Professor Richard Lynn said the Scots average IQ of 97 was well below the England and Wales average of 100.5 and on a par with the Republic of Ireland. London and the south-east of England scored top in the UK, with an average IQ of 102.
Scotland - when split from the rest of the UK - comes two-thirds down the table of European average intelligence, beating Russia by one point and France by three.
But Prof Lynn, a lecturer at the University of Ulster, said he was not surprised by the poor Scots showing. He blamed the centuries-old tradition of Scots seeking their fortune south of the Border or overseas for a literal brain drain.
Prof Lynn said: "I believe this has been caused by selective migration. Over the course of two centuries, there has been a trend for more intelligent Scots to migrate to London and beyond, to New Zealand, Australia, the United States and Canada.
"Once in London, for instance, they have settled and reared children, who have inherited their high intelligence and transmitted it to further generations."
Prof Lynn claimed that Scotland was not alone in suffering from a long-term brain drain.
He said Corsica had a lower IQ than Paris because people gravitated to the economic powerhouses of major cities. Prof Lynn said: "This has hit Scotland particularly hard, so I was not surprised with this finding."
The survey showed huge variations in IQ across Europe, as well as within the UK. Scotland is 16th out of the 23 European nations, with eastern Europe filling most of the places below that.
Russia, Greece, France, Bulgaria, Romania, Turkey and Serbia all score lower averages than Scotland, with Serbians notching up just 89 IQ points on average.
The European bright sparks, according to Prof Lynn, are the Germans and the Dutch (107 IQ points) and Poland (106 IQ points). The UK as a whole comes eighth, behind Sweden (104), Italy (102), Austria (101) and Switzerland (101).
Gabriela and Monica Irimia, 23, the Romanian pop twins the Cheeky Girls, who live in Britain, said their homeland's poor ranking was an insult. Gabriela said: "Our father is head of the ambulance service in Romania. He's a doctor of medicine and extremely intelligent."