It is often said that it is better to forgive and forget.

But psychologists say actually getting angry can be the best way to solve relationship problems.

James McNulty, associate professor at the University of Tennessee, found that forgiving may actually build up resentment.

He said the 'short-term discomfort of an angry but honest conversation' can benefit the health of a relationship in the long term.

'I continued to find evidence that thoughts and behaviors presumed to be associated with better wellbeing lead to worse wellbeing among some people - usually the people who need the most help achieving wellbeing.'

McNulty therefore set out to examine the potential costs of positive psychology. In a set of recent studies, he found that forgiveness in marriage can have some unintended negative effects.

'We all experience a time in a relationship in which a partner transgresses against us in some way,' he said.

'For example, a partner may be financially irresponsible, unfaithful, or unsupportive.

'When these events occur, we must decide whether we should be angry and hold onto that anger, or forgive.'

His research found a variety of factors can complicate the effectiveness of forgiveness, including a partner's level of agreeableness and the severity and frequency of the transgression.

'Believing a partner is forgiving leads agreeable people to be less likely to offend that partner and disagreeable people to be more likely to offend that partner,' he said.

Additionally, he claims, anger can serve an important role in signaling to a transgressing partner that the offensive behavior is not acceptable.

'If the partner can do something to resolve a problem that is likely to otherwise continue and negatively affect the relationship, people may experience long-term benefits by temporarily withholding forgiveness and expressing anger.'

However, McNulty found there was no single answer to the problem.

There is no 'magic bullet,' no single way to think or behave in a relationship.

'The consequences of each decision we make in our relationships depends on the circumstances that surround that decision.'