There may be another human’s DNA trapped inside of you. While it may sound like a weird science sequel to The Exorcist, according to Peter Kramer at the University of Padua, “A very large number of different human and non-human individuals are all incessantly struggling inside us for control.”

Kramer and his colleague Paola Bressan just published a paper about this strange phenomenon — microchimerism — that influences our behavior. Basically, we pick up DNA perhaps in the womb from a twin, a subsumed twin, an older sibling, our mothers, or later in life from a pregnancy. In one study, 63 perhaps of women who’d had children were harboring male cells in their brains.

“A very large number of different human and non-human individuals are all incessantly struggling inside us for control.”

While it’s been studied that the bacteria in our guts can influence everything from our mood to our appetites, less is known about how this shadow human DNA may sway our decisions and feelings. This foreign DNA could potentially influence which hand is dominant or the propensity to develop Alzheimer’s. In one study of Danish women who’d been pregnant, epidemiologists determined that the leftover Y chromosomes improved the overall health of the women.

What’s stranger still is that microchimerism of male cells in women occurred even when they’d never given birth to a son — the DNA possibly transferred by an older brother or even sexual intercourse with a man. It seems then that the human body is far from fixed at birth and instead picks up DNA the way our browser does cookies.

Bryan Sykes, founder of Oxford Genetics, says everyone should be proud of their own genes.