STEVE INSKEEP, host:
It's MORNING EDITION from NPR News. Good morning. I'm Steve Inskeep.
Now, all through this Thanksgiving week, we're going to report on your siblings - or maybe not precisely your siblings, but those with brothers and sisters may recognize something in the stories to come.
Today in Your Health, we'll look at a mystery of siblings. Many people wonder how, with the same parents and the same family, siblings often seem so different. NPR's Alix Spiegel introduces us to two brothers who say they share almost nothing, except love and a last name.
ALIX SPIEGEL: The fight happened a long time ago, when they were still in school. But for both Tom and Eric Hoebbel, the fight was a defining event - the kind of family story they trot out for new acquaintances because it conveys something important about who they are. Eric Hoebbel begins.
Mr. ERIC HOEBBEL: He had come home from college. We were at home and we were just having one of these, you know, eleven o'clock at night, twelve o'clock at night conversations.
SPIEGEL: They talked sports. They talked school. Then the conversation turned to money. Tom's position was that money was inconsequential.
Mr. TOM HOEBBEL: I just said it wasn't that important. I said I could just, you know, take out the dollar bill and just burn it and that wouldn't really matter. And he said, but you wouldn't do that.
SPIEGEL: To Eric the idea of burning money was just stupid.
Mr. ERIC HOEBBEL: You know, a dollar bill is very valuable. Even if it's only a one dollar bill you can do stuff with it. And he said, no, it's not work anything at all. It's just paper.
SPIEGEL: And to prove his point Tom offered a demonstration.
Mr. TOM HOEBBEL: I took out a dollar bill in my mom's kitchen and lit it on fire and burned it. And he literally kind of freaked out.
SPIEGEL: Eric Hoebbel.
Mr. ERIC HOEBBEL: I'm just, you know, freaking out. I was probably being held back.
SPIEGEL: So there they were - two brothers of roughly the same height and weight, with the same hair color and the same last name. But as they looked across the table from one another, Tom Hoebbel says, what they saw was unrecognizable.
Mr. TOM HOEBBEL: I think he couldn't conceive why I would possibly do that, just like I couldn't conceive why it was such a big deal to him.
SPIEGEL: Today, Tom and Eric Hoebbel are middle-aged men. And their differences have only expanded. Tom's an artist. Eric's a financial planner. Tom's a rebel. Eric's a joiner. Then there's the wardrobe issue. Tom says his formalwear is limited.
Mr. TOM HOEBBEL: You know, I have one tie. I wear it when I have to. When I need to do something that's, you know, suit and tie oriented.
SPIEGEL: Then there's Eric.
How many ties do you own?
Mr. ERIC HOEBBEL: How many ties?
Mr. ERIC HOEBBEL: Oh, 150.
SPIEGEL: Tom and Eric Hoebbel are an extreme example of a common occurrence. Many siblings have very different personalities. But it wasn't until the 1980s, when a researcher named Robert Plomin published a famous paper, that the extent and importance of those differences were understood.
In the paper, Plomin reviewed the three main ways that psychologists had studied siblings up until that point. They'd looked at personality, intelligence and physical characteristics. And Plomin says that in terms of physical characteristics, siblings are very similar.
Mr. ROBERT PLOMIN (Psychologist): When you look at siblings and height and weight, they differ somewhat, but they're a lot more similar, on average, than they are to children picked at random from the population.
SPIEGEL: Plomin says, on average, cognitive abilities are also really similar.
Mr. PLOMIN: The surprise is when you turn to personality.
SPIEGEL: Turns out that on personality tests, tests which measure stuff like how extroverted you are, siblings are practically like strangers.
Mr. PLOMIN: Children in the same family are more similar than children taken from random from the population, but not much more.
SPIEGEL: In fact, in terms of personality, we are similar to our siblings less than 20 percent of the time. Siblings shared genes, homes, routines, parents. How could that possibly be?
Mr. PLOMIN: It was a real puzzle. What is it that makes children in the same family different?
SPIEGEL: To understand, Plomin and others did study after study, trying to tease out what role genes played in the personality differences they saw, and what role the environment played in those differences.
