Not quite - Identical Twin research (which is why there are registries of them) is used to try and separate out the effects of nurture (what happens after birth) from nature (what you are born with), which should be 'identical'.
More commonly a goup of people are randomly allocated to different exposures (eg bra or no bra), but this is not always possible, especially in social and behavioural science.
Therefore the next step is to take people with and without an exposure (bra or no bra) and compare some common outcome (breast position) after collecting as much information as possible that might afect the outcome (eg race, age, weight, height, cup size) and adjusting for that, so that only the question you are reall interested in (bra status) remains.
Unfortunately there are so few completely bra free women as to make this impossible to do also. That leaves measuring degree of freedom as the question of choice (eg how many years and how many hours a day).
Bra 'fit' is probably an important factor too. Misfit is common and a major cause of illness.
Michael
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