Aged six: Flesh and Blood How can children from the same family be so different, why do some siblings have better relationships than others, and what are the consequences of being an only child?

It has long been believed that parents have the most significant influence on a child's life. But it now seems that our brothers and sisters might be even more important. After all, our relationship with our siblings is the longest most of us will ever have.

We measured how altruistic our children were with their siblings by giving them a cookie and their sibling a pretzel. Were they prepared to sacrifice something they loved for the sake of their brother or sister?

Temperament is more important than birth order, spacing or gender in determining the quality of our relationship with our siblings. To reveal our children's personalities, Dr Linda Blair asked our parents to favour one child over the other for five minutes to see how the child coped with differential treatment.

How siblings cooperate on a shared task reveals important differences in dominance and communication. We asked our children to build a Lego house together, one using red bricks and the other using yellow, to see who could pull together under pressure.

The children

James has an older sister, Bernie. They'd had an unsettled year because Carol's ex-boyfriend was back on the scene. This meant James spent more time at his grandmother's. Was James and Bernie's relationship strong enough to hold them together?

Competitive William has two older siblings, twins Alice and Charles. He'd made peace with his mother, but was now making Charles's life a misery.

The relationship was very physical and William had the upper hand. His parents didn't want to intervene, but was William's aggression having a more damaging effect on Charles than they'd imagined?

Het was spending most of her time on her own and her parents were worried they might have spoiled her because she found it hard to share with others. Her mother had invited her sister over from India to instil discipline.

The youngest of our children, twins Alex and Ivo, were about to start school after being held back a year. Their parents had to make a difficult decision about whether they should be in the same class. The boys had strong ideas of their own on the subject, so who would decide?

Matthew and his older brother, Robert are close in age. We showed that Matthew was quick to turn to Robert when he was in trouble. But how did Matthew react when Robert faced some serious problems of his own?

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