Iam astonished by how different a friend’s daughters are. While one is an extrovert, the other is reserved and withdrawn. While one is brimming over with self-confidence, the other is diffident and unsure of herself. While one is neat and fastidious, the other is abysmally untidy. While one is hardworking and ambitious, the other is totally laidback, content to take each day as it comes. And so it goes on, an endless list of differences. Only a family resemblance indicates that they are sisters. Yet these girls are not unusual. In family after family, siblings are seen to be totally different to each other in temperament, attitude and behaviour.

How does one explain this apparently strange phenomenon? After all, siblings have the same genes, the same parents and are raised in the same house.

The answer, according to at least one psychologist, is that siblings don’t have the same parents! Parents are at different stages in their lives when their different children are born, and very often, so is the state of their marriage. The start of parenthood has a radical effect on most women. Equally, the father reacts in his own way to the birth of a child. He may, for instance, be delighted by the new arrival or he may feel excluded. When the second baby arrives, the family has already been changed by the first. The first child may have brought the parents closer, or subjected them to new pressures that have pushed them apart. The second child may present them with new financial anxieties. All in all, the emotional contentment and the preoccupations of each parent changes from those present at the birth of the first child.

While the parental behaviour changes according to the order in which their children are born, the children also react to changed attitudes and their place in the family hierarchy. For example, a firstborn is quick to assert prior claims and attempt to dominate. Being bigger and stronger, this makes the child more self-assured. Younger siblings, anxious to carve out a niche for themselves in their parents’ affections, cultivate personality attributes different from their older brothers and sisters.

Children also respond to parental expectations. Firstborns, for instance, are required to act as the responsible ones in the family. In a continuing effort to live up to their parents’ ambitions, they become over-responsible, high achievers.

And what of genes? They play a part in the make up of an individual by contributing to appearance and conferring such emotions as humour, anger and sadness. But their role in determining behavioural differences between siblings is a different matter altogether. These are fashioned by upbringing and parents, who, perhaps unwittingly, treat their different children differently.