Every parent of twins faces the question "Are your twins identical or fraternal? Many think that being identical twins refers to how twins look and not how they form. Take a look at the biology that determines the sex combinations that are possible with identical and fraternal twins. The terms identical and fraternal don't describe what the twins look like, but actually how they form.
The psychologist John Money oversaw the case and reported the reassignment as successful and as evidence that gender identity is primarily learned.
Testosterone and oestrogen can cross membrane barriers in the uterus. A female twin foetus is therefore exposed to a brother's testosterone and a male twin foetus to a sister's oestrogen. However, male and female foetuses have similar oestrogen levels, so a female is more likely to be affected.
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