For many of us, sibling relationships have a profound effect on our lives – whether we love our brothers and sisters or find them almost impossible to get along with. Over 80% of human beings have at least one sibling, and our sibling relationships are very often the longest lasting relationships that we experience, so the potential for influence is huge. Sibling relationships involve high familiarity, and are emotionally uninhibited, and so often very intense – which can bring both problems and benefits to brothers and sisters.

As one seven-year-old complained to me recently, about his sister, "She's awfully good at annoying me… She knows exactly what to do to make me cross, and I think she does it on purpose." When I asked him what was good about having a sister, he thought for a while and then commented thoughtfully, "Well, she's good at helping me when I can't do stuff, and she does understand how boring Mum and Dad are sometimes"(!)

What this little boy's comments capture nicely is the dual nature of many sibling relationships. There can be rivalry and competition, especially for parental attention, but at the same time, brothers and sisters can offer each other support, love, friendship and understanding.

Our relationships with our brothers and sisters are generally unlike the relationships we have with our parents or friends – especially when we are children. Parent-child relationships are unequal in terms of power, and involve complementary roles. Friendships are more equal, and involve more reciprocal roles.

Sibling relationships fall somewhere in between – there's usually some difference in power, because one child is older than the other, but there is similarity (both being children in the same family) and a more reciprocal relationship too. One thing that is very clear is that wide individual differences exist among sibling relationships.

What happens when a sibling arrives?
Professor Judy Dunn's extensive work on sibling relationships has shown that reactions to a new sibling can be variable and sometimes ambivalent. Children's reactions reflect their own temperamental traits, as well as their age, and the way their parents behave. Most research on this topic has focused on first-born children reacting to the arrival of a new baby brother or sister. It's common for the older child to show jealousy and be clingy or deliberately naughty when the new baby arrives, although not all do this. For some it takes a while to realise that their new brother or sister is a permanent fixture, and it is only later that their behaviour reflects the major change they have just experienced in their lives. Positive responses occur too though! Children may show affection and concern for their new sibling, and by the time the new brother or sister is a year old, siblings have often developed a meaningful relationship, playing and spending time together.

Interactions between siblings
Children can't choose their siblings, but are simply faced with living with them, whether they get along or not. Sibling relationships develop soon after the birth of a new sibling, and are fairly stable in nature throughout childhood. They may be close or characterised by conflict, but either way, provide a huge source of interaction and opportunity for learning. Older siblings in particular provide several very useful sources of support for their little brothers and sisters – the younger child is treated to a ready source of companionship, help and learning. The older child gains the chance to develop responsibility, and learns to share and protect others. Both are likely to get a very intense initiation into the human social world of needs, intentions and feelings.

Because sibling interactions are frequent, uninhibited and intense, they offer an unrivalled context in which to develop and hone social skills – from comforting, sharing and co-operating, to deceiving, manipulating and arguing. Children with more siblings have been reported to develop an understanding of the link between what we think and how we behave in advance of those with fewer siblings. Those with positive sibling relationships tend to display greater moral maturity and more positive peer interactions. Siblings can also provide strong emotional support for each other, and they often grow closer in the face of difficult situations like parental divorce or remarriage.

Conflict between siblings is almost inevitable, unless the age gap between the children is very large. Parents may find this stressful, but again, there are real advantages for children in terms of their social development. Unlike your friends, your siblings are stuck with you, so children in conflict with their siblings have an opportunity to learn about the causes and mechanics of conflict, along with how to manage it and (hopefully!) how to negotiate solutions. As a whole, same-sex siblings tend to get into more conflict and show more aggression than opposite-sex siblings, although it's worth noting that same-sex siblings can also be exceptionally close to one another.

Sibling situations
One of the problems researchers face when studying sibling relationships today is the variety of siblings that they may encounter. Full siblings, half-siblings, step-siblings, twin siblings, foster-siblings and adoptive siblings all exist, and many children have several different types of sibling. Family context can also be important in determining the nature of a sibling relationship. Differential parental treatment is one example of this, but other factors are also important, such as having a disabled versus non-disabled sibling, family structure (step versus biological parents), and how many other siblings a child has.

Parents often wonder about the effects of age gap and birth order on sibling relationships. A very large age gap may lead to limited relationships between siblings in childhood. It has also been suggested that an age gap of two to four years tends to produce more conflict and rivalry, because siblings this close have especially similar lives. However, research to date indicates that how well – or how badly – siblings get along is actually more to do with the children's individual temperaments, and how well these do, or don't, fit together, than it is to do with the age gap between them.

Research into the effects of birth order has produced similar findings. Birth order often matters a lot to children, and there is evidence that suggests that birth order has an effect on us as individuals. However, as far as relationships between brothers and sisters go, the bottom line again appears to be that children's actual personalities are more important than their birth order.

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