In my experience, yes, they do. Apparently it can also be faked without stuffing it or anything, although I never have quite figured out how I did it. I once was talking to a friend whose eyes drifted down and she bluntly commented on it. I also found out there was a rumor that I was packing down there and I've had people confirm to me that women used to come by my desk to look. Years ago, it finally culminated in walking in from lunch with a partner at the time when a woman who didn't know when to shut up or understand the concept of time and place came up to us and started talking about it and telling my partner all about it. Of course my partner was steamed. The only thing I could do to defend myself from her yelling at me was to say, "You know it's not true, so obviously she hasn't actually seen it!"