English equivalent Polish saying "Dumb people are not sown, they're born"

The saying "there is one born every minute" is close to yours:

• There are many fools and dupes in the world.

Origin

• The earliest form of the phrase that I can find in print comes from The European magazine: and London review, published by the Philological Society of Great Britain, 1806:

• It was the observation of one of the tribe of Levi, [that is, a Jew] to whom some person had expressed his astonishment at his being able to sell his damaged and worthless commodities, "That there vash von fool born every minute".

• The "sucker born every minute" is widely said to have been conjured up by P. T. Barnum. Like many other sayings attributed to Barnum, they are just that - there's no evidence that he coined this phrase.

• Whilst probably not having been coined by Barnum "a sucker born every minute" is certainly an American phrase. The first record that I can find of it in print is in The New York Times, December, 1883:

• 'There's a sucker born every minute,' as the gamblers day.

(The Phrase Finder)

If I were trying to capture the pungency of the original expression, I would say something like this:

Stupidity grows wild; nobody has to cultivate it.

But that isn't an idiomatic phrase in English. The main idiomatic elements are "grows wild" (which means something like "arises without the help or intervention of human effort") and "cultivate" (which has both an agricultural meaning related to planting and maintenance, and a cultural or societal meaning related to social refinement).

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An American saying that expresses a rather different view of stupidity had become popular in recent years:

Everyone is born ignorant, but you have to work hard to stay that way.

The underlying idea here is that becoming more sensible as we mature is every bit as natural as starting out knowing nothing. The possible source of this saying is discussed at Quotation ascribed to Benjamin Franklin, "We are all born ignorant, but one must work hard to remain stupid."

This would usually be found in contexts where the person in question actively defeated a mechanism to stop them from doing something stupid, but nicely conveys the inevitability in your original phrase, if not the fact that dumb people are created naturally.

You can make it idiot-proof, but someone will always build a better idiot.

"Idiot-proof" means that it cannot be defeated by idiots, in the same way that bulletproof armour cannot be defeated by bullets and childproof packaging cannot be defeated by children. "Build a better idiot" is a reference to the Emerson quotation "Build a better mousetrap, and the world will beat a path to your door."

There is an English idiom that matches your "long explanation" almost exactly: "Dumb people are born, not made".

It is also used when talking about someone with an exceptional ability to do something (for example "Olympic athletes are born, not made") as well as to an exceptional lack of ability.

Experience keeps a dear school, yet fools will learn in no other.

http://www.notable-quotes.com/f/franklin_benjamin.html#x8UABGg5MTxViORg.99