Canadian darling Sarah Polley's bombshell that she was born of an affair stunned fans this week, but more eye-opening was the reaction of her father – the father who raised her – to the revelation that his late wife had been unfaithful, and that he had zero biological investment in his youngest daughter. In Ms. Polley's words, it was "extraordinary."

"His chief concern, almost immediately, was that my siblings and I not put any blame on my mother for her straying outside of their marriage," Ms. Polley wrote in a blog post for the National Film Board of Canada. "He was candid about his own lack of responsiveness toward her and how that may have led her to the point where she sought out the affection of another person."

There was none of the screaming or chair-tossing often found in mainstream portrayals of cuckoldry, as men's paternity is announced or renounced from a manila envelope, Maury Povich style.

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"There's this idea people have that men's commitment to fatherhood is this fragile thing, easily shattered," said Noah Brand, editor-in-chief of the Good Men Project, an online magazine. "It's not an absolute rule."

As a child raised primarily by a stepfather, Mr. Brand wasn't surprised about Michael Polley's absolute support for his daughter: "He raised a kid that wasn't biologically his own: so does everyone who adopts a child. That's parenting. He put in the work, that's being a father."

While testing DNA for disease in the last decade, geneticists have stumbled on figures that suggest roughly 10 to 15 per cent of children are not fathered by the men they believe to be their dads. A 2003 study by the American Association of Blood Banks pegged it higher still, finding that in 30 per cent of 354,000 blood tests done to determine paternity, the man tested was not the biological father.

From a cultural standpoint, Mr. Polley's acquiescence to news of his wife's affair was most startling of all, says Robin Milhausen, an associate professor in Family Relations and Applied Nutrition at the University of Guelph who has researched cheating.

"We're going against traditional scripts where it's men who are more likely to be sexually unfaithful. Women's sexual infidelity tends to bother men more than emotional infidelity and that's why the public reacts with more judgment and surprise that the man is taking it in stride."

A more callous view would call it resigned cuckoldry, especially as there were jokes in the family about Ms. Polley's lineage. But Mr. Brand sees it as a more modern masculinity.

"It was the most manly-in-a-good-sense part of it. He was taking responsibility because it was his marriage. That is deeply admirable in my mind."