1865 Sin creates a proclivity to sin; it engenders vice by repetition of the same acts. This results in perverse inclinations which cloud conscience and corrupt the concrete judgment of good and evil. Thus sin tends to reproduce itself and reinforce itself, but it cannot destroy the moral sense at its root.

1866 Vices can be classified according to the virtues they oppose, or also be linked to the capital sins which Christian experience has distinguished, following St. John Cassian and St. Gregory the Great. They are called "capital" because they engender other sins, other vices.138 They are pride, avarice, envy, wrath, lust, gluttony, and sloth or acedia.

1867 The catechetical tradition also recalls that there are "sins that cry to heaven": the blood of Abel,139 The sin of the Sodomites,¹⁴⁰ The cry of the people oppressed in Egypt,¹⁴¹ The cry of the foreigner, the widow, and the orphan,¹⁴² injustice to the wage earner.¹⁴³

1868 Sin is a personal act. Moreover, we have a responsibility for the sins committed by others when we cooperate in them:
- by participating directly and voluntarily in them;
- by ordering, advising, praising, or approving them;
- by not disclosing or not hindering them when we have an obligation to do so;
- by protecting evil-doers.

1869 Thus sin makes men accomplices of one another and causes concupiscence, violence, and injustice to reign among them. Sins give rise to social situations and institutions that are contrary to the divine goodness. "Structures of sin" are the expression and effect of personal sins. They lead their victims to do evil in their turn. In an analogous sense, they constitute a "social sin."144