Why is it called “adultery” when being unfaithful isn’t a particularly “adult” thing to do?

A celebrity marriage mystery has raised some very grown-up questions about the words we use to talk about love, commitment and desire.

Gossipmongers have been transfixed the past few days by the curious romantic situation of three TV stars: Elisabeth Moss of “Mad Men” wed Fred Armisen of “Saturday Night Live” (the guy who plays President Obama) in 2009 after a whirlwind romance. In the past few days Armisen has been spotted with fellow “SNL” cast member Abby Elliot behaving like more than friends.

This isn’t “People” magazine or “Dear Abby,” but perhaps we can use this salacious scenario as an excuse to look at a dilemma of language that is as enigmatic as the state of this celebrity marriage: the complexities of “adult” and “adultery.” How can such similar words have such different meanings?

Remarkably, the answer is that the words don’t share a common ancestor. “Adult” comes from the Latin verb adolescere, “to grow up, mature.” Students of Latin will understand what we mean when we say that adultus is the pluperfect of adolescere. Adultery, on the other hand, derives from a French word, avoutre, which in turn evolved from a distinct Latin verb, adulterare, “to corrupt.” The verb adulterate, “to debase or make impure by adding inferior materials or elements,” stems from the same source.

The sense of “adult” that means pornographic emerged as a kind of  reverse assumption that adult and adultery have more direct links than they do.

Let’s look at the dictionary definition of adultery: “voluntary sexual intercourse between a married person and someone other than his or her lawful spouse.” In fact, there are two types: single adultery (with an unmarried person) and double adultery (with a married person.) Here is the definition of adult: “having attained full size and strength; grown up; mature.”

What conclusions can we draw regarding relationships from these twists of etymology? Maybe that a real-life definition of adult is the condition of being responsible for our choices, and that the choice of whom to love and honor is probably the most adult decision of all.

(On the topic of “Mad Men,” do you know what the “mad” in the title stands for? Here’s the answer.)

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