The word's biggest study of children born as a result of fertility treatment has dispelled fears that they will suffer from psychological problems caused by their unnatural conception and unusual biological relationship to their parents.

Findings from the study to be released today show that the children of IVF treatment and artificial insemination by a sperm donor are, if anything, happier and more balanced than those conceived naturally.

The research, which involved interviewing more than 400 families from four countries over a period of 12 years, also found that nine out of 10 marriages of couples who had undergone fertility treatment were still intact – a significant improvement on the one in three marriages in the general population that end in divorce.

But the research also found that a high proportion of parents who had used donor insemination had not told their children about the treatment on the grounds that it might make them less loving towards their father, even though these men were found to be good parents.

Professor Susan Golombok, director of the City University Family and Child Psychology Research Group in London, said there is a danger that such secrecy might store up trouble in the future if children learn that their father is not their true biological father.

Professor Golombok told the European Society of Human Reproduction and Embryology in Lausanne: "Many parents have told other people and this creates a risk that the children will find out from someone else."

The second phase of the European Study of Assisted Reproduction Families – involving couples in Britain, Spain, Italy and the Netherlands – has nevertheless supported the view that fertility treatment is not detrimental to parents or their children. Professor Golombok said: "The assisted-reproduction parents were found to have had better relationships with their children than the natural conception parents."