It is amazing that some identical twins separated at birth have the same traits. We would expect them to have the same physical traits, but what about the length of their hair or their favorite ice cream? There have been cases where there are things they have in common that seem almost creepy. I think however that the number of things they don't have in common probably far outweigh these, and we just latch on to the things that are common. There will be these random statistical variations among any two people.
In my observations I would have to say that there is more to the nurture part as far as the impact on how we develop. While our DNA may determine many of our physical traits I firmly believe that our personality is determined by our environment.
I must voice my agreement with ask966. Although dna studies are fascinating, and a number of similarities have been shown between identical twins separated at birth; there is simply not enough evidence to indicate a relationship. Environment is always a factor, and we don't really understand the influence of heredity completely, even though we have mapped the genome. It is doubtful that any meaningful conclusion will be derived until such time as we fully understand the impact of environment as well as heredity; and that, I fear, is a long way off.
I must disagree to some extent with Pohnpei. I'm not sure he has proved his point. Identical twins are born with the same dna, but his argument would only be valid if dna determines personality. This I don't believe is true. Think back to the news story of one identical twin plotting the murder of her sister. Both girls were raised in the same home with the same influences, and yet one gave life to the murderous plot. It is quite likely that identical twins have some sort of connection not clearly understood by science, but we still must argue "nature vs. nurture." I would love to see studies on what determines "nature"
The old nature vs. nurture chestnut! There are of course studies that show that twins that have separated at birth and who then re-find each other later on have remarkable similarities in terms of the kind of lives that they are leading. This would indicate apparently a massive vote in the favour of nature as the most important factor in our upbringing. However, critics would argue for a more nuanced examination of such cases, looking at the facts individually and in the social context, arguing that it is our nurture, our environment that plays a bigger role in our development.
It also allows for "layered" studies on environment. What effect does a two parent versus a one parent household have? What about different income levels? Urban versus rural? There are dozens of demographic factors that we can get a clearer picture of their effect when the observations are made on twins.
Identical twins have essentially identical genetic materials. Therefore, they should have inherited the same traits. Therefore, differences between two identical twins must come from the environment. Also, if identical twins are raised separately, similarities between them may well be heriditary.
Adopted siblings have none of the same inherited traits. But they are raised in the same family. Therefore, similarities between them may well come from their environment.
This means that researchers might be able to tell what traits are inherited and what traits are caused by "nurture" by looking at twins and adopted siblings.