Forget the castor oil. My OB had delivered several thousand babies (he started in practice before WWII) and he said it just made you miserable. He thought the idea originated in the practice of sympathetic magic, because when you really do go into labor your bowels empty quite thoroughly - clearing the decks as it were. So folks were artifically producing the consequence in hopes of also producing the cause . . .
Well, given how powerful even the placebo effect can be, we simply looked at the likely worst result: discomfort for a bit...; but I was initially in the same skeptic ball park.
But the pressures of late, well.... I am trying to be flexible.