It is one of the world's most persistent myths: that one in 10 children are illegitimate without their legal fathers knowing that they have been cuckolded. Now scientists believe they have arrived at the real figure. One in 25.
Scientists analysed the Y chromosomes of 1,600 unrelated men with 40 surnames to test the idea that there could be a link between the Y chromosome and surnames, which are both passed on to boys down the male line.
For certain, uncommon surnames there was a clear association between the unique genetic markers carried on the Y chromosome with the surname, suggesting that many of the men sharing the same surname had a common male ancestor many centuries ago.
This also enabled the scientists to estimate the probability of illegitimacy among the men carrying the same surname and found that it came out at between 1 and 4 per cent.
"People often quote a figure of one in 10 for the number of people born illegitimately. Our study shows that this is likely to be an exaggeration. The real figure is more likely to be less than one in 25," said Professor Mark Jobling of Leicester University, who carried out the study with colleague Turi King. Common surnames, such as Smith and Brown, do not show any significant link with the genetics of the Y chromosome, but some less common names, such as Attenborough, Haythornthwaite, Herrick, Stribling and Swindlehurst, showed a definite association with the male chromosome, indicating that as many as 80 or 90 per cent of the men with these less familiar names shared a common ancestor.
"Surnames such as Smith come from a person's trade and would have been adopted many times by unrelated people. Less common names were more geographically specific and possibly adopted by only one or two men, so we would expect people with these surnames to be more closely related," Dr King said. "Attenboroughs, for example, essentially form one big family of distant relatives. The Y chromosome type was the same even across spelling variants, which confirms that the spelling of names were formalised only relatively recently," she said.
Surnames were introduced into the British Isles with the Norman invasion and had become widely used by about 1500. Previous research demonstrated a link between Y chromosomes and surnames, especially in Ireland where scientists had found that more than one in five men in north-west Ireland were descended from a single medieval ancestor.