Any studies that have proven any benefit for prayer (to a god) have also been duplicated with studies of meditation. They have also been duplicated in a number of different religions.
Since meditation is internal, and doesn't involve any plea to a higher being.. if it is equally effective as prayer to a higher being that seriously implies that any benefits are internal, and not a result of "god answering your prayers".
Furthermore, there are just as many accounts of "miracles" for every other religion. From stories of people healed by Indian medicine men, to stories of Muslims healed on their religious pilgrimages. So if we accept these anecdotal stories as evidence that "god answers prayers" then you must also accept that OTHER gods answer prayers.. and therefore htere is more than one god.
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