Why Do Siblings Have Such Different Personalities?

Why do siblings - despite having much of the same genes and upbringing - grow up to be have such different personalities?

NPR's Alix Spiegel explores:

Then in the 1980s, a researcher named Robert Plomin published a surprising paper in which he reviewed the three main ways psychologists had studied siblings: physical characteristics, intelligence and personality. According to Plomin, in two of these areas, siblings were really quite similar.

Physically, siblings tended to differ somewhat, but they were a lot more similar on average when compared to children picked at random from the population. That's also true of cognitive abilities.

"The surprise," says Plomin, "is when you turn to personality."

Turns out that on tests that measure personality — stuff like how extroverted you are, how conscientious — siblings are practically like strangers.

"Children in the same family are more similar than children taken at random from the population," Plomin says, "but not much more."

In fact, in terms of personality, we are similar to our siblings only about 20 percent of the time. Given the fact that we share genes, homes, routines and parents, this makes no sense. What makes children in the same family so different?

http://www.npr.org/2010/11/18/131424595/siblings-share-genes-but-rarely-personalities - via Cliff Pickover's Reality Carnival

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Adopted children don't share genes with their siblings, unless their siblings were adopted from the same family. I'm as different as night and day from my older (adopted) brother, but have quite a bit in common personality-wise with my younger brother. Both brothers are my parents' biological children. I did meet my biological family about 10 years ago, and have a great deal in common with two of my siblings (the two closest to my age) while having little in common with the oldest and the youngest.
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I did follow the link, and they actually didn't discuss birth order. They spoke about how the different ages of siblings effects the timing through which they experience events such as divorce, which may result in differing impacts of these events on children's personalities e.g. a divorce may affect a 5-year old very differently compared to a 10-year old.

This is not quite the same thing as birth order. Birth order is not just about the differing ages of children impacting their experiences, but rather how they stand relative to their siblings in the order of their births, and how this affects the family dynamic.
Obviously birth order does not determine personality, but it can certainly influence it.

This is a complicated issue, and it is likely that all the points they mentioned and more contribute towards the cause of differing personalities in siblings.

I am the youngest of 6 and I can say that everyone in my family has totally different personalities.
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I agree with Jessss. Birth order plays a bigger role in personality development than most people think. While I don't completely agree with everything they attribute to each category (First born, second born, middle child, youngest, only, etc), there is a lot of valid information to be had by studying birth order.
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I'd argue that past a certain age they don't share mostly the same environment. They may go to the same school, but their different peers make it a different experience.

HERE'S A CONUNDRUM: What about conjoined twins? They share identical genes, identical birth order, and identical environments. The only thing that varies environmentally between them is the way in which they are attached and their health i.e. one may be the "weaker" twin or may be attached in such a way that puts them at a disadvatage compared to the other twin. Yet every story I've read about conjoined twins implies that they have different personalities. Maybe it's something they subconsciously create together so as to be seen as individuals.
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