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Consanguinity is a diriment impediment of marriage as far as the fourth degree of kinship inclusive. The term consanguinity here means, within certain limitations defined by the law of nature, the positive law of God, or the supreme authority of State or Church, the blood-relationship (cognatio naturalis), or the natural bond between persons descended from the same stock. In view of the recognized descent of all men from one common stock, there is a general blood-relationship between all men; hence the limitation mentioned has reference to the nearest root or source of consanguinity. This bond or union of blood takes place in one case through the descent of one person from the other; this is called the direct line. In another case it takes place because the common blood is drawn from a common root, the same ancestor, from whom both persons descend, though they do not descend one from the other, and are therefore not in a direct but in a transverse or collateral line. By the law of nature, it is universally conceded, marriage is prohibited between parent and child, for the reverential relation between them is recognized as incompatible with the equality of relations engendered by the bond of marriage. The universal sentiment of peoples is likewise opposed to marriage between all persons related in any degree in the direct line, thus between grandparent and grandchild.

Because of the acknowledged derivation of the human race from the common progenitors, Adam and Eve, it is difficult to accept the opinion of some theologians that the marriage of brother and sister is against the law of nature; otherwise the propagation of the human race would have begun by violation of the natural law. It is readily understood that, considering the freedom of intercourse between such persons, some effort would soon be made (in the interest of the social welfare) to prevent early corruption within the close family circle by placing a bar to the hope of marriage. Hence among all peoples there has arisen a natural repugnance to the marriage of brother and sister. Some theologians suppose herein a positive Divine law, but it is not easy to point out any such early Divine enactment. Abraham married Sarah who was his sister by his father, though of a different mother (Genesis 11; cf. Genesis 20:12). Marriage was allowed at Athens with half-sisters by the same father (Plutarch, Cim., iv; Themist., xxxii), with half-sisters by the same mother at Sparta (Philo, De Special. Leg., tr. Yonge, III, 306), and with full-sisters in Egypt (Diodorus Siculus, I, 27) and Persia, as illustrated in the well-known instances of the Ptolemies in the former, and of Cambyses in the latter, country (Herodian, III, 31). For a good summary of non-Christian customs in this respect see Melody, "Marriage of Near Kin" in "Catholic University Bulletin" (Washington, Jan., 1903, pp. 40-60).

In the earlier history of the human race there was a tendency in a family group to keep marriages of its members within the group. Of this we have examples in the marriage of Isaac and Rebecca (Genesis 24) and Jacob and Lia-Rachel (ibid., xxix). We know from Exodus 6:20, that Amram took Jochabed, his father's sister, to wife, and she bore him Aaron and Moses. The Mosaic Law, however, introduced important modifications into the arrangements of marriage or carnal intercourse between near relations by blood as also by affinity; these modifications were founded mainly upon the sharpened instincts of human nature and the importance of guarding against the dangers of corruption from the intimacy of very near relations, which prompted the cutting off all hope of covering past impurity by subsequent marriage. Undoubtedly this danger increased the instinctive natural repugnance to marriage between those connected by the closest ties of blood and family affection. These prohibitions relating to consanguinity, between a man and the "flesh of his flesh", are contained mainly in Lev., xviii, 7-13, and xx, 17, 19. Specific prohibitions are here made with regard to marriage or carnal intercourse with a mother, granddaughter, aunt by blood on either side, sister, or half-sister, whether "born at home or abroad". This expression has generally been understood as equivalent to "in or out of wedlock". Yet, as late as David's time, the language of Thamar towards her half-brother Amnon (2 Samuel 13:13) seems to imply the possibility of their union with consent of their father, perhaps because he was also king (for a contrary opinion see Wernz, Jus. Decretalium, Rome, 1894, II, 634). Some theologians held the daughters of Lot (Genesis 19:30-38) somewhat excusable because they thought that the human race had been swallowed up by fire, and could be continued through their father alone (Kenrick, De Imped. Matr., ch. v, p. 318).

