There are actually two myths here: 1) that laws allowed men to beat their wives as long as the stick was small, and 2) that this is the origin of the phrase “rule of thumb.”
First things first. In the various American colonies, laws differed from place to place and year to year. In Maryland, at least in the 1600s, beating one’s dependents with a small stick was allowed. Dependents included indentured servants, slaves, children, and theoretically, wives. “The community expected him [a husband and landowner] to keep good order and the law allowed him to correct any of his charges with physical punishment, provided that any stick used in beating was no thicker than a man’s finger at its thickest end. Beating even his wife was permissible.” Lois Green Carr, “From Servant to Freeholder” Maryland Historical Magazine (Fall 2004), p. 298. Legal reference not cited. No similar references have been found that refer to thumbs and beatings, only to fingers.
Beatings with this size stick could be severe, and several cases appear against masters who beat their slaves or servants excessively, “Whereuppon mr Ouerzee beate him wth some Peare Tree wands or twiggs to the bignes of a mans finger att the biggest end, wch hee held in his hand” and the slave died. (Maryland Provincial Court Proceedings 1658) but I found no mention of wives or children being beaten with sticks, perhaps because none sued in Court. Again, the reference to stick size is a man’s finger, not a thumb.
Now for Part 2) the origins of the phrase Rule of Thumb. Our old reliable friend, the Oxford English Dictionary, disputes this claim. The Rule of Thumb, it says, is “a method or procedure derived entirely from practice or experience without any basis in scientific knowledge; a roughly practical method.” The earliest known instance of the term is 1692: “What he doth, he doth by rule of Thumb, and not by Art.” More examples from later years follow, but nothing pertaining to anything like beating one’s wife.
In her 1994 book, Who Stole Feminism?, Christina Hoff Sommers spends 5 entire pages discussing the origins, legal and journalistic, of the phrase. It is too long for me to retype the entire section and the book is not available on line to cut and paste, so you’ll have to read pages 203-207 at your local library if you want more detail. Suffice to say that Sommers’s exhaustive research uncovered no link between the phrase and the law, other than misguided journalists quoting one another in magazine and newspaper articles. Which is how such myths are spread.
In a nutshell, Sommers debunks the oft-repeated statement that rule of thumb laws permitting wife-beating can be found in the famous legal commentaries of William Blackstone (1723-1780), which is the basis of much U.S. common law, and that these laws prevailed in state courts throughout the 19th century. Blackstone does state, “The husband, by the old law, might give his wife moderate correction . . . in the same moderation that a man is allowed to correct his apprentices or children . . . But with us, in the politer reign of Charles the Second, this power of correction began to be doubted; and a wife may now have security of the peace against her husband.” This conforms to the Maryland law, mentioned above. (Charles II ruled 1660-1685)
“In America,” Sommers says, “there have been laws against wife beating since before the Revolution. By 1870, it was illegal in almost every state, but even before then, wife-beaters were arrested and punished for assault and battery. . . . The Massachusetts Bay Colony prohibited wife-beating as early as 1655. The edict states: No man shall strike his wife nor any woman her husband on penalty of such fine not exceeding ten pounds for one offense . . . ”
But a couple careless judges stated in their opinions the erroneous belief that this myth was true, and these men are often quoted as proof. In Mississippi in 1824 and in North Carolina in 1874, judges referred to an “ancient law” by which a man was allowed to beat his wife with a stick provided it was no wider than his thumb. There is no ancient law. And even here, neither judge referred to the supposed law as the “rule of thumb.”
Even the much-maligned Wikipedia has got this right. But there are many online sites that cheerfully and wrongly explain the origin of this phrase.