Interestingly my son has an identical twin! I like to joke that one is the control and sleeps in the house and the other has to sleep in the storage shed out back, and when they're 18 we'll publish a paper comparing their emotional development. :)

I always thought DNA determined pretty much everything, and we raised them exactly the same, but they have distinct personalities and some different physical features, although of course they're much more similar than they are different. My other son shows some symptoms of anxiety but not as much, yet.