Q.: "Is there a certain sex or ethnic group that Siamese Twins occur more in?"
Siamese (preferred term= conjoined) twins are a form of monozygotic twins. Monozygotic twinning rates are the same throughout the world; they are not affected by heredity, ethnicity, infertility treatments, maternal age, or any of the other factors that affect dizygotic twinning. No one really knows what causes monozygotic twinning. So, the answer to your question is no.
However, the majority of conjoined twins are female. Again, there is no identified reason for this. It is not known whether more female twins are actually conceived, or if simply more female conjoined sets survive the pregnancy.
For more information about monozygotic twinning, see my article on Determining Zygosity.
Conjoined twins, also known as Siamese twins, are a rare type of monozygotic (identical) twins. Find more answers to Frequently Asked Questions About Conjoined Twins. (If you have a question that is not answered here, please e-mail me: multiples.guide@about.com.)
