The Dilemma: You want to assail someone's intelligence, but you don't know quite which word to use, which calls into question your own intellect.
People You Can Impress: Well, idiots and morons both, for starters. But also psychologists. And you really, really need to impress psychologists, because - as you'll see - you don't want them to think you're an idiot.
The Quick Trick: These days, the words are completely synonymous. But back in the dark days of psychology (which is to say until about 30 years ago), there was a difference, and here's the quick trick psychologists used: Ask a question. If your subject answers, they're a moron at worst. If they don't answer, you might have an idiot on your hands.
The Explanation: Anyone who says that political correctness never accomplished anything worthwhile should take a long, hard look at the lot of the idiot.
In 1911, French psychologists Alfred Binet and Theodore Simon created the first modern intelligence test, which measured intelligence (hence the "intelligence quotient") based on whether children could accomplish tasks like pointing to their nose (honestly) and counting pennies.
The concept of "IQ" followed soon after, and psychologists fell so deeply in love with the scientific nature of the tests that they created classification systems. Any child with an IQ of above 70 was considered "normal," while those with scores above 130 were considered "gifted."
To classify scores below 70, psychologists invented a nomenclature of retardation. Those with IQs between 51 and 70 were called morons. Morons had adequate learning skills to complete menial tasks and communicate. Imbeciles, with IQs between 26 and 50, never progressed past a mental age of about six. And the lowest of all were the idiots, with IQ between 0 and 25, who were characterized by poor motor skills, extremely limited communication, and little response to stimulus.
The moron/imbecile/idiot classifications remained popular, amazingly, until the early 1970s, when people started to note that the developmentally disabled have enough difficulties without being saddled with condescending labels.
Today the classification system is one category broader - moron, imbecile, and idiot have been replaced with mild, moderate, severe, and profound retardation - and diagnostic factors other than IQ are considered in making a diagnosis.
Good to Know: The doubly offensive term "Mongolian idiot," which in the 19th and early 20th centuries was an actual, literal diagnosis, derives from people's belief that individuals with Down syndrome - with their wide set eyes and round faces - resembled Mongolians. In fact, before the British physician J.H.L. Down (1828 - 1896) lent his name to the chromosomal syndrome, Down syndrome was known merely as "mongolism."
From the book: What's the Difference by mental_floss, published in Neatorama with permission. Please visit mental_floss' website and blog for more fun trivia!