optimism may reduce the motive to address problems, the authors suggest, though this mechanism was not assessed in the study.

The authors also point out that their findings relate only to controllable stresses as in marital friction. The benefits of positive thinking are more firmly established for uncontrollable negative experiences, such as cancer and grieving over deaths.

They also warn that the findings are correlational and cannot support strong causal conclusions.