In early Roman times marriage of cousins was not allowed, though it was not infrequent after the Second Punic War. Marriage between uncle and niece was unlawful among Romans. Consanguinity in the direct line, to any extent, was recognized by the Church as an impediment to marriage. Worthy of notice is the declaration by Nicholas I (858-67) in his letter to the Bulgarians, that "between those persons who are related as parents and children marriage cannot be contracted, as between father and daughter, grandfather and granddaughter, or mother and son, grandmother and grandson, and so on indefinitely". Billuart, however, calls attention to the fact that Innocent III, without distinction of lines, allows indiscriminately infidels converted to Christianity to retain their wives who are blood-relations in the second degree. Other theologians take it for granted that this declaration of Innocent III has no reference to the direct line. In the early ages the Church accepted the collateral degrees put forward by the State as an impediment to marriage. St. Ambrose (Ep. lx in P.L., XVI, 1185) and St. Augustine (City of God XV.16) approved the law of Theodosius which forbade (c. 384) the marriage of cousins. This law was retained in the Western Church, though it was revoked (400), at least in the East, by Arcadius, for which reason, doubtless, the text of the law has been lost. The Code of Justinian permitted the marriage of first cousins (consobrini), but the Greek Church in 692 (Second Trullan Synod, can. liv) condemned such marriages, and, according to Balsamon, even those of second cousins (sobrini).

This discipline continued throughout the Church till the eighth century. We then meet with the canon (c. 16, C. 55, q. 2), attributed to various popes and embodied in a letter of Gregory III (732), which forbids marriage among the Germans to the seventh degree of consanguinity. Wernz (Jus Decretal., IV, p. 624), says that at this date so severe a prohibition cannot be based on the canonical computation, but rather on that of the Roman law; it is, therefore, no proof of so early an acceptance by the Church of the Germanic computation. For a fuller exposition of the theory that the canonical computation is borrowed from the Germanic system see Von Scherer, "Handbuch des Kirchenrechts" (Graz, 1898), II, 291, and the excellent exposé of Wernz, "Jus Decretalium", IV, 616-25, especially p. 621, where he sets forth with moderation both the free and original action of the Church in establishing the degrees within which it was forbidden relations to marry and her natural tendency, so often exhibited in other matters, to accept whatever was good or useful in the manners and institutions of newly converted peoples. Von Scherer calls attention (op. cit., II, 296-9) to the influence of the ninth-century Pseudo-Isidore (and the canonical collections based on him, e.g. the "Decretum" of Burchard) in familiarizing the West with the Germanic computation, and says that it does not appear in any genuine papal decretals before Alexander II, and that its exact character is not yet thoroughly ascertained. The Roman canonist De Angelis (Prælectiones Jur. Can., Bk. III, tit. xiv) holds rightly that the computation of degrees was originally the same as that of the Roman civil law for inheritance. He states that in the eleventh century Alexander II (c. 2, C. 35, q. 5) adopted the now usual system of computation, which established for collateral consanguinity the principle that persons were remote from one another by as many degrees as they are remote from the common stock, omitting the common stock (Wernz, however, op. cit., IV, 623, believes that this system, de facto the Germanic computation was adopted at some earlier period, though doubtless not so early as Gasparri maintains). In this way the degrees of relationship were determined by the number of generations on one side only; while in the Roman civil system the number of degrees resulted from the sum of the generations on both sides. In the Roman system (computatio Romana civilis) first cousins would be in the fourth degree, while in the new computation they would be in the second degree of consanguinity. This, as is seen, would extend the impediment of consanguinity.

