giraldus cambrensis writing c 1200AD (translated from latin)' the britons do not sing their their tunes in unison,like the inhabitants of other counries,but in different parts. so that when a company of singers meets to sing, as is usual in this country, as many different parts are heard as there are singers....in the northern parts of the country,beyond the humber and on the borders of yorkshire, the inhabitants make use of a similar kind of symphonious harmony in singing, but with only two differences or varieties of tone and voice, the one murmuring the under part, the other singing the upper in a manner equally soft and pleasing". pretty conclusive evidence,eh? personally i think you should sing in harmony if (a) you likeit and (b) the audience like it, and be damned to any theorists who say different: but if you specially need a bit of history to back you up, there it is greg