Some people know, with absolute certainty, that they were born the wrong gender.
A girl sees that she has no phallus, yet she feels deeply, unambiguously male. A boy is equipped with a penis, yet he feels fundamentally, unarguably female.
Such discord often gets chalked up to the physical - prenatal hormone exposures, abnormal brain structures, gay genes. Or to the psychological - repressed homosexuality, absent dads, overbearing moms, parents who wanted a baby of the opposite sex.
But there is a new explanation: Some transgender men claim to possess phantom penises. From the time they were little girls, they say they had vivid sensations of a penis between their legs. Others develop such a phantom when they begin taking testosterone therapy.
Similarly, transgender women who are born male and later undergo sex reassignment surgery generally do not report having a phantom. They say that their penis was never part of their body image.
V.S. Ramachandran, a neurologist and psychologist at UC San Diego and a leading authority on phantom limb sensations, says it has long been known that some people who are born without arms have vivid phantom arms. They can swing them around, wave goodbye and make complicated gestures.
This suggests that an intact body image - the maps of the body laid down in the brain before and after birth - can develop without actual limbs. So-called mirror neurons that map the actions and intentions of others into one's own brain may help bring the phantoms to life, Ramachandran says.