Reincarnation is the concept that after biological death, some part of of a person (soul, spirit, etc.) continues to exist and is born again as either another human being or non-human living thing.
Lack of evidence
It is claimed that some part of a person, such as a spirit or essence, survives death and continues to exist into another life. However, there is no reliable observable conservation of any singular and/or distinct identity, essence or spirit into each new life. While testimony for this exists, it has not been reproduced in a controlled environment. If this effect is unobserved, what exactly is retained between lives?
Mechanics of reincarnation
Assuming a spirit is immediately reincarnated upon death, reincarnation fails to consider population growth. This can be rescued if one assumes that the conscious beings born as humans may have been intelligent aliens in their previous life. The population of the universe as a whole may be steady.
Alternatively, spirits may have to wait to be reincarnated or be reincarnated in the past by time travel.
Difference between beliefs
Hinduism, Buddhism, Jainism and Sikhism all teach reincarnation as part of their religion. However, they differ on how this works, either regarding each other or internally between sects. This is not even to mention other beliefs regarding it, such as those of New Agers. So even if reincarnation existed, that would not by itself support any particular religion unless these differences were cleared up.
- Skeptic's dictionary on Reincarnation
- Alex Lickerman, Happiness in this World, Psychology Today, October 14, 2012