Because the genetic blueprints of identical twins are the same, you might expect them to have a lot in common – and they do.

They are as identical as human clones would be. But are identical twins completely identical?

Identical twins are genetically the same, but the laws of genetics don’t completely determine your physical appearance. The prints of identical twins are more alike than the prints of two completely unrelated people. Still, they’re not identical.

Each one of us started out as a fertilized egg. In the case of identical twins, two people arose from the same egg. After the egg was fertilized, it split in two – so identical twins are the same sex. They often have nearly the same height, weight and hair color. But look closer, and you’ll find differences in twins. Common differences revolve around features like birthmarks, moles, hair patterns and teeth development. Look below the skin, and you’ll find even bigger differences in their personalities.

For centuries, scientists have debated whether we are the way we are because of our nature – that is, our biological heritage – or because of the environment in which we were raised? The “nature vs. nurture” debate often revolves around studies of identical twins. Because they share the same genetic blueprint, differences in how twins turn out should be due – at least in part – to differences in their environments. Likewise, their similarities might be due to their shared genetic makeup.

One of the best-known twin studies began in Louisville, Kentucky in 1957 and continues to this day. Researchers there have collected detailed physical and psychological information on hundreds of pairs of identical twins from birth to adulthood. Twin studies such as this suggest that our genetic blueprints do play a large role in alcoholism, reading disorders and susceptibility to disease. Likewise, a Swedish twin study is finding strong evidence that genetics affects how long we live. And twin studies are finding genetic links to obesity, baldness, personality, and intelligence.

But genetics isn’t everything. For example, scientists don’t think your genetic makeup makes you more or less likely to use tobacco on a regular basis.