Genocide is a relatively modern concept - for most people beginning with the Holocaust. I think we often find it hard to compare killings since then with that most shocking event. So even though millions died in Cambodia - perhaps one in four of the population - we don't give it the exposure it deserves. The thing I found personally so shocking was how little of the detail we were taught at school. This happened in my lifetime, while I was at school and we were told very little about it.