The older girl was smart, neat and perfectly behaved in school; in her spare time, she won dance trophies. At every check-up, her mother would tell me what a good girl she was.

She is the oldest, her mother would say, so she gets lots of attention, and she works very hard. When her younger sister turned out to be an equally good student, the proud mother explained that naturally she wanted to be just like her older sister.

Then a long-looked-for baby boy was born. When he was a toddler, I began to worry that his speech seemed a little slow in coming.

His mother was perfectly calm about it. He is the only boy, she said, so he gets lots of attention, and he doesn’t have to work very hard.

Everyone takes it personally when it comes to birth order. After all, everyone is an oldest or a middle or a youngest or an only child, and even as adults we revert almost inevitably to a joke or resentment or rivalry that we have never quite outgrown. Children and parents alike are profoundly affected by the constellations of siblings; it is said that no two children grow up in the same family, because each sibling’s experience is so different.

But that doesn’t mean the effects of birth order are as clear or straightforward as we sometimes make them sound.

Indeed, birth order can be used to explain every trait and its precise opposite. I’m competitive, driven — typical oldest child! My brother, two years younger, is even more competitive, more driven - typical second child, always trying to catch up!

... contd.