In families where there are at least two children, parents can expect to see sibling rivalry rear its competitive head from time to time. It's totally natural and to be expected, and if handled properly, it can have an entirely positive outcome for all concerned.
Michael Fitzgerald, who is Henry Marsh Professor of Child and Adolescent Psychiatry at Trinity College Dublin, says that sibling rivalry is inherent in the psyche and is manifested in children as young as two.
"Often it's the older child who feels displaced by the younger," he says. "In response he may attack or hit the younger sibling, or throw his toys aside."
Dr June Atherton, a Jungian psychologist who practises in Dublin, believes that sibling rivalry is usually triggered by one child feeling that another is getting more praise and attention.
"It can manifest in fighting or withdrawal," she says. "Both are ways of expressing pain."
As to whether rivalry is more common between sisters, between brothers, or sisters and brothers, it's not really an issue, according to Michael Fitzgerald.
"Personality is the most important factor," he says.
"Some are more aggressive and impulsive by nature. Some tend towards rivalry and competitiveness, others are more passive."
In her practice, June Atherton has come across parents who admit pitting one child against another to toughen them up.
"They actively encouraged rivalry in the mistaken belief that this would benefit them," she says.
Most of the time though, parents are doing all they can to try and keep the peace at home, and try to nurture a healthy, supportive relationship between their little ones. But it's difficult.
"Parents tend to favour those of their children who are most like themselves, and those who arrived after a long wait," explains June Atherton.
"This causes insecurity and jealousy in siblings, who expect to be treated fairly.
"It's easy for a parent to punish the child who makes trouble at home, but often that's the child who is in pain, and crying out for attention."
In her experience, rivalry is usually not an issue between siblings who have more than five years between them.
"Their interests are different so they're not in competition," she says. "It's when they're close in age, but not twins, that it can be tricky to manage. When there is a group of them close in age it gets trickier again."
Author Natasha Mac a'Bhaird, from Cabra in Dublin, says that sibling rivalry is not something that's apparent between her identical twin daughters, Rachel and Sarah (four).
"You'd imagine that there would be intense rivalry between twins, as they're in direct competition with each other for the same things, but ours are more likely to co-operate rather than compete" she says.
"Mostly, they oblige each other rather than clash, and there's no one-upmanship between them.
"Of course there's the occasional tussle if they both want the same toy at the same time, or if there's disagreement about whose turn it is, but that's only natural."
June Atherton isn't surprised by this, as she believes that sibling rivalry is rarely exhibited in identical twins.
"By their very nature they are mirror images of each other, so quite often a benign narcissism exists between them," she says.
"It's not normally an issue in non-identical twins either, as they usually share so many traits and interests that they don't tend to compete with one another.
"Parents should intervene sensitively when they see blatant signs of rivalry between their children. They should also ensure that what they perceive to be nothing more than normal sibling rivalry is not bullying," says Professor Fitzgerald. "It's quite common for parents to mistake the two.
"It's important to allow siblings to vent a natural competitiveness between themselves, as I've seen cases where entrenched, intense rivalry with an identical twin was suppressed in childhood, and was buried so deep that it was the slowest and biggest thing to emerge in therapy," he says.
Natasha says that she and her husband, Aidan, make an effort to have time alone with each of their daughters, and recognise the wonder of each child's individuality. This is exactly what parents are meant to do to help combat the rivalry among their children.
"Some try harder than others to recognise that each child has his or her individual strength," says June Atherton.
"For parents who don't understand this concept, it can be a help to consider how they might feel in a work situation if a colleague received more praise and recognition, while their contribution was not appreciated."
When does sibling rivalry end? "It stops at death," laughs Michael Fitzgerald.
But, he says, it does play a hugely positive role in a child's development.
"It is the catalyst for a massive drive of creativity in the person who feels peeved, one that propels him to strive to triumph over the sibling rival; to outperform him or her on the football pitch, in the classroom, and in later life, with academic and professional success."
Finally, Michael Fitzgerald has this advice for parents: "Be aware. Don't endlessly criticise your children. Don't create demons versus gods among them. Don't constantly call one child bold as this will lead him to believe that that is what he is.
"Treat them equally. Be sensitive and tolerant, and be sympathetic towards the child who is displaying signs of rivalry."
Natasha's latest book, Olanna's Big Day, is published by The O'Brien Press