There's a one-in-100 chance your dad isn't your real dad and you don't even know it.
Researchers in Europe looked at centuries of DNA evidence to figure out how common affairs that result in children are, and what drives them.
They found while fewer than 1 percent of middle- and upper-class people are the result of extramarital affairs, that rises to 6 percent of those born into poverty in densely populated cities.
The researchers call this 'extra-pair paternity'.
"Our research shows that the chance of having extra-pair paternity events in your family history really depends on the social circumstances of your ancestors," said Maarten Larmuseau of Belgian university KU Leuven, who led the study.
Farmers and low-income people living in small towns and rural areas had a cheating rate of about 4 percent.
"If they lived in cities and were of the lower-socioeconomic classes, the chances that there were extra-pair paternity events in your family history are much higher than if they were farmers."
The researchers started with more than 500 men with a common male ancestor, so should have carried the same Y chromosome - but a few didn't, proving somewhere in their family tree was someone who shouldn't be there.
No significant differences were found between religions - socioeconomic factors were the biggest influence.
Larmuseau said they were surprised the rate of extra-pair paternity events was so low.
"The evidence puts average rates at around 1 percent."
The study was published in journal Current Biology.