Since identical twins are relatively rare, they often attract a great deal of curiosity. It is estimated that there are just 10 million identical twins in the world, which means about 1 in 80 births is the result of a twin pregnancy.

Identical twins, also known as monozygotic twins, are born when a mother’s single fertilized egg is split into two parts after conception. Since identical twins are formed from the same zygote, their DNA is almost indistinguishable. While identical twins have the same basic genetic makeup, however, it is interesting to note that they do not have identical fingerprints.

Recently, researchers have also proven the existence of “semi-identical” twins. These twins are born when one egg is fertilized by two sperm before splitting into two parts. This results in twins that have identical genes from their mother, but different genes from their father’s side. However, this form of twin pregnancy is quite uncommon.

Although identical twins have many similarities, there are subtle differences. For example, many pairs of identical twins have different weights or heights. This can happen as the result of dietary differences, serious illness, or a different connection to the placenta while the twins were still in the womb.

Identical twins also have personality differences as well. Twins may have different hobbies and interests. One twin may be more outgoing, while the other is quiet and shy. In fact, some researchers believe that identical twins will take on the stereotypical birth order characteristics associated with first born and youngest children--despite the fact that twins are usually born just minutes apart.

Parents of identical twins often contribute to the personality differences in their children. While it was once popular to dress identical twins in matching outfits and treat them as part of a pair, many parents are now committed to helping their children develop strong individual identities. They encourage their twins to develop a close bond, but celebrate the characteristics that make each child unique.

If the differences between identical twins are difficult to understand, think of twins as two chocolate chip cookies made from the same recipe. Even though the same basic ingredients are used for the dough, it’s impossible to create two virtually identical cookies. Subtle differences in size, color, and texture are simply inevitable since a cookie’s position in the oven will determine how evenly it is baked. In this way, both nature and nurture contribute to the differences we see in identical twins.