Sisters 'make people happy'
Sisters "make families more open and willing to talk about problems"
Sisters spread happiness while brothers breed distress, experts believe.
Researchers quizzed 571 people aged 17 to 25 about their lives and found those who grew up with sisters were more likely to be happy and balanced.
The University of Ulster said having daughters made a family more open and willing to discuss feelings.
They said the influence of girls was particularly important after distressing family events such as marital break-ups.
The findings are due to be presented at the British Psychological Society in Brighton on Thursday.
During the study, participants filled in psychological questionnaires which researchers used to assess a range of issues, including whether they had a positive outlook and any mental health problems.
Emotional expression is fundamental to good psychological health and having sisters promotes this in families
Professor Tony Cassidy, lead researcher
Lead researcher Professor Tony Cassidy said: "Sisters appear to encourage more open communication and cohesion in families.
"However, brothers seemed to have the alternative effect.
"Emotional expression is fundamental to good psychological health and having sisters promotes this in families."
He said many of the participants had been brought up in families where parents had split and the impact of sisters was even more marked in these circumstances.
"I think these findings could be used by people offering support to families and children during distressing times.
"We may have to think carefully about the way we deal with families with lots of boys."
Geri Burnikell, co-ordinator of the charity Support Line, which offers counselling to young people and families, said: "This is very interesting and certainly chimes with our experiences.
"Boys tend to internalise problems and in families where there are lots of sons, I can see that can cause problems.
"I think the most important thing in these circumstances is to give people someone independent to talk to outside the immediate family unit."