The theory of reincarnation is used to explain certain things. Reincarnation helps in understanding enlightenment-the full blossoming of consciousness in Buddhism and in Hinduism too it’s used to explain the Karmic process which culminates in Moksha, which is equivalent of Nirvana in Buddhism. The difference between Nirvana and Enlightenment (Buddhatva) in Buddhist context is just that until enlightenment happens there is no chance of Nirvana and after the death of a person who has attained final liberation there are no cycles of birth and death because there were no Karmic remnants.
In his book ‘Born Again,’ Walter Semkiv proposed that he came to know after his extensive research that Christians used to believe in the theory of reincarnation but after a few centuries of systematic removal of these doctrines they started to disbelieve them. I do not know about the veracity of his claims, neither do I find his book to be very convincing on the matter of reincarnation. Metempsychosis is the name given to this idea in Greek philosophy and it’s found in Jainism and Kabbalah as well. Reincarnation has also been called ‘Rebirth’.
The sceptical inside you asks an important question: How did this seemingly never ending process of birth and death begin? If this perpetual-birth-after-death-after-birth melodrama is because of Karmic debts, then what in the first place, in the name of holy Ass Hub started this process? Why there was Karma in the very beginning? Is that Karma a fraction of ‘Original Sin’; a pinch of salt given by his highestness? Buddhists will not digest it but they also don’t have any answer for this question as far as I know; therefore you’re independent to speculate as long as you want to. Buddha had patented that he was entitled to answer only a certain number of questions and mind you even if you are in possession of the highest truth, you will answer only as many number of questions as you consider would suffice for dissolution of pain and suffering and so did Buddha and that is what knowing the highest truth is all about.
Many sceptics have also posed a question which points towards ever increasing human population. They ask that if humans are increasing in the number on this planet, wherefrom these many souls are coming into existence. In the name of holy Sandal Hunk, a few more people have come forward suggesting that they have observed certain decrease in the number of inhabitants of earth in non-human species. What these wise people fail to remark on is the most perspicuous fact that we have myriads upon myriads of planets like earth where spirits can incarnate; therefore drawing a line and trying to find answer a dull question like this is not a very wise thing to do in my opinion.