A chimera brain could have serious consequences. For instance, we know that the arrangement of different brain regions can be crucial for its function – but the presence of foreign tissue, being directed by different genes carrying a different blueprint, may throw that intricate design into disarray. This may explain, for instance, why twins are less likely to be right-handed – a simple trait that normally relies on the relative organisation of the right and the left hemispheres. Perhaps chimerism has upset the balance.

Even if you do not think you ever had a twin, there are many other ways you might be invaded by another human’s cells. It’s possible, for instance, that you started off as two foetuses in the womb, but the twins merged during early development. Since it occurs at such an early age of development, the cells can become incorporated into the tissue and seem to develop normally, yet they are carrying another person’s genetic blueprint. “You look like one person, but you have the cells of another person in you – effectively, you have always been two people,” says Kramer. In one extreme case, a woman was surprised to be told that she was not the biological mother of her two children (See “Brother from another mother”, left). Alternatively, cells from an older sibling might stay around the mother’s body, only to find their way into your body after you are conceived.

However it happens, it’s perfectly plausible that tissue from another human could cause the brain to develop in unexpected ways, says Lee Nelson from the University of Washington. She’s currently examining whether cells from the mother herself may be implanted in the baby brain.