Straightening children's crooked teeth with braces may improve their smile but it is no guarantee of happiness and improved self esteem.

A 20-year study by scientists in Britain that looked at the impact of braces on more than 300 children in Wales showed that having straighter teeth had little positive impact on their psychological health later in life.

When they were questioned as adults, none of them regretted having had their teeth improved and most were satisfied with the way they looked. But there was no difference in their psychological wellbeing when compared to other people who have never had braces.

"On the basis of our research if there are irregularities, and especially if they are not severe, then there will be no harm to dental health and it wouldn't change their life happiness in the future if they don't wear braces," said Professor William Shaw, an orthodontist at the University of Manchester in England.

Each year countless children around the globe receive orthodontic treatment to align their teeth or correct other irregularities.

Parents can spend a small fortune on their children's dental work. In Britain and other countries braces can cost 2,000 pounds ($3,915) or more depending on the severity of the problem.

Shaw and his team had assumed that there would be some social or psychological benefit for the children who had their teeth straightened in 1981. But when they gave them a battery of standard psychological questions 20 years later to assess their well-being, the researchers found nothing that seems to have been derived from having orthodontic treatment.

All the people questioned in the study published in the British Journal of Health Psychology had quite significant dental irregularities and had received braces free through Britain's state-funded National Health Service (NHS).

"It can be concluded that, although in general participants' self-esteem increased over the 20-year period, it was not as a result of receiving braces and didn't relate to whether an orthodontist treatment need existed in 1981," Shaw told Reuters.

Dr Pamela Kenealy, a psychologist at the University of Roehampton in London who worked on the study, said the importance of having the perfect smile changes as people age.

"Teeth are important to an individual's self-perception during adolescence, but by adulthood other factors have greater significance," she said in a statement. "So while it may make a minor contribution to an individual's perception of self-worth, orthodontics cannot be justified on psychological grounds alone."