What is the equivalent expression to "old wives tales"?
The common translation seems to be:
- absurd superstition (英和対訳)
- old wives' tale (英和対訳)
- 迷信 (和英辞典)
To make sure, you are referring to this, right?
There is an expression which looks similar (おばあちゃんの知恵; lit. "wisdom of old women"), but this has a positive connotation in Japanese.
The kind of "theories" listed in the Wikipedia article is plainly referred to as:
- 古【ふる】い迷信【めいしん】 (lit. "old superstition")
- 俗説【ぞくせつ】 (lit. "layman's theory", usually implies scientifically false ones)