The following is lifted from a revised version of Here Comes Granddad Again. I recommend two pop texts that survey the twin research and present an annoyed defense of genetics research
Lawrence Wright, Twins and What They Tell Us about Who We Are.
William Wright, Born that Way: Genes, Behavior, Personality.
"Similarities and Differences.
"Twins, because they usually share highly similar environments as well as nearly identical genetic material, surprise no one when they act like each other. Interpretations of the causes of similarity usually stress either biology or rearing. The problems occur for "rearing explanations" when twins grow up away from each other and still act alike.
First, Bailey (1998) summarizes that environment acts primarily to make siblings different from, not similar to, each other. Wright (1997) surveys the Minnesota Twin Project highlights the conclusion that identical twins reared apart are more similar than identical twins reared together. Second, Plomin (1986) concluded that the family environment is more similar for identical twins because each member of a set of twins elicits similar behavior from parents rather than the opposite effect that similar environments pull similar behaviors from the children (Bailey, 1998). Thus, the explanation that "like households produce like behaviors" is challenged from two directions
"Details and More Details.
"Some of the traits found by Bouchard in one set of twins reared apart (Wright, 1997; Wright 1998) include: the way dinner knives were held, bitten fingernails, living in the only house in the block, each house with a white bench surrounding a backyard tree, anxious sleep patterns, woodworking (miniature rockers or miniature picnic tables), stock car fans, leaving love notes around the house, bruxism while sleeping, and dimensions of personality (sociability, flexibility, tolerance, conformity, self-control). A similar level of detail but for different traits was found in every set of twins.
"Details in the similarity of physical traits seem less surprising. Lawrence Wright (p 92-93) shares findings by David Teplica, a plastic surgeon in Chicago, findings about 6000 twins. Teplica commented,
'... freckle patterns, hair whorls, the first gray hairs, the first wrinkles on the human face, even the development of acne on the same location on the nose at exactly the same time -- all these things seem to be in some way genetically predetermined. Why would two women from Texas develop basal cell carcinoma in exactly the same spot on their left ears within a year of each other? Why would two young men from Ohio have the same extra hairs on their cheeks and the same cupped-ear deformities? How can it be that two cell clusters that were separated fifty years ago have enough information to determine where your blackheads would develop when you are fifty or sixty years old?'"