When I was a child, my older brother told me stories about tarantulas living under beds and rocks that tasted like candy. When I believed him (as I always did), he’d howl with laughter and tell me how gullible I was. In a fit of fury, I’d call him a liar.

My mother was quick to intervene. “We don’t call people liars,” she’d say sternly. That’s how seriously the word was taken. “Liar” was the worst insult that could be hurled.

That attitude wasn...