I often wonder if there is any similar English idiom to Polish saying "Głupich nie sieją, sami się rodzą" literally meaning "Dumb people are not sown, they're born."
This saying is similar in meaning to There are always dumb people and you can't do anything about it, you must deal with that.
When I try to explain the meaning behind this saying I always explain it like this: imagine we have cereal. Cereal can be sown randomly and naturally by wind or wild animals that are transporting the seeds in their fur. That just happens. But there is a need for cereal to feed all the people so they're planting them on purpose. If we compare that to a baby, we can say that no one is ever planning to give birth to an idiot because no one wants idiots in society and there is no need for them. Dumb people are not 'sown'. That just happens and they're born (or to be more exact they're becoming dumb along the way).
This saying is commonly used to describe a person that made something obviously stupid or wrong only because he or she believes it is right despite the fact that majority of society finds it simply stupid. Be sure that it is not used to describe a person that have done something wrong because of unawarness. It must be done on purpose and because of ignorance. It can be done because of stereotype but doesn't have to. I guess this saying can be used to describe at least teenager. Below that age you'd rather use it to describe parents than a child.
- Jack did you hear what Mike did? He thought the snow is thick enough and he jumped out of balcony and he broke his leg.
- Are you serious? Dumb people are not sown, they're born.