Why aren’t identical twins identical? Their genes are the same – basically like having the hardware of a computer. These genes can change their expression, much like software can change the way your computer works – or doesn’t work. Various life forces and choices affect our genes – this is the fascinating field of epigenetics. Basically there are tiny molecular “switches” than can turn on or off the gene’s expression. Here’s a video which is a great introduction to the field of epigenetics.
As an identical twin, I can confirm there were some differences, but very few. We never had a teacher from kindergarten through college who could tell us apart. True, we married different women, and he was an attorney (I a physician), but our interests remained the same. Once, at about age 55, we both picked up the same book at an airport shop, some 3000 miles apart, on the same day.
But yes, there were of course some differences.
Tom Preston, 1005