Wilful Ignorance and Sin

Not every kind of ignorance is the cause of sin, but that alone which removes the knowledge which would prevent the sinful act…This may happen on the part of the ignorance itself, because, to wit, this ignorance is voluntary, either directly, as when a man wishes of set purpose to be ignorant of certain things that he may sin the more freely; or indirectly, as when a man, through stress of work or other occupations, neglects to acquire the knowledge which would restrain him from sin. For such like negligence renders the ignorance itself voluntary and sinful, provided it be about matters one is bound and able to know (Saint Thomas Aquinas, Summa Theologica, I-II, q. 76, a.1, a.3).