I have wondered that myself. Based on changes in information and communication technologies since I was a kid in late 50s, there probably was a lot more than we knew about. But I also think it is worse now.
My gut feeling is that the percentages are the same, but because we have more people in the U.S., there are more of everything. Without an instant nation-wide and world-wide media like we have now, we just didn't hear about all the bad things 30 - 40 - 50 - 60 years ago.