That's a good point Astral about boys and girls possibly being raised differently in the same home due to gender. It also raises the issue of children from the same family, with different personalities, being raised in the same way possibly not allowing one to be able to 'be' which is what janeainsworth pointed out earlier
There have over the years been some questionable studies of twins and triplets separated at birth. I'm thinking of an extensive study done in America where babies given up by their mothers at birth, were deliberately given to separate adoptive parents and were then 'studied' at regular intervals so sibling comparisons could be made.
It came out into the open when a young man went to uni and on the first day, by several students, was mistaken for someone else. His brother who was unknown to him was contacted by a friend, went to the uni to see for himself and they discovered they'd been separated by the adoption process.
It was jumped on by the media which resulted in a third brother coming forward; they were triplets. There's was just one of many cases of twins and triplets being deliberately separated during the adoption process in the name of 'science'.
As well as the stark physical similarities and mannerisims were their personality traits, taste in music, fashion and hobbies.
The documentation that relates to this study, which wont be unavailable for public scrutiny for several years even to the 'participants', was to discern what has the most influence, nature or nurture.
Until I watched the documentary on this study, I always believed it was 50/50. Having seen it and read other more ethically carried out studies, where it hadn't been possible to keep the siblings together for adoption, I'm now of the opinion that even if it's 52/48, nature seems to have the greater input.