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Show Me the Science with Luke O'Neill

The Science of Sibling Rivalry

07 May 2026

Transcription

Transcript generated automatically by AI and may contain errors.

Chapter 1: What is the main topic discussed in this episode?

1.297 - 3.66 Unknown

Show Me the Science with Professor Luke O'Neill.

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4.441 - 14.956 Professor Luke O'Neill

Hello, Luke O'Neill here and welcome to Show Me the Science, my weekly podcast. And I try to pick stuff that interests me and hopefully will interest you. And there's sometimes an immunology twist because I'm an immunologist.

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15.097 - 31.343 Professor Luke O'Neill

And this week, there's a great one if you're an immunologist because it's all about the science of siblings and does where you come in the family affect your future prospects? And guess what? It might be tied to infections is what they found in this really interesting study. Now, of course, we're all fascinated by this.

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31.603 - 48.675 Professor Luke O'Neill

Like if you're the eldest or the youngest or in the middle or if you're an only child and what you might end up doing for a career or maybe your personality. And of course, there's all this bickering between siblings, although we love each other really, don't we? But it's really interesting statistically because it's possible to look at this question.

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48.695 - 65.164 Professor Luke O'Neill

And the scientific question we're asking is, does where you come in the family matter? The birth order, we call this, whether you're eldest or youngest or whatever it is. And it's easy to look at in a way because you can say, OK, let's take a middle child. What are the chances of that middle child becoming, say, a politician or whatever?

Chapter 2: How does birth order influence career choices and personality?

65.184 - 82.066 Professor Luke O'Neill

And they sometimes look at professions differently. They look at personality type as well, like is the eldest going to be more dominant or whatever it is. And then you can look at millions of people and then come to an average. Now, again, importantly, there's no hard and fast rules here. You can have the youngest sometimes being like the eldest in certain families.

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82.267 - 96.931 Professor Luke O'Neill

So you're looking for an average response here. And what's really interesting is there are these trends emerge when you look at all these big numbers. And in science, the bigger the numbers, the better. So the more people you look at, the more likely it is your bottom line will be true in terms of the average.

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97.332 - 109.924 Professor Luke O'Neill

And it had been known for a while, a big one, first of all, to do with performance and education. And you can look at that. You can look at A-level scores, for instance. And the eldest is inclined to do better at A-levels. That was a UK study.

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109.904 - 130.857 Professor Luke O'Neill

Another UK study looked at IQ measurements and again the eldest was inclined to have a higher IQ score than a younger sibling and that seems to stand up statistically. So for some reason the older one seems to do better in education and then has a slightly higher IQ. the middle one and the younger one less, right? And that was interesting anyway.

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130.897 - 151.892 Professor Luke O'Neill

And in fact, that was the very question that these scientists wondered about. Why would that be? And they were scientists in Denmark and they examined this closely and they noticed that because of this sort of educational advantage is part of it, the eldest child or the older child is inclined to earn more money in their life than the next child. And it's 1.9%.

151.872 - 161.027 Professor Luke O'Neill

Now, what that means is over your lifetime, if you're the older sibling, you will earn 1.9% more than the next child, the second child in the order.

Chapter 3: What statistical trends emerge regarding sibling rivalry?

161.047 - 178.335 Professor Luke O'Neill

Then you looked at the elder one and younger one. And in Ireland, for example, the average industrial wage is €52,000 a year. The eldest will earn about €1,000 more per year than the younger one. And the Danish scientists had seen this, and they wondered why that was, and they came up with a hypothesis.

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178.395 - 194.44 Professor Luke O'Neill

Now, if you're a scientist, you have to come up with what are called hypotheses, which means an idea. And their idea was maybe it's to do with infections. Now, you might wonder why they would have hypothesized that. It had been known that the second child is more likely to end up in hospital with a respiratory disease than the older child.

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194.5 - 214.551 Professor Luke O'Neill

And again, that's statistical, and this is a really robust piece of data. So if you look at the rate of hospitalization, from respiratory diseases like flu, RSV, infects infants and younger children, the younger child is more likely to end up in hospital than the older child. And there are studies showing three times more likely. Now, that's a big difference.

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214.571 - 231.772 Professor Luke O'Neill

And the Danish guys knew this and they wondered, was that part of it? And they dug in a bit more deeply. Why would the second child be more likely to end up with a serious infection than the older child. And of course, if you get a serious infection, it means you're out of school or you're in hospital and you're being treated differently as an infant because you're sick.