And when they began they assumed, like everybody else, that being raised in the same environment would be one of the things that made children similar. But that's not what they found.
Mr. PLOMIN: The environment works in a very odd way. It's making two children in the same family different from one another, not similar to one another, different.
SPIEGEL: The question was why. Why is it that being raised in the same family pushes children in opposite directions? There are three major theories. The first is a view popularized by a Darwin scholar named Frank Sulloway. In Sulloway's view, in a family, the main thing that is happening is that children are competing - competing for the love, time and attention of their parents.
Dr. FRANK SULLOWAY: And when organisms compete there tends to be a phenomenon that Darwin long ago identified in the "Origin of Species," called the principle of divergence. The role of divergence is basically to minimize competition so it is not direct. And that leads to specialization in different niches.
SPIEGEL: So if one child in a family seems to excel at academics, to avoid direct competition, the other child - consciously or unconsciously - will specialize in a different area, socializing say.
Sulloway says he saw a very small version of this happen in his own family.
Dr. SULLOWAY: My older brother was quite good in tennis and actually became a professional tennis player. I simply was never going to be as good in tennis as he was. And in the course of my high school experience, I discovered I was much better in track than I was in tennis. So I quit the tennis team.
SPIEGEL: Now the second theory might initially sound a little funky. It's this: though from the outside it appears that we are growing up in the same family as our siblings, in very important ways we really aren't.
Susan McHale is a sibling researcher at Penn State University.
Professor SUSAN MCHALE (Researcher, Pennsylvania State University): Children grow up in different families because most siblings differ in age, and so the timing with which you go through your family - parent loses a job, parents get divorced, you know, you're three or five years behind your sibling - the experience of a five-year-old whose parents divorce is very different from the experiences of a nine-year-old or a 10-year-old.
SPIEGEL: Also, McHale says, children in the same family are rarely treated the same by their parents.
Prof. MCHALE: Children have different needs. They have different interests. They have different personalities that are eliciting different treatment from parents.
SPIEGEL: The third and final theory, is that basically, families comparison machines - comparison machines that greatly exaggerate even minor differences between siblings. Susan McHale.
Prof. MCHALE: So you have two children who are born in a family and one of those children is incredibly extroverted, and the other one is very sociable. And, you know, in another context this would be a sociable child. But in this family, she's the introvert.
SPIEGEL: And once a child sees him or herself as an introvert, it influences the choices that they make.
Prof. MCHALE: And so we pick different groups of friends, we spend our time in different ways that only reinforces what may have been a very small difference to begin with.
SPIEGEL: When asked the brothers, Tom and Eric Hoebbel, why they thought they were different, both were clear. In their case, it wasn't Darwinian. Eric, especially, was convinced he didn't gravitate toward the path he was on to be different from his brother.
Mr. ERIC HOEBBEL: No, I don't think it was a reaction to him at all.
SPIEGEL: Tom agrees.
Mr. TOM HOEBBEL: Yeah. I don't really see that.
SPIEGEL: Instead, they point to the death of their father. When their father died, Tom was 17 and heading off to college - but Eric was only 12. So, in a sense, they grew up in different homes. Tom, the radical, grew up in a secure, two-parent home.�But Eric the financial planner, as Tom points out, spent many years as the only man in a house destabilized, both emotionally and financially, by death.
Mr. TOM HOEBBEL: And as he, you know, grew and went to college, I think maybe that was, for him, his primary motivation - is to be the provider.
SPIEGEL: So, anyway, this Thanksgiving Thursday, look across the table. There, you'll see a brother, a sister; and maybe they're your friend, and maybe they're your enemy, but one thing is for sure, their very existence has had a profound influence on your life.
Alix Spiegel, NPR News, Washington.
INSKEEP: And that's Your Health on this Monday before Thanksgiving.
(Soundbite of song, "I Want You Back")
INSKEEP: It's NPR News.
NPR transcripts are created on a rush deadline by a contractor for NPR, and accuracy and availability may vary. This text may not be in its final form and may be updated or revised in the future. Please be aware that the authoritative record of NPR's programming is the audio.