§ 3. On Incest

I. Notion. Incest is said from in and castus, and is intercourse between relatives or affines within the degrees which constitute an impediment of marriage. It is said:

1. Between relatives, that is, relatives either naturally, or consanguineously,¹ or spiritually,² or legally;³ likewise between affines.⁴

2. Within the degrees which constitute an impediment of marriage, whether by ecclesiastical law alone or also by natural law itself. Namely, as St. Thomas teaches, "in the mingling of joined persons there is something which is in itself unbecoming and repugnant to natural reason, as that mingling should take place between parents and children, whose kinship is of itself and immediate. For children naturally owe honor to parents... But other persons who are not joined according to themselves, but by order to parents, do not have unbecomingness thus from themselves. But concerning this, becomingness or unbecomingness varies according to custom and human or divine law, because the use of venereal things, which is ordered to the common good, is subject to law" (II-II, q. 154, a. 9, ad 3).

One who, after a dispensation has been obtained for entering marriage, sins with a relative or affine, quite probably does not commit incest (St. Alphonsus III, 452).

¹ In the direct line of consanguinity, marriage is invalid between all ascendants and descendants, both legitimate and natural. - In the collateral line it is invalid up to the third degree inclusive, yet in such a way that the impediment of marriage is multiplied only as often as the common stock is multiplied (can. 1076, §§ 1 and 2).

² Only that spiritual relationship of which canon 768 speaks invalidates marriage (can. 1079). Canon 768, however, establishes: From baptism only the one baptizing and the sponsor contract spiritual relationship with the baptized.

³ Those who by civil law are held incapable of entering marriage with one another on account of legal relationship arising from adoption cannot validly contract marriage with one another by force of canon law (can. 1080).

⁴ Affinity in the direct line invalidates marriage in any degree; in the collateral line up to the second degree inclusive (can. 1077, § 1).

II. Morality. 1. Incest, besides the sin of lust which it involves for the same reason as fornication (n. 33) and adultery (n. 37), also contains a sin against piety. Incest, however, between those spiritually related seems to be opposed to religion rather than, or equally with, piety, because the Church seems to have established the law both from the motive of reverence due to those joined together and from reverence due to the sacrament (cf. St. Thomas, II-II, q. 154, a. 10, ad 2; St. Alphonsus III, 450). I said: incest contains a sin against piety; for it is opposed to reverence due from piety, according to these words of St. Thomas: "Naturally man owes a certain honor to parents, and consequently to other blood-relatives, who draw their origin proximately from the same parents... But it is manifest... that in venereal acts there especially consists a certain shamefulness contrary to honor; hence men are ashamed concerning these things; and therefore it is unfitting that such venereal mingling should take place between such persons with one another" (II-II, q. 154, a. 9). These things can also be applied to those legally related, because the adopter and the adopted are in the likeness of parent and child, and therefore the Church, in making the law, is judged to have been moved by the same motive.

2. Incests between those joined in diverse ways differ thus:

a) Incest between those naturally related, or consanguineous, those spiritually related, and those legally related differ in species. The reason is that the said relationships arise from an altogether diverse root, namely natural relationship from intercourse, spiritual relationship from the sacrament, legal relationship from adoption; hence they themselves, and consequently their violations through incest, seem morally diverse. With respect to incest between those spiritually related, it is added that this also seems opposed to religion.

b) Incest between affines, those spiritually related, and those legally related, for the same reason explained under a), differ in species.

c) Incest between consanguineous persons and affines more probably does not differ in species. The reason is "because a person who is affine is joined to someone on account of a person joined by consanguinity; and therefore because one is on account of the other, consanguinity and affinity make an unbecomingness of the same nature" (St. Thomas, II-II, q. 154, a. 9, ad 2; St. Alphonsus III, 449; VI, 469).

3. Incests between lines and degrees of diverse connections differ thus:

a) With respect to consanguinity, incest in the first degree of the direct line¹ seems to differ in species from others. But incest between other consanguineous persons seems not to differ in species. The reason is that among consanguineous persons in the first degree of the direct line the character of piety is found per se, whereas among the rest it is found only by participation, that is, by relation to parents. But what is such per se seems to differ in species from what is such by participation. But things which are such by participation seem to differ not in species, but only in degree (St. Alphonsus III, 448; VI, 469, 470; cf. St. Thomas, II-II, q. 154, a. 9, ad 3). St. Alphonsus (VI, 470) and others judge that a sin with a person ascending in the direct line also differs in species from a sin with one descending, and therefore they teach that it must be explained whether a father sinned with his daughter, or a son with his mother.

b) With respect to affinity, incest in the first degree of the direct line, that is, between stepson and stepmother and between father-in-law and daughter-in-law, in the same way as incest in the first degree of the direct line of consanguinity, seems to differ in species from others. "The reason is that the debt of reverence which is the special motive prohibiting intercourse of parents with children is also the motive prohibiting intercourse between stepmother and stepson, or father-in-law and daughter-in-law." Thus St. Alphonsus (VI, 469) and others.²

Whatever is admitted theoretically, in practice the confessor, in order to consult the peace of the penitent, if he has a founded suspicion that incest was committed in the first degree of the direct line, whether of consanguinity or affinity, should ask about it; and he should proceed for the same reason if the discussion is of the first degree of the collateral line.

Corollary. A parish priest who sins with his parishioner, likewise a confessor who sins with a penitent, are not guilty of incest, because spiritual relationship, which is an impediment of marriage, is lacking (cf. can. 1079 compared with can. 768). Nevertheless the parish priest sins against justice, because from his office, to which a salary is annexed, he is bound to provide for the spiritual good of the parishioner (St. Alphonsus III, 451).

¹ Many assert the same concerning the first degree of the collateral line (cf. St. Alphonsus III, 448; VI, 470).

² Salmanticenses, tr. 26, c. 5, n. 31; Lugo, disp. 16, nn. 316 and 319.