Scientists who normally ignore the "paranormal" will occasionally -- and with some apparent relish -- break their silence to speak out against psychic research. The latest episode appears in this week's Nature, the British science journal.

The report concludes that psychic research has some fundamental problems:

"The failure of paranormal investigators to produce a single repeatable effect despite 100 years of published research is a serious matter. The hoped-for results have been described in thousands of reports, but not one can be repeated in a properly controlled replication."

The author, psychologist David Marks of the University of Otaga in New Zealand, compiled a list of assumptions and arguments made by psychic researchers.

By definition, paranormal phenomena are only those that can't be explained by "normal" science. Science has found explanations for several popular paranormal claims, but has failed to convince believers.

One example is Kirlian photography, which makes pictures of colorful flame-like electrical discharges surrounding objects in high-voltage fields. Believers in the paranormal call the discharges auras and give religious or metaphysical explanations. "Normal" scientists say the phenomenon has well-understood scientific explanations.

Marks challenges another assumption of psychic research: that the usual rules of evidence in science are replaced by such factors as the "sheep-goat effect" and the "shyness effect."

The sheep-goat effect, according to paranormal believers, governs the outcome of an experiment. Observers who believe in the paranormal are sheep. Those who do not are goats. Thus, negative results can be dismissed, and skeptics invariably fail to confirm paranormal findings.

"Similar in kind is the shyness effect -- the tendency of metal not to bend psychically while it is being observed," Marks writes.