"Whatever does she see in him?" is a common refrain from mystified friends as yet another acquaintance settles for a strange choice of partner. The conventional explanation is that "love is blind" but new psychological research suggests that long after the first flush of passionate love has ebbed away, distorted, unrealistic perceptions of one's partner are the key ingredient to a successful marriage.

This startling explanation for the secret of marital success has arisen because relationship psychologists have begun to move away from their favourite group of research subjects, students in short-term relationships, to the more middle aged who have survived, and indeed thrived, though many decades of married life.

Focusing on this group has uncovered what appears to be a self-deception mechanism that lies behind successful marriages. The secret, it seems, is to see your partner as a lot nicer than he or she really is.

The latest research, just about to be published in the academic journal Social Behaviour and Personality, measures a phenomenon referred to as "marital aggrandisement".

Marital aggrandisement entails an idealised appraisal of one's spouse and marriage to the exclusion of any negative beliefs and perceptions. Those who aggrandise their marriages tend to endorse items on personality tests that are extremely unlikely to be completely true, for example, "My spouse doesn't make me angry" or "I do not recall arguments with my spouse".

Marital aggrandisement was previously thought by psychologists to be most prevalent amongst newlyweds, reflecting the novelty of married life and an incomplete knowledge of one's spouse.

Early on in a relationship, for instance, one may endorse statements such as, "I have never known a moment of sexual frustration during my marriage", because the experience has yet to occur.

But research by psychologists Norm O'Rourke and Phillipe Cappeliez at the university of Ottawa in Canada has uncovered surprisingly high levels of marital aggrandisement in a sample of 400 married couples who had been married an average of almost 40 years.

They found that those who appeared the most happy with life in general and with their marriages tended to be the couples who aggrandised their partnerships, leading them to be labelled "Pollyannas" by the researchers. This group accounted for 52 per cent of the sample.

The rest of the sample, who appraised their relationship history more realistically and were honest about past negative events, tended to score significantly lower on marital satisfaction.

Psychologists believe that what they are observing in couples who endorse these and similar sentiments are strongly selective memories that ignore inevitable negative events over the course of marital history. Maybe a distorted view of your marriage that emphasises the positive and forgets the negative is crucial to accounting for who stays and who flees when it comes to relationship endurance.

Similar findings have recently been outlined by another group of psychologists who found that the people in more satisfied marriages saw more virtue in their partners than their friends or other close acquaintances did, more even than their partners perceived in themselves.

Sandra Murray and psychologist colleagues at the State University of New York have published a series of studies in recent years that are an attempt to get to the bottom of whether we stay with the person we are married to because we tend to see them in a more positive light than fits with strict reality. The theory is that being relatively blind to their faults and vices helps us live with them and keeps the marriage going for much longer than if we were more dispassionately aware of their flaws.

Psychologists compared what spouses thought of each other with the opinions of close friends and others who knew the spouses well.

Murray concludes that the reason these illusions about partners are so common in sturdy marriages is that to be happy we strongly need to believe we are in the right relationship with the right person, someone who will be there for us come what may.

But sustaining this belief in the face of partners who sometimes disappoint seems to require a protective buffer, one that is afforded by the perception of special virtues in the partner. Once achieved, Murray argues, this positive bias allows satisfied intimates to dispel potential doubts or reservations almost in advance of their occurrence. After all, few decisions have higher stakes than the one to commit to a particular romantic partner, and in perhaps no other context do we voluntarily tie the satisfaction of our hopes and goals to the goodwill of another. To feel happy and secure in the face of such vulnerability we need to believe our relationship really is a good one and that our partner can be counted on to be caring and responsive across time and situations.

Another example of delusional thinking at the heart of highly successful marriages comes from a further recent study published by Sandra Murray's group that examined our preference for marrying a "kindred spirit". A kindred spirit is someone who appears especially to understand us and uniquely share our experiences, probably because they see the world they way we do and are therefore, in important respects, just like us.

Murray's group measured marital partners' personalities, values and day to day feelings and compared these to marital satisfaction. Those in the happiest and most stable marriages were those most likely to believe their partners were most like them - that is, "kindred spirits" - even when objective comparison of personality found that the similarity was much more imagined than real.

This series of new psychological studies suggests the real issue in marriages is not the faults you notice in your spouse and try to point out. Rather they suggest that the best therapy for couples is to encourage more deluded thinking.

If you are going to insist on being realistic, then maybe marriage is not for you.

Raj Persaud is Gresham professor for public understanding of psychiatry and director of public engagement centre at the institute of psychiatry at London university.