This is hidden brain.
I'm Shankar Vedantam.
In October 2014, psychology professor Nancy Siegel heard a story.
It was about her specialty, the study of twins.
Nancy had made a career of studying unusual cases of twins.
But she had never heard a story like this one.
It's December 1988.
A woman from a remote farming village in Colombia makes a day long trek to a hospital.
She gives birth there to a pair of identical twin boys.
One of the twins needs more advanced medical care that isn't available.
One of them was extremely sick.
Since the mom is still recuperating, a relative takes him to another hospital, 6 hours away in Bogota, the capital.
At the Bogota hospital, another set of identical twin boys has just been born.
Their mother is a seamstress who lives in the city.
The scene was quite chaotic.
The hospital staff is tired and overworked.
It was approaching Christmas time.
People were thinking about the holidays.
Now, nobody knows exactly what happens next, but something goes wrong.
The baby from the countryside is switched with one of the babies born to the mother from the city.