I believe we remember lives in better documented eras because knowing of them can trigger our memories from that time period.I recall lives both during well known periods and less known ones.I've researched the less known ones and found as much or even more authentication than from better documented times.And since someone mentioned why no one remembers living in a less known time like the Song Dynasty I'd like to mention my best life was lived in that era.
We also must remember times like ancient Egypt,the Roman empire, Tudor England, Victorian England, Revoltionary France, Civil War or Colonial America had high %'s of our population alive then.Ancient Egypt lasted for millenia.Anyone who's been reincarnating since the beginning of recorded time should probably recall lives in ancient Greece and Egypt etc. as for several thousand years they were the only heavily populated areas of the world at the time.The roman empire lasted 1000 years and boasted a very large population so probably at least a third to half the world should recall lives then.Alot of people should also recall lives in China and India as they've always had the greatest populations of all. These places probably are most remembered because everyone tends to remember what is more interesting or that they better understand.Remote places such as africa or the phillipines were not as advanced as Rome and Rome could support a larger empire.For about 1/5 of our history rome controlled nearly half the known world and supported the most people(except perhaps china and India).These are considered historic times because there was alot of people and activity going on then.History is made quicker when there are alot of people making it.In more recent times there was alot more history being made because of technology,communications and above all a large population.
I recall lives in all these times and places but I also recall lives lived in more obscure less known eras.In the pre-roman era I recall lives in Egypt,Greece,Turkey,china,india,the middle east,africa and even some native american lives and one in England/ireland.I recall many lives in the roman era but not all were in the roman empire.I also recalled lives in asia and the americas during that time as well.My best series of lives was lived between the years 450-1150AD which is an era in history that is not recorded in great detail.Pretty much all of those lives were in africa,asia and the americas with only one in the middle east and one in europe.The middle ages lasted about 500-1000 years,depending on when you believe they started.It is desputed to be as early as the fall of rome and as late as the 1200's don't know why no one agrees it's really opinion I guess.I'd expect many to recall medieval lives.Perhaps that's why medieval fantasy is so possible now.Most lives recalled are from between 1500 and present because most remember more recent lives.These times are also better documented.
I feel that although we reincarnate across many races there does seem to be some evidence there's a particular area of the world or racial group we prefer.Lives in Europe in historical times are probably most common because most of the history learned about is in Europe because Europe had the most wars etc.Also most of the people who are writing about remembering past lives are in america and ethnically are usually of a european ethnic background so I'd expect more lives in Europe and the americas from a person in america and Europe.In asia where it's more accepted it's not as written about because it's not seen like anything special.
For a good book on statistics about population trends and lives remembered I suggest the book Reliving Past Lives by Helen Wambach.She conducted a massive survey of who was living where in what era of time to see if it matched known population.People often recalled living in socities that were more populated in the times they lived.
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"Doesn't expecting the unexpected make the unexpected the expected?"