I asked this question in a gardening NG but didn't get far so I am
trying here.

I see posts in NGs and references in books and web sites to companion
planting. There seems to be a fairly widespread belief that some plants
do better when in proximity to others and that some combinations are
harmful. Some sources list a number of feasible mechanisms that could
be the causes of this phenomenon but other than the obvious (ie the tall
upright plant shades the prostrate one) there is little evidence
included that shows the effect actually happens. Also many sources
state without attribution things like:
"it has long been known that companion planting is beneficial....." or
"scientific studies have shown that companion planting is ....." or
"research in this area has consistently shown that companion plants
offer no pest control benefits under controlled conditions."

These references are rather unsatisfying as they don't name the original
research that the author appears to be relying on. At this point I have
an open mind on the subject and I would like to know more but I want
more than just what was handed down by grandpa even if he was a wizard
farmer.

Can anybody give me references to particular studies which approach this
issue in a thorough and scientific way? I am interested in studies that
might attempt to prove/disprove that this actually happens or to show
that proposed mechanisms actually work or not. I would expect such a
study would include some sort of measure of just how beneficial or
harmful given combinations might be, as I can't see how you could say a
combination was significantly good/bad without being able to measure the
benefit/harm.

David