Infidelity is common in the animal kingdom. Governor Mark Sanford of South Carolina admitted to adultery. /MARY ANN CHASTAIN/ASSOCIATED PRESS

“Infants have their infancy,” wrote Professor David P.Barash. “And adults- Adultery.”

Dr.Barash, who teaches psychology at the University of Washington, wrote a book called “The Myth of Monogamy,” and was interviewed by Times reporter Natalie Angier for an article on animal infidelity.

Is monogamy really a myth- In the animal world, it is, even for species that mate for life. “Sexual promiscuity is rampant throughout nature, and true faithfulness a fond fantasy,” Ms.Angier wrote.

“Social monogamy is very rarely accompanied by sexual, or genetic, monogamy,” Ms.Angier reported. “Assay the kids in a given brood, whether of birds, voles, lesser apes, foxes or any other pair-bonding species, and anywhere from 10 to 70 percent will prove to have been sired by somebody other than the resident male.”

In politics, infidelity is hardly a shock. Italian Prime Minister Silvio Berlusconi, 72, is accused of having relationships with very young women, including Noemi Letizia, “whose 18th birthday party he attended,” wrote Rachel Donadio in The Times, “and who has said she calls him Daddy.” After his wife told newspapers she wanted a divorce, he demanded that she apologize.

South Carolina Governor Mark Sanford disappeared recently, telling his staff that he had gone hiking. He’d actually gone to visit his lover in Argentina, ostensibly to break up - which he had also tried to do in New York earlier this year, with his wife’s permission. “I think I speak for us all when I say that no spouse is allowed more than one officially sanctioned goodbye visit to his mistress,” the Times columnist Gail Collins wrote.

But even when a spouse cheats, the marriage usually survives, wrote Benedict Carey and Tara Parker-Pope in The Times. In a University of Virginia study, 10 percent of married people admitted having affairs in any given year. Years later, 76 percent of the adulterers were still married and living with their spouses.

“Historically, the institution of marriage has not succumbed to infidelity so much as coexisted with it, like a body does with the flu virus: weakening at times, yet developing some immunity from long exposure,” wrote Mr.Carey and Ms.Parker-Pope.

In Governor Sanford’s case, it was not his infidelity but the fact that he seemingly could not stop talking about it that most “weirded out” his colleagues, as Ms.Collins put it.

He held a rambling, apology-filled news conference, and later told The Associated Press that he had had flirtations with other women as well, but didn’t cross “the ultimate line.”

Mr.Sanford’s wife, for her part, has said she is willing to work on the marriage.

Like some human s, animals often retaliate against their wayward partners. In an experiment with scarab beetles, which pair up and build a ball of dung in which to lay eggs, researchers tethered a female beetle near her mate, Ms.Angier wrote. The male then released pheromones to attract other females. When his mate was released, she pushed him onto his back.

“She’d roll him right into the ball of dung,” Dr.Barash told Ms.Angier, “which seemed altogether appropriate.”