These are some comments I wrote in liveonearth's LJ regarding belief in reincarnation.

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I think whether reincarnation is true or not is irrelevant to me. Because we lose most/all of our memories in it anyway, it wouldn't matter to me if it were true.

I don't think it's true because I'm not a dualist -- i.e. I believe you can't separate the material realm from the experiential realm: every thought, feeling, memory, etc. has a corresponding physical element, so I can't imagine a mechanism which would transfer a person between bodies without also transferring something physical to hold the information.

It's possible the transfer works through some unknown physical method of course -- such as electromagnetism or something undetectable to the senses. But I doubt there's a hidden physical mechanism for another reason: unless we assume the "souls" are reproducible, i.e. that anyone born must have a soul and that it must have come from some previous life -- that would mean that the number of souls in existence would have to remain constant, which is contrary to what we know about the human population (which has increased over our history) or the earth's population if we allow transfer between species (which has also increased over the earth's history).

We could also allow interplanetary reincarnation, as the Scientologists believe, but that still requires either a steady-state universe where the number of lives living in the universe is constant, with no increase or decrease in that number (which is contrary to what cosmology we know) -- or a nearly limitless number of free souls with sometimes long delays between incarnations, or soul generation and destruction of some sort (perhaps souls could be converted into matter or energy under extreme conditions, the way matter and energy are interconvertable under extreme conditions).

One problem with interplanetary reincarnation too is that souls would have to travel long distances instantly, faster than the speed of light, which is in violation of what we have observed about the universe. Or else the souls would need to know ahead of time where they're going to be reborn (where there's going to be an increase in the total need for souls on a planet) and travel there at or below the speed of light to arrive there on time.

Another reason I doubt it is that I can't imagine any use for it. Why would the universe or life require a reincarnative mechanism? What's the benefit of it? If we imagine two alternate universes, one with reincarnation and one without, would life in the one with it have any type of advantage that would allow them to evolve it? If not, then it's something that didn't evolve from life, but was either introduced by some external agent outside of the universe, or arose from the nature of the laws of the universe. I find the idea that reincarnation could evolve with life as an advantageous trait interesting, and more conceivable than the idea that consciousness or experience is somehow special or built-in to the laws of the universe.

But as I said, if it's true, I don't really think it matters. It'd be interesting in the abstract sense, but not something that matter one way or the other -- i.e. it won't change the way I live or think or how I act.

I think a lot of people believe in reincarnation because they use it as a rationalization to explain why they have problems in life, believing it's their karma left over from their last life. Or they just believe in it because they don't like the idea that they or others will be gone forever -- which would be true even if reincarnation is true in any case, because most of their memories and personality are gone forever either way.

Or they sometimes use it like the Christians use hell, as a belief that they'll be rewarded in the next life and the others who don't act the way they like will be punished by being born as a lesser animal (this is a common belief among Hindus for example, and oddly mirrors how Christians use the idea of heaven and hell).