The question of whether siblings' birth order shapes their personality has puzzled families and psychologists for years. But the evidence isn't as straightforward as you might think.

As the eldest daughter of two, I often identify with the traits stereotypically associated with being the oldest sibling: responsible, conscientious, a perfectionist. My mum is an eldest daughter, too, and also shares those traits. My younger sister, on the other hand, is a bit more carefree – even though she and I grew up in the same household with the same parents, and are close, our personalities are quite different.

I wondered whether that difference could be due to our birth order – is there any science to the idea that being the oldest, the youngest, or an only child, shapes who we are, and how we navigate the world?

Despite fascinating the scientific community and public for more than 100 years, the question of whether our birth order amongst siblings shapes our personalities is very much still up for debate.

Historically, research has produced inconsistent findings. There are several reasons why this is, though to put it simply: it's hard to measure. Rodica Damian, an associate professor in psychology at the University of Houston, Texas, in the US, explains that previous studies have often included small sample sizes. In addition, since personality tests are often self-reported, they may be affected by bias.

"The Big Five"

The five-factor model of personality is used to determine general personality structure. Psychologists researching birth order measure siblings on the following traits:

• Extraversion (or introversion)

• Agreeableness

• Conscientiousness (or intelligence)

• Neuroticism (or emotional stability)

• Openness to experience

Recent studies point out that a number of confounding variables can make it hard to investigate if birth order is systematic, meaning that it affects every person in the same way. The total number of siblings may be a factor, for example: one would expect the overall dynamics to be different in a family with two siblings compared to a family with seven siblings. Being the eldest or youngest child in these differently-sized families might be a very different experience, and not directly comparable.

Family size and the experience of being a child in any given family may in turn be entangled with many other factors, such as a family's socioeconomic status (wealthier families with higher socioeconomic status tend to have fewer children, for example). And then there is a person's age and gender, which could influence their experience within the family and beyond.

Within this context, researchers have not been able to conclude that birth order has any consistent, universal impacts on our personalities. But that doesn't mean birth order is irrelevant. It could play a role within certain families, or cultures.

"I think people have a lot of beliefs that are kind of outdated, or that were never well supported in the first place," says Julia Rohrer, a personality researcher at Leipzig University in Germany. "For example, the 'eldest daughter syndrome' thing is a big one – of course, women often still have different roles and are expected to provide more care. And then, first-borns are expected to take care of younger siblings," she says. "For some women, this might perfectly match their experience but for others it doesn't because every family is different." In other words, not every eldest daughter will be responsible and caring – but for some, the idea of an "eldest daughter syndrome" may ring true because they really did grow up having to look after their younger siblings and feel that this experience shaped them.

Rohrer and her colleagues have found that birth order "does not have a lasting effect on broad personality traits" after examining three........

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