I don't think that believing in the testimony of the holy ghost is rational, and I'm going to explain why in this thread. If you really press most Mormons they will admit that they believe because of the testimony of the Holy Ghost. They don't have historical evidence that proves Joseph Smith was telling the truth, nor do they have anything resembling a plausible account of reality. So in order to find some reason for their belief or to make people believe, the appeal to the religious experiences that they have when they pray or read the scriptures. If this proves one thing, it is that Joseph Smith was a very gifted conman; by arguing that people should trust these feelings he absolved himself of the need to prove that he was telling the truth using actual physical evidence.
I made this thread because I believe that there are good reasons to believe that the "testimony of the holy ghost" can be explained using natural causes. If religious experiences can be explained by natural causes, and we don't need to appeal to god in order to explain them, then we ought to reject the religious explanation for their existence. That follows because of Occam's Razor (the natural explantion is more parsimonious). Do not posit the existence of an entity that is not necessary to explain the data since it increases the probability that your account is mistaken. So in order to prove that accepting the testimony of the holy ghost is stupid, all we need to do is show that natural explanations for religious experiences are plausible. Can religious experiences be explained in terms of natural phenomena? Of course they can. We have the ability to induce religious experiences in people by manipulating certain parts of the brain (google artificial religious experiences) so we can know that stimulating the brain using natural means can cause these feelings to exist. From there can conclude that natural explanations for religious experience are possible, and the god hypothesis can be out back on the crazy shelf where it belongs. That doesn't mean these feelings aren't real, but it does mean they don't prove anything.
In other words
Nietzsche wrote:A casual stroll through the lunatic asylum shows that faith does not prove anything