Growing up with a sister makes people more optimistic, more ambitious and better balanced, psychologists have found.

A study of 571 families comprised of brothers, sisters, a mixture of both and only children found that having a sister in the home led to siblings of either sex scoring more highly on a range of standard tests for good mental health.

They were found to be better at coping with setbacks, more highly motivated, had more friends and a better social life than those who grew up with just brothers.

The research was conducted by psychologists at De Montfort University and the University of Ulster and will be presented today at the British Psychological Society’s annual conference in Brighton.

Liz Wright, research fellow at De Montfort, said the study began after a previous project showed that girls with sisters appeared to suffer less distress when they encountered trouble in their lives.

“We wanted to see if the positive impact of sisters went further than just girls and found that it did. One of the most interesting findings was the impact of female siblings when parents split up,” she said.

“It seems their natural inclination was to express themselves, talk about the separation and encourage other family members to do so as well. It seems to help keep family relationships going. Their was markedly less distress in broken homes with a sister.”

Psychologists have long believed that “emotional expression” at times of upheaval, is fundamental to good psychological health.

“Sisters appear to encourage that. However, brothers seemed to have the opposite effect, perhaps discouraging others to talk.”

The tests used a standard scoring method and covered how much social support people felt they had, how much control over their lives they felt they had, levels of optimism, achievement motivation, or drive, and ability to cope with setbacks.

The researchers said the difference when a sister was in the home were “significant”. It may help to explain the success of the Williams sisters, who have coped with huge upheaval and pressure in their lives on the tennis circuit, and Sienna Miller, the actress, and her sister Savannah, the fashion designer. Sienna Miller bounced back swiftly after the end of her engagement to the actor Jude Law. She even went on stage the night the news broke.

The mental health of only children lay between those children with a sister and those with only brothers.

“It seems many only children had built up significant social support outside the home by the time they reached their late teens which helped them in a crisis and in other areas of life,” Ms Wright said.

The findings suggest parents with boys who separate should be aware their sons may struggle to come to terms with the family break up. They will also going to be used in the treatment of eating disorders in the future.

The next projects will be to identify more precisely what sisters are contributing to family life to make for the better outcomes.

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