Studies have shown that, despite differences in gender and ethnicity, people experience the five stages of grief – denial, anger, bargaining, depression, and acceptance – in the roughly the same order, and for the same length of time.
I think that humans as a whole are more common than we think. Biologically and neurologically the human brain has evolved to process information in this way, probably resulting from the coevolving of human society as we became smarter. While there may be outliers in this data about grieving, we are still data at the end of the day. This overlying trend is an example of this. Human data almost always follows a common trend and I'd agree with these studies just based on personal experience as well.
