The announcement last week that the Duke and Duchess of Cambridge are expecting a third child is welcome news, although it means big changes for the children, as each takes on a new role - eldest, middle and youngest.
Does birth order position really matter and are there typical profiles of a firstborn, middle and youngest?
Although the brain isn't fully organised until we're adults, the most spectacular and dramatic neurological development occurs during our early years because of the brain's plasticity.
Furthermore, because most sibling pairs are born within a few years of one another and because many, if not most, of a baby's interactions involve members of their family, our siblings are instrumental in shaping our character.
When we think about birth order, a number of "typical" qualities spring to mind. For example, firstborns are generally thought of as organised, responsible, nurturing and eager to please.
This is because they quickly realise that these qualities, mainly in terms of helping parents with younger siblings, allow them to regain some of the attention they lose whenever a new baby is introduced. They usually do well in school - partly because they received so much exclusive parental attention as babies (particularly language input), and partly because parents prize academic achievement. They often worry more than their siblings. Again, this makes sense because babies pick up parental moods.
Middle children, growing up with siblings on either side of them, are generally the most socially skilled.
They learn how to get on with others and how to come up with compromises. Wisely unwilling to compete with an older sibling who prioritises academic achievement, they often develop talent in music, sport or drama. Inadvertently overlooked at times because of their birth order position, middles are more likely than other siblings to go through a phase of dressing bizarrely, or championing some overlooked group or cause with whom they may identify.
Youngest children are often considered the "babies" of the family even when grown up, so it feels natural for them to attract attention by being "cute" and charming.
By the time a third child is born, parents are usually more relaxed about discipline, so the youngest has more latitude and, as a result, takes more risks. They have to work hard to find a talent that sets them apart from their siblings and gains parental attention, so they're often the most creative.
Of course, these portraits are an over-simplification. Individual experiences, parental behaviour and values, and our unique genetic make-up also help shape us, as do gender distribution and number and spacing of children in the family. Birth order position is only one determinant of character - but, none the less, a very powerful force.
Linda Blair is a clinical psychologist and author of Birth Order: What Your Position in the Family Really Tells You About Your Character.