Objection 1. It would seem that sin cannot be in the reason. For the sin of any power is a defect thereof. But the fault of the reason is not a sin, on the contrary, it excuses sin: for a man is excused from sin on account of ignorance. Therefore sin cannot be in the reason.
Objection 2. Further, the primary object of sin is the will, as stated above (Article 1). Now reason precedes the will, since it directs it. Therefore sin cannot be in the reason.
Objection 3. Further, there can be no sin except about things which are under our control. Now perfection and defect of reason are not among those things which are under our control: since by nature some are mentally deficient, and some shrewd-minded. Therefore no sin is in the reason.
On the contrary, Augustine says (De Trin. xii, 12) that sin is in the lower and in the higher reason.
I answer that, The sin of any power is an act of that power, as we have clearly shown (1,2,3). Now reason has a twofold act: one is its proper act in respect of its proper object, and this is the act of knowing the truth; the other is the act of reason as directing the other powers. Now in both of these ways there may be sin in the reason. First, in so far as it errs in the knowledge of truth, which error is imputed to the reason as a sin, when it is in ignorance or error about what it is able and ought to know: secondly, when it either commands the inordinate movements of the lower powers, or deliberately fails to check them.
Reply to Objection 1. This argument considers the defect in the proper act of the reason in respect of its proper object, and with regard to the case when it is a defect of knowledge about something which one is unable to know: for then this defect of reason is not a sin, and excuses from sin, as is evident with regard to the actions of madmen. If, however, the defect of reason be about something which a man is able and ought to know, he is not altogether excused from sin, and the defect is imputed to him as a sin. The defect which belongs only to the act of directing the other powers, is always imputed to reason as a sin, because it can always obviate this defect by means of its proper act.
Reply to Objection 2. As stated above (Question 17, Article 1), when we were treating of the acts of the will and reason, the will moves and precedes the reason, in one way, and the reason moves and precedes the will in another: so that both the movement of the will can be called rational, and the act of the reason, voluntary. Accordingly sin is found in the reason, either through being a voluntary defect of the reason, or through the reason being the principle of the will's act.
The Reply to the Third Objection is evident from what has been said (ad 1).
Source: Summa Theologica (New Advent)