There's all kinds of stereotypes about what your position in your family say about you. Oldest kids are the most responsible, the ones in the middle are rebels, and, since I'm the youngest, I must be spoiled. And sorry only children, according to society, you're selfish.

It's called Birth Order Theory, and it might sound like an appealing way to explain why your older brother is so overbearing and competitive, or why your older sister always has to get what she wants.

But even though psychologists have used birth order theory to explain personality traits for decades, recent studies are finding that it doesn't actually seem to matter where you are in your family.

The theory dates way back to 1908, the early days of psychology when it was proposed by Austrian psychologist Alfred Adler, a buddy of famous psychoanalysts, Freud and Yung.

All three of them agreed that experiences a person had as a kid could have big impacts on their personality as an adult.

Adler took it one step further and argued that a child's position in their family would have a big impact on their development, because each kid would experience a different psychological environment.

In his writings he theorized that the first born child starts out as the spoiled only child, and is basically dethroned when the next sibling comes along. After getting bumped, the first child ends up overbearing and neurotic, since they feel responsible for looking after their younger siblings. 

Middle born children, on the other hand, they're likely to feel overlooked, and become the "rebels" of the family. 

And the youngest children might be babied by their parents. So, according to Adler, they typically end up the most spoiled.

Adler never used any scientific research to justify his theory, but he used it as a therapeutic tool when working with his clients anyway.

As the idea became more popular, psychologists realized they should probably do some actual research to find out if birth order theory was actually true. 

Personality is a complex thing to try and quantify, and it can change over time. But scientists have  figured out a way to sort of measure personality, using what's known as the five factor model, or just the Big Five.

It's not so much a theory of personality as it is a way to categorize for traits on sliding scales. 

People can take a test to see where they fall on the scales of five traits. Openness to experience, conscientiousness, extraversion, agreeableness and neuroticism.

Based on a hand full of early studies, some psychologists are pretty convinced that you can connect birth order to specific Big Five traits.

Like, oldest children are more conscientious and neurotic, while youngest children are more open to new experiences.

And this approach is really appealing because it sounds totally plausible, right? Like, I have an older brother who is pretty neurotic.

But those earlier studies have a lot of flaws, and the more recent research, with bigger and better designed studies, hasn't found much of a connection between birth order and Big Five personality traits.

One of the problems with those smaller studies was that they often depended on one sibling self-reporting their personalities, and those of their siblings. Like what I just did.

So if the researches were interviewing an oldest child, for example, the subject was more likely to report that they were more responsible, while their free-spirited little brother was more open.

But the difference between the way someone sees themselves and how they see someone else can really affect the result of the survey.

These studies had other flaws too. For instance, they mostly looked at the differences between siblings within the same family, and they interviewed everybody around the same time, which meant that the people born first were always older than the siblings they were being compared to. 

So, if the researchers found that first-borns tend to be more responsible, was that because they were born first or because people just tend to get more responsible as they get older.

So, even though, some of those early studies did seem to find a link between birth order and personality, their flaws meant that the results weren't super reliable.

Now, more recently, researchers have been using much bigger data sets to analyze differences in large groups of kids from multiple western countries.

And it turns out that once you start comparing traits across families and looking at more subjects, the effects tend to go away.

In two huge studies published in 2015, scientists were able to compare tons of different kids across lots of different families.

One of the studies looked at almost 400,000 US high school students, and the other included over 20,000 students from the US, UK and Germany. 

The one difference they found, and if you are an older sibling, I expect you to hold this over your younger siblings, was a small change in intellect, which is considered as one of the sub-traits of openness in the Big Five.

Siblings born first tend to have a very slightly higher IQ than siblings born later. We're talking like, a couple of points. 

It could come from first-born kids getting a little bit more direct attention from their parents at an early age, but as we talked about before here on Scishow Psych, IQ is just a way to measure learning ability.  And that's only one part of the complex concept of intelligence, and it can be affected by lots of different things, like how motivated you are to do well ont the test.

And other studies looking at big sets data haven't found an effect of birth order on IQ at all.

Other than that, these gigantic studies found no significant effect of birth order on any of the other Big Five traits.

So, most psychologists treat birth order theory with a healthy amount of skepticism these days, although it hasn't been totally disregarded.

Birth order might be an appealing way to explain differences in the behaviour in siblings, but when you look across large groups of families, the effects just aren't here. You might know individual people who fit the stereotype of an oldest sibling who's more responsible, or a youngest who seems spoiled. But there are also plenty of responsible youngest children, or oldest children who act like total brats.

Over time and across population, those traits balance put.

So birth order theory doesn't actually explain why your middle sister ran off to start a rock band. That's just cause she's awesome.

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