Sisters can make their siblings and family members happy, while brothers are more likely to cause distress, psychologists have revealed.
Research discovered people who have at least one sister present while growing up tend to become more happy and balanced as adults. However, only having brothers can mean the effect is not so positive.
A total of 571 people, between the ages of 17 and 25, had their psychological wellbeing tested by experts. Some of the volunteers only had brothers or sisters and some had both.
It was found that people with at least one sister had higher scores on indexes which are considered key for good mental health.
Those with sisters appeared to have more optimism, better social support, and more positive coping, compared with people who only had brothers.
Professor Tony Cassidy, from the University of Ulster in Northern Ireland, 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. This is important information for those working with families and children."
His findings were presented at the British Psychological Society's annual meeting.