Rate of Non-Paternal Events

When you attempt to eat a grapefruit, there is a chance that you will press the wrong way when prying out a section and squirt yourself in the eye. Since this is startling and painful you remember it and, if asked how often it happens, you tend to exaggerate its frequency. Similarly, when you discover a non-paternal event in your family, it sticks in your mind. I’ve come across four documented instances in my own family history and it seems like a lot until I realize that I have around 4200 individuals in my database.

What is the real rate of NPEs? Well, it depends. Estimates range from one in a hundred to one in five. Studies that have used DNA or blood types indicate that the rate of NPEs varies across ethnic, religious and economic groups. When cohesive religious groups (such as Mormons or Orthodox Jews) are studied the rate of NPEs is between 1% and 1.5%. Lower socioeconomic groups have higher rates of NPEs than wealthier groups. A study of working class families in the British Midlands that used blood types showed that a fifth of the children could be NPEs, although the criteria used to establish that were somewhat loose.

An interesting angle on this question has been taken by researches looking at sibling pairs used in medical research on genetic markers for various disorders. Since full siblings share more of their genetic makeup than half siblings, it is important to know the rate of NPEs. In order to establish a base rate for NPEs, the researchers studied genetic variation among men with the same surnames. In Britain, common people adopted surnames sometime after the Norman Conquest (1066). Some of the surnames were taken from a man’s occupation (e.g. Smith), others from their father’s name (e.g., Johnson), and so on. It is not too surprising, then, that occupational surname lines had a lot more founding fathers than surname lines based on a unique location or unusual physical characteristic. When the researchers looked at genetic variation among men with different surnames, they found that the variation was high among common surnames and low among rare surnames. This is because rare surnames are likely to have had just one founding father. For uncommon surnames, they were able to estimate the rate of NPEs at between 1 and 1.5%. Now it is unlikely that people with uncommon surnames have a lower rate of NPEs than those with common surnames, so we can take this as an overall estimate of NPEs in the general population.

Family history researchers have different reactions to NPEs – some horrified, some titillated and some gratified that their ancestors were human after all. The rate of NPEs revealed by these studies suggests that it is an infrequent but not unheard of event.