Now, Julia, I'm sure plenty of people listening right now are going to be like, hold on, no way, that's not true. My older sister is like this, my younger brother's like that. But I will say there have been other big studies as well as yours, including one of almost 400,000 American high school students that found the same as you, that there isn't this link between birth order and personality, except for this tiny effect on intelligence. Of course, one intelligence is, and now it's defined as a whole other issue for another podcast. But I am curious of their theories as to why this effect is seen in the data.

There are multiple explanations for that that are all quite plausible. So one obvious difference is that when there is just one child around, the parents can give like full attention and provide more cognitive stimulation and so on. Another one that I kind of really like because it's a positive twist on things is the idea that actually first-borns get a boost because of their smaller siblings, because they become a teacher for them and explain things to them and so on. And explaining things to somebody else is a lot more challenging than just understanding things. And then last but not least, it could actually be also differences in education. So maybe because of expectations and because of their role, first-borns do end up studying more challenging degrees. And we do know that education also affects intelligence. So we have all sorts of plausible explanations and it could very well be that all of these contribute a bit to the tiny mean level difference that we observe.

So, Yulia, we can kind of put aside personality traits. But I wonder, does birth order play any role in our life outcomes? So

there are some very interesting findings from Sweden that shows that there does seem to be an association between birth order position and people's careers. First-borns are more likely to become managers and are more likely to go into engineering or medicine, whereas younger ones are more likely to study, for example, journalism or go-to art school. I think that again points to that idea that there might be different roles that might actually have to do with how you grew up with your family and so on that maybe determines that you become the doctor and the younger sibling can do whatever they want without necessarily actually affecting people's personality.

So that's more of a kind of social-cultural thing going on. You know, there are expectations placed on the first-born that might not be there for the second-born.

Yes, I very much assume that it's like social-cultural and I also assume that that means that it won't necessarily generalize to other contexts. So we did conduct a study in Indonesia where we were actually not able to confirm the intelligence effect that we have found in so many western samples and that might actually be because sometimes first-born are taken out of school so that they can start working earlier and provide for later-borns. So then maybe the second child gets the boost and so on. So this is all within the historic context and it needn't necessarily be universal laws in any way. So

are there any questions around the impact of siblings and the role siblings have in your life that you're still investigating or that you're interested in

looking at? One thing that I find more interesting these days are gender dynamics. So for example, does it make a difference for a girl if she grows up with a brother as opposed to with a sister? There again, we do find that it does seem to make a difference for the later life course. In economics, there's something called the brother earnings penalty and that is the observation that women who have a brother as opposed to a sister earn less later in life. So it does seem like there's something going on with respect to gender family dynamics of the type that if there is a brother around, then maybe the girl gets into a more stereotypically female role which also affects what she's going to work later in life and whether she's going to stay in the workforce once she has children and so on. But again, it looks like it's not really about personality, it's more about roles in general.

Yulia, the fact that studies like yours have proved quite conclusively now that there just isn't any link between personality traits and birth order apart from this very small boost in IQ that you've talked about. Why do you think this concept just resonates so strongly with people and it's still so persistent? I believe

that people are just super interested in understanding why they are the way they are and siblings and child who plays a big role in that right? We all believe that this is like the most important time where our personality is formed.