I'm having lunch tomorrow with a colleague and her identical twin daughters, who were my students this year. When the year started I was so worried that I would never be able to tell the two of them apart - they are exactly identical, no discernable physical difference between them at all. I guess that is what identical means, right? They are only seven, so they're too young to have terribly different fashion sense, and for most of the year I just avoided calling them by their names unless we were in the classroom, where they have assigned seats.
Although that strategy failed when they changed seats.
But over the course of the year, as I got to know them, their personality differences became so striking that was much easier to tell them apart (as long as I could observe them for a couple of minutes before using their names outside of the classroom). One is more reserved and proper, more concerned with following the rules and doing what is right and being nice to others. The second one is more of a free spirit, ready to make mischief or joke around, tell stories about her classmates, not as bound by the rules as her sister. There are also similarities in their personalities - they are both extremely shy, so it can take a while to get them to come out of their shells and show their personalities, they are both easily made uncomfortable, they are both good, serious students.
I started thinking about the personality differences between the two girls as they relate to the nature versus nurture argument. They are identical twins and therefore have identical DNA, so the nature component, or genetic component, can't really play a role here (unless there are some heavy duty mutations happening). But neither can nature - they were raised by the same set of parents, have always had the same babysitters and day care programs, the same teachers, they do the same activities. They lead identical lives. I'm sure they've had some different experiences, but I don't think there's been anything major that happened to one and not the other.
The personality differences seem to come from each girl processing their common life experiences differently. The questions remaining are why and how? Perhaps the chemicals in their brains that create memories work in different ways. Perhaps they reason through life experiences in a different manner. No matter how it happens, there isn't a strict nature or nurture reason for their personality differences, because their nature (genes) and nurture (evnironment) are exactly the same. The issue of 'why we are the way we are' seems to be much more complex than that.
Of course, the above paragraph doesn't take into account my ignorance of the exact experiences they have had together and apart. That may be the missing piece of the puzzle, things that have happened to one and not the other. I was friends with half of a set of identical twins in high school, but I never knew her sister well enough to observe their similarities and differences. I'm sure they were very different too, just like my students.
I don't know much about the chemical basis of memories, but this article and this other one are good starts for some basic reading. Here is a good introduction to the issue of nature vs nurture. I also found this fascinating paper about a 13th century romance that anticipates feminism by several hundred years and deals with the issue of nature and nurture.