Is the proverb “never ask a barber if he thinks you need a haircut” used and understood?

The saying “never ask a barber if he thinks you need a haircut” means “don’t ask a person about their own activity, because they are in a conflict of interest and can only answer in one way”. Thus, it is an equivalent to “don’t ask silly questions”.

Looking it up on Google Books, I’ve found that the oldest occurrence dates back to 1998. It is therefore quite recent, if we are allowed to give some credit to this piece of information. I would like to know if this saying is actually used and established in everyday language, and if it can be understood by any native speaker.

Solution 1:

Although I wasn't familiar with this saying, it appears in Charles Doyle, Wolfgang Mieder, Fred R. Shapiro, The [Yale] Dictionary of Modern Proverbs (2012):

Don't ask a barber if you need a haircut.

1972 Daniel S. Greenberg, "Don't Ask the Barber Whether You Need a Haircut," Saturday Review: Science 55, no. 48 (Dec.) 58 (the article is subtitled "Greenberg's First Law of Expertise"). 1973 Arnold H. Vollmer, "The Numbers Game," in Environmental Impact: Proceedings of the ASCE Urban Transportation Division, Specialty Conference ... 1973 (New York: American Society of Civil Engineers) 85: "One of the basic laws of reasoning, discourse o argument can be summarized as 'Don't ask the barber whether you need a haircut.' Try though we may to be completely objective, there is no denying that an engineer has an an inherent bias toward and a vested interest in engineering." ...

So the expression goes back to 1972 (at least), is sometimes credited to Daniel Greenberg, and has attained the status of a modern proverb in the estimation of the Yale modern proverb collectors. I would caution you, however, never to ask someone who is publishing a collection of proverbs whether a particular saying is a proverb.

Malcolm Berko, "Broker Not to Blame for Bad Stock Picks" extends this advice to other fields:

I have a philosophy: never ask a painter if your house needs painting, never ask a lawyer if you should sue a defendant and never do business with a lawyer who advertises for business in newspapers, radio or TV.

Nevertheless, you should have no qualms whatsoever about asking strangers at English Language & Usage questions about English language and usage.