When I graduated from the University of Düsseldorf, Germany, in 1996, I had a topic for my Ph.D. dissertation all worked out. Fifteen years later, I can hardly remember what it was (something to do with a corpus investigation of different sentence types). However, around the same time, I got involved through personal connections in the transcription of a corpus of Oral History interviews with former citizens of Düsseldorf: German Jews, who had had to flee from Germany during the Nazi regime around sixty years earlier, and had lived in English-speaking countries ever since. Their testimonies were being collected for documentation purposes by a Holocaust museum. I was asked to help with the transcriptions, since I had some experience in working with spoken data.

Living with these narratives was a fascinating and moving experience in many ways, and I encountered many stories – about loss, pain and terror, about ignorance, intolerance and cruelty, but also about courage and generosity. Among the diversity of voices, characters and personalities that I came to meet through my headphones, however, one thing began to stand out to me: the astonishing range in the degree of confidence and proficiency which people had retained in their mother tongue. In some cases, if I hadn't known better, I would have sworn that they had never lived outside Germany for a day. In others, I would have been equally certain that they had learned German as a foreign language late in life.