1869 [S]in 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."
Catechism of the Catholic Church 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.
Formal cooperation refers to agreement in the will regarding the evil act. We may never assist another person in an external sinful act, and intend its sinfulness. Such formal cooperation is always sinful, a sin against charity (scandal), as well as of the same kind as the act in which one cooperates. For example, to willingly drive someone to get an abortion, to pay for it, encourage it (if it determines their will to do it), defend the doing of it etc. is gravely sinful, both against charity and against life.