On 6th December 2004 a baby girl named Yan was born. Her father, an internet entrepreneur, is called Shen Tong. Yan was Shen’s first child, and you might have expected him to have an excitable, sleepless night. But oddly the opposite occurred. He slept better than he had done for 15 years, six months and two days. It’s possible to be exact about the timing because 15 years, six months and two days earlier was 4th June 1989 and on that day Shen had been on a boulevard just off Tiananmen Square in Beijing. He was a 20-year-old student, and like thousands of others he was demonstrating in favour of political reform.

After martial law was declared, Shen watched as the army drove through the city. Between outbursts of shooting, students tried to reason with the military. Shen approached a truckload of soldiers; he wanted, he says, to calm the surrounding crowd. Suddenly an officer pulled out a pistol. Parts of the rest of the story are hazy. Shen was dragged back by others. A shot was fired, and a female student, roughly Shen’s age and standing just behind him, was hit in the face. She died. Shen remembers her covered in blood. He is convinced that the bullet was intended for him. Shen moved to the US, but violent images recurred in his dreams for many years—until, that is, the arrival of Yan. Not only did he sleep well that night, but the following night, and the night after that.

Post-traumatic stress disorder (PTSD) is a condition that can occur after a distressing event. It involves a traumatic memory which comes back to mind repeatedly and involuntarily. It’s associated with chronic anxiety and hyper vigilance. The numbers affected are contentious. By one mid-range estimate tens of thousands of US veterans of Iraq and Afghanistan suffer from it. As do British veterans of the Falklands war—more of whom have committed suicide than died in active service. The Pentagon has sunk hundreds of millions of dollars into PTSD research. But of course, as Shen Tong knows, you don’t have to be a soldier to experience it.

Investigation of PTSD has been an important cause of new research into memory. And our understanding of memory is, in turn, propelling a debate about what is known as enhancement, or the boosting of human capacities beyond a normal level. The first issue to emerge…

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