Girls are top! That is the conclusion of yet another study, this time by US researchers at Brigham Young University in Utah, that purports to show that having loving siblings of either gender - but especially girls - is the best protection against adolescent unhappiness.

No matter whether the sister is younger or older, or the age difference between you, they increase well-being and even your penchant for doing good deeds in the world - above and beyond that which even loving parents manage to promote.

The report follows a study involving 571 British families by De Montfort University, Leicester and the University of Ulster published last year, which showed that siblings who lived with their sisters scored higher on the standard range of tests for good mental health.

The project was prompted by an investigation that showed girls with sisters suffered less distress when they encountered trouble later in their lives than those without. Researchers wanted to find out how extensive the effects of sisterhood could be. Looking back on the complexities of my own sororal relationship makes me glad that I am not a sociologist charged with unpicking the multiple strands of joy and woe with which such bonds are made. My first memory of my sister is of a shattered peace.

For three years it had been just me, mum and dad. But then they started hankering after Kipling's "family square" and one day brought home a red-faced, squalling bundle that changed everything, and not for the better.

It is notable that the studies locate the benefits of sisterhood as arising in older childhood - once the initial sibling rivalry and smarting pain of the knowledge that from now on there will always be someone younger and cuter than you around to grab the attention has worn off.

Because wear off it does, and it did. Most children bond in the face of what is interpreted as - wholly correctly in our case, I will insist until my last breath - increasing parental unreasonableness. I vividly remember our first overt alliance, when we united in the face of our mother's strictures about not having a drink with soup ("Soup's a meal and a drink!") to maintain that we were both dangerously dehydrated and required two glasses of water instantly. We didn't get them, but it was a beautiful moment, that first exercise of mutual support, and gave us something to build on as we became thirsty for more heady freedoms in the years to come.

Most of the reports suggest that it is girls' greater capacity for emotional expression that provides most of the benefits to the family that has them. As Tony Cassidy, of the 2009 study from the University of Ulster, puts it: "Where there are a number of boys together, there is almost a conspiracy of silence."

Of course, this is to speak in generalisations, but when I think of how differently I talk to my closest male friends and female friends, I understand how difficult it would be to replicate with a brother my almost daily communications with my sister. We have a shorthand and an understanding that arises not just from our shared history, but also from our shared gender. When I say, "I lay in bed this morning crippled by a nameless sense of guilt and couldn't rise until I had allocated some of it to specific tasks left undone and worked out how I was going to punish myself for the undefined rest", she doesn't try to have me see a shrink, as a man might (and my husband wishes to); she comprehends instantly and absolutely.

Of course, it would be better if I had a sister who wasn't taller, blonder, cleverer and had better boobs than mine, but you can't have everything. Sometimes love and understanding just has to be enough.

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