The sole survivor may carry some trace of the hormones of the opposite sex.
The girls are tomboys
The boys are sissys
The women are manly,
the men are feminized.
If there is a massive transfusion of bloods between the twins, the sole survivor may feel as if he or she is in the "wrong body" and may want to change sex. This feeling is known as "gender dysphoria".
Read about it here.
Now I wonder: how many of the people who write about gender dysphoria think about womb twin survivors? I decided to find out.
This case report describes the psychosexual development of a child with true hermaphroditism who was assigned to the male sex at birth, but reassigned to the female sex at age two months. Given this child's excessive exposure to male sex hormone in utero, relative to physically normal females, one would predict a biological predisposition to behave in a masculine manner. This has occurred since two years of age. In addition, this child has had periodic episodes of gender disturbance, dysphoria, and ambivalence. In contrast, a physically normal fraternal twin sister has been conventionally feminine. A number of psychosocial factors appeared to have exacerbated the biologic predisposition to behave in a masculine manner, and thus may have been responsible for "pushing" this child into varying degrees of gender identity conflict. These included a closer father-daughter than mother-daughter relationship, parental tolerance of cross-gender behavior, and a mother who has been psychologically disturbed since the birth of her children. The heuristic value of this case will be discussed in relation to contemporary models of psychosexual development which emphasize the interaction of biological and psychosocial factors.In 1987 there was no attempt to explain why this little girl has been exposed to male hormones when her twin sister was not.
I was unable to find a single scholarly article that related gender dysphoria to the lost of a twin in utero. An oblique and tentative one sentence reference to the unborn twin on a transgender therapy site in 2000 here. Thats all I found today, anyway.
So. You heard it first here, everyone!