Since the middle ages, common law decreed a woman was her husband’s property.

Like anything he owned, animals or children, he had to discipline her from time-to-time.

Although, he must use reasonable force and not kill her.

In 1782, Judge Francis Buller was required to sit in judgement on such a case.

He referred to an earlier precedent, where a man had been found guilty of beating his wife to death with a pestle (a heavy, club-like kitchen tool).

At the time, the judgement was: “Though a husband by law may correct his wife, the pestle is no instrument for correction”.

Judge Buller felt the guidelines needed to be more specific, so he ruled: “A husband may thrash his wife with impunity provided he uses a stick no bigger than his thumb.”

Subsequently the cartoonist, James Gilray, caricatured him as “Judge Thumb”.

But the rule persisted, in 1824 in Mississippi a court ruling stated a man was entitled to enforce domestic discipline providing he used a cane no wider than his thumb.

In North Carolina, in 1868, a defendant was found to have used a cane on his wife “about the thickness of his finger”.

He was allowed to go free because a finger was thinner than a thumb.

That judgement was later upheld by the Supreme Court.

In 1917, legal scholar Beirne Stedman cited the old common law rule allowing a husband to use: “moderate personal chastisement on his wife” so long as the rod was no larger than his thumb.

In 1976, this law was used to illustrate the cliché by women’s-rights advocate Del Martin: “The common law doctrine was modified to allow a husband to whip his wife provided he use something no bigger than his thumb – a rule of thumb, so to speak.”    

In 1977, a book on battered women mentioned: “The reason nineteenth century British wives were dealt with so harshly by their husbands and their legal system was the ‘Rule of Thumb”.

Which is how, we are told, the phrase ‘Rule of Thumb’ passed into common usage.

Wikipedia defines Rule of Thumb as: “An easily learned and easily applied procedure or standard, based on practical experience rather than theory.”

Interestingly, Wikipedia later uses ‘rule-of-thumb’ to define heuristics.

“A heuristic is a mental shortcut that allows people to solve problems and make judgements quickly and efficiently. These ‘rules of thumb’ strategies shorten decision-making time and allow people to function without constantly stopping to think about the next course of action.”

In Behavioural Economics, heuristics are referred to as cognitive biases.

Cognitive is defined as: “Involving conscious intellectual capacity such as thinking or reasoning.”

Bias is defined as: “To feel inclination for or against someone or something”.

So, cutting through all the long words, rule-of-thumb means heuristic, which means prejudiced thinking.

In behavioural economics, cognitive biases include: confirmation bias, availability bias, sunk-cost bias, anchoring bias, framing-effect bias, actor-observer bias, and many more.

And of course, all these are designed to sound intimidating to outsiders.

But remember, they are all actually just common-sense dressed up in long words.

They all come back to the rule-of-thumb.

And the rule-of-thumb goes back to a silly medieval law.

So don’t be frightened by long words, just unpack them and find out what they’re actually disguising.

It’s a good rule-of-thumb, or heuristic, to say: “Excuse me, what does that mean?”

You’ll be surprised how often the people using the long words don’t actually know what they mean, or where they came from.