Having a sister makes you happier and more optimistic, a new study shows, but the same is not true for having a brother.

Growing up with at least one girl in the family also makes people more able to cope with their problems, according to the study.

Daughters tie loved ones closer together and encourage them to communicate their emotions more effectively, the researchers believe.

Prof Tony Cassidy, from the University of Ulster, who carried out the study with researchers from De Montfort University in Leicester, said that having a sister helped to promote good mental health.

He said: “Sisters appear to encourage more open communication and cohesion in families. However, brothers seem to have the alternative effect. Emotional expression is fundamental to good psychological health and having sisters promotes this in families.”

Girls who had sisters also tended to be more independent and keen on achievement, according to the findings.

The effects were stronger among children from broken homes, suggesting that sisters might lean on each other more for support when their parents divorce.

Prof Cassidy added that the lowest scores were among boys who had only brothers.

“It could be that boys have a natural tendency not to talk about things,” he said. “With boys together it is about a conspiracy of silence not to talk. Girls tend to break that down.”

The study questioned 571 young adults, aged between 17 and 25, about the make-up of their families and their emotional well-being.

Only children tended to score in the middle range for happiness and optimism.

Liz Wright, the co-author of the study, said: “With only children we found that they had lots of strong communication outside of the home. It appears that they have as much social support as those with siblings, but it does not come from within the family.”