For instance, he said in a 2013 interview with “Voices From Oxford” when he was mayor of London that female suffrage happened only because men realized that women could “run them down” with cars.

“I think that women’s liberation, female suffrage, probably wouldn’t have happened, if it hadn’t been for the motorcar,” he opined.

Mr. Johnson was also raked over the coals for a 1995 article in which he described the children of single mothers as “ill-raised, ignorant, aggressive and illegitimate,” and referred to single mothers as “uppity and irresponsible women.”

In this latest case, a senior reporter at the newspaper that published Mr. Johnson’s prediction, Rosamund Urwin, promptly called it “nonsense” during on a segment on the BBC on Sunday. Others called it “silly.”

There’s one very clear reason to doubt the idea of an Olympic baby boom, as commentators on social media were quick to point out. While 2012 was indeed a big year for British babies — there were 729,674 live births in England and Wales, according to Britain’s Office for National Statistics, the most in over four decades — the London Games were a summer event, too late to affect that total.

Any Olympics-inspired babies would have arrived in 2013, when births declined by more than 30,000.

What’s more, the high in 2012 was not a one-off boom but the culmination of a yearslong trend. “It’s just been gradually building up,” Paul Vickers, a statistician at the O.N.S., told The Guardian at the time.