Objection 1. It would seem that folly is not a sin. For no sin arises in us from nature. But some are fools naturally. Therefore folly is not a sin.
Objection 2. Further, "Every sin is voluntary," according to Augustine (De Vera Relig. xiv). But folly is not voluntary. Therefore it is not a sin.
Objection 3. Further, every sin is contrary to a Divine precept. But folly is not contrary to any precept. Therefore folly is not a sin.
On the contrary, It is written (Proverbs 1:32): "The prosperity of fools shall destroy them." But no man is destroyed save for sin. Therefore folly is a sin.
I answer that, Folly, as stated above (Article 1), denotes dullness of sense in judging, and chiefly as regards the highest cause, which is the last end and the sovereign good. Now a man may in this respect contract dullness in judgment in two ways. First, from a natural indisposition, as in the case of idiots, and such like folly is no sin. Secondly, by plunging his sense into earthly things, whereby his sense is rendered incapable of perceiving Divine things, according to 1 Corinthians 2:14, "The sensual man perceiveth not these things that are of the Spirit of God," even as sweet things have no savor for a man whose taste is infected with an evil humor: and such like folly is a sin.
This suffices for the Reply to the First Objection.
Reply to Objection 2. Though no man wishes to be a fool, yet he wishes those things of which folly is a consequence, viz. to withdraw his sense from spiritual things and to plunge it into earthly things. The same thing happens in regard to other sins; for the lustful man desires pleasure, without which there is no sin, although he does not desire sin simply, for he would wish to enjoy the pleasure without sin.
Reply to Objection 3. Folly is opposed to the precepts about the contemplation of truth, of which we have spoken above (Article 16) when we were treating of knowledge and understanding.
Source: Summa Theologica (New Advent)