Earlier quoted context omitted.
I'm not aware of any studies showing that it has no effect either. If I had to guess I would estimate a prayer to be as effective as a placebo, but to proclaim that as fact seems a bit unscientific.
There has been quite a bit of study of intercessory prayer. i.e. for people that don't know they are being prayed for. No affects have been found. > In 1872, the Victorian scientist Francis Galton made the first statistical analysis of third-party prayer. He hypothesized, partly as satire, that if prayer were effective, members of the British Royal Family would live longer than average, given that thousands prayed fo…
there's been a number of studies showing lots of effects. Most studies seem to show slight positive effects from prayer, some show none and some show negative effects.