Bobby Neel Adam's Age Maps.

One prominent theiry they didn't mention is birth order. Apparently the order in which we are born relative to our siblings can influence our personalities due to the changing context of the family dynamic with eldest siblings often being more extroverted, middle children being more introverted etc...

Another is the influence of our peers. While children do have generally identical home environments, it has been argued that past a certain age, friendship groups have a far greater influence on behaviour and personality than family.

Because genes aren't everything.

Shocking, huh..

Of course genes aren't everything - but remember that siblings share both genes AND environment. Well, many of the same genes and mostly the same environment anyhow, but they can have diametrically opposite temperaments and personalities.

And then, there's the Enneagram, which makes more sense to me than any other explanation. Check it out.

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.

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.

If you guys had clicked the link and listened to the NPR report, they address birth order and a lot of other issues.

@Dani
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.
http://en.wikipedia.org/wiki/Birth_order
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.

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.