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232.433 - 251.615 Professor Luke O'Neill

And the question then was, would that affect them? They also are able to, again, a bit of speculation, I suppose, but they know, we know that if you have an infection, you get this thing, inflammation, that I often talk about. Your body's under stress and they've hypothesized that that inflammation diverts nutrients away from the brain. Because you're trying to fight the infection.

251.956 - 270.697 Professor Luke O'Neill

And therefore, maybe the brain is slightly different. And again, that's a bit of speculation. But it's not unreasonable to speculate that if you have an infection, something happens and maybe your brain develops slightly differently. And as a result, then you don't do quite so well in exams. And that might translate into poorer job prospects down the line is the idea.

270.857 - 289.318 Professor Luke O'Neill

So it's a complicated enough kind of notion, I suppose. But I do like this idea that the second child is more likely to get infections. And that could well then mean that certain things happen that mean they don't do as well in education. And then they end up in a slightly worse job. Now, the evidence again got even more compelling. Why would the second child get more infections?

289.438 - 307.516 Professor Luke O'Neill

Well, it's very simple. The older child goes to school or goes to the creche, brings back the infection that they've picked up with the other children. And the infant or the young child, the second child then, is more likely to be infected because they've got a weaker immune system. And this is well known. When you're a baby, your immune system hasn't fully developed.

307.676 - 324.196 Professor Luke O'Neill

So if your older brother comes back from the school with an infection, you're more likely to pick it up and get really sick. So that's the idea. Whereas the older child didn't have that exposure. They didn't have an older sibling to bring back the infection. And that's why the risk of serious infections is higher. in the second child than in the first.

Chapter 4: How does being the eldest child affect educational outcomes?

374.821 - 388.375 Professor Luke O'Neill

The second thing they talk about that's really interesting is very often the older child will teach the second child something. It can be anything, but very often they show them how to play a game or whatever it might be. And that gives the older child an advantage because there's a great line about teaching.

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388.636 - 404.199 Professor Luke O'Neill

In fact, there was Richard Feynman, the physicist, who said, if you want to master something, teach it. So the older child is learning to master things by teaching the younger child. That gives them an advantage as well. So again, these are other reasons why this would be. And you can see now we're all fascinated by this, aren't we?

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404.74 - 420.506 Professor Luke O'Neill

And then another one, the older one is in plenty more ambitious, maybe because the parents are pushing them. And they're 7% more likely to go to university than the second child. And that just shows you maybe there's a bit of a push on from the parents there. So that's another factor, the parents being a bit more ambitious for the first child than the second one.

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421.007 - 437.671 Professor Luke O'Neill

And then, of course, what this all means is vaccinate the child. Because if you vaccinate the children, they're less likely to pick up these infections to give them a disadvantage. So yet again, it's yet another reason to vaccinate, not just to stop the child getting sick. It can help them in their lives later on. It's a remarkable part of that.

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438.092 - 457.56 Professor Luke O'Neill

Now, the thing to say is some infections are good, remember. Don't be keeping children too clean. Because if you keep them too clean, their immune systems aren't being educated. A bit of dirt is good. It's more the severe end that we worry about, hence the need for vaccines for these things like RSV. Great breakthrough with vaccines there, actually. And flu for children, very important.

457.58 - 477.32 Professor Luke O'Neill

But it's okay to get infections now and again because you need them. Now, the last thing I want to tell you about that I found really fascinating is the types of jobs these children do in different order. The eldest child is more likely to be a leader. Now, what does that mean? Senior role in management, maybe in politics, maybe the military. They've shown this even, right?

477.34 - 495.888 Professor Luke O'Neill

So the eldest is inclined to become the captain or whatever it is. Then the second or third child... And of course, leaders earn more than the grunts, don't they? You know, senior people in management earn more. And that might be another factor here for the earning aspect to this. The middle child, interestingly, is more inclined to be sort of a mediator.

496.469 - 513.173 Professor Luke O'Neill

They've had to learn how to mediate between the older child and the younger child. And they go into more mediating type jobs is the way to think of it, you know. And social awareness is better in them, better negotiation skills. And that translates into the jobs they do. And then the youngest, guess what? They're inclined to be creative.

513.774 - 533.554 Professor Luke O'Neill

There's a correlation with, say, the third child or the youngest child getting into the arts. You know, they're more creative. They are inclined to be entrepreneurs and risk takers. And that's because the parents have kind of the less strict and they get a bit more freedom and they're inclined to become much more creative. And we all know being in the creative arts is notoriously badly paid.

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