Some have called the new computation Germanic (computatio Germanica) because it has a similarity to the peculiar Germanic system of determining inheritance, and whose technical terms were borrowed from the seven joints of the body (on both sides) from the neck to the finger-tips. But Santi-Leitner calls attention (ed. 1905, III, 241, against Gasparri) to various discrepancies between the ecclesiastical (computatio canonica) and the Germanic systems which often led the newly-converted Franks and other Germans to oppose the system of the Church. The latter system was more directly connected with the natural relations of marriage, and Alexander II (1061-73) treated it as peculiarly ecclesiastical law (c. 2, C. 35, q. 5) and threatened severely all advocates of a return to the Roman, or civil, calculation. The reception and extension of this severe discipline regarding the impediment of consanguinity came about gradually and by custom, says Wernz, from the sixth and seventh centuries (when first the third and then the fourth degree, i.e. respectively second and third cousins was the limit) to the eleventh and twelfth centuries; in the eleventh century the controversy of St. Peter Damian ("De parentelæ gradibus" in P.L., XLIV, 191 sqq.) with the Roman legists of Ravenna, decided in his favour by Alexander II, helped to fix the popular view in the sense of extreme strictness. It is, however, doubtful whether the sixth and seventh degrees of consanguinity were ever a diriment impediment, at least everywhere. It is not improbable that even the fifth was only a preventive impediment (Wernz, op. cit., IV, 626). While in the twelfth century the theory of the remote degrees was strictly maintained by canonists, councils, and popes, in practice marriages ignorantly contracted within them were healed by dispensation or dissimulation (Wernz, loc. cit.). Finally, in the Fourth Lateran Council (1215) Innocent III restricted consanguinity as a diriment impediment to the fourth degree. He explains that it was found difficult to carry out the extension to further degrees. in those days of imperfect registration it was, of course, often impossible to ascertain the distant degrees of relationship. (For a defence of his illustrative reference to the current theory of the "four bodily humours", borrowed from the ancient physiology, see Santi-Leitner, op. cit. III, 248; cf. Wernz, op. cit., IV, 629.)

Gregory I (590-604), if the letter in question be truly his, granted to the newly converted Anglo-Saxons restriction of the impediment to the fourth degree of consanguinity (c. 20, C. 35, qq. 2, 3); Paul III restricted it to the second degree for American Indians (Zitelli, Apparat. Jur. Eccl., 405), and also for natives of the Philippines. Benedict XIV (Letter "Æstas Anni", 11 Oct., 1757) states that the Roman pontiffs have never granted dispensation from the first degree of collateral consanguinity (brothers and sisters). For converted infidels it is recognized that the Church does not insist upon annulment of marriages beyond this first degree of consanguinity. (For further details of the history of ecclesiastical legislation concerning this impediment see Esmein, "Le mariage en droit canonique", Paris, 1891, I, 335-56; II, 258, 345; Santi-Leitner, op. cit. below, 247-48; and Wernz, "Jus Decretal", II, 614 sqq.)

The Church was prompted by various reasons first to recognize the prohibitive legislation of the Roman State and then to extend the impediment of consanguinity beyond the limits of the civil legislation. The welfare of the social order, according to St. Augustine (City of God XV.16) and St. Thomas (Suppl. Q. liii, a. 3), demanded the widest possible extension of friendship and love among all humankind, to which desirable aim the intermarriage of close blood-relations was opposed; this was especially true in the first half of the Middle Ages, when the best interests of society required the unification of the numerous tribes and peoples which had settled on the soil of the Roman Empire. By overthrowing the barriers between inimical families and races, ruinous internecine warfare was diminished and greater peace and harmony secured among the newly-converted Christians. In the moral order the prohibition of marriage between near relations served as a barrier against early corruption among young persons of either sex brought habitually into close intimacy with one another; it tended also to strengthen the natural feeling of respect for closely related persons (St. Thomas, II-II.154.9; St. Augustine, City of God XV.10). Nature itself seemed to abhor the marriage of close kin, since such unions are often childless and their offspring seem subject to grave physical and mental weakness (epilepsy, deaf-muteness, weak eyes, nervous diseases), and incur easily and transmit the defects, physical or moral, of their parents, especially when the interbreeding of blood-relations is repeated (Santi-Leitner, op. cit., IV, 252; Huth, "The Marriage of Near Kin, considered with respect to the Law of Nations, the results of Experience and the teachings of Biology", London, 1875; Surbled, "La morale dans ses rapports avec la médecine et l'hygiène", Paris, 1892, II, 245-55; Eschbach, "Disputat. physiologico-theolog.", 99 sqq.; Luckock, "The History of Marriage, Jewish and Christian, in relation to divorce and certain forbidden degrees", London, 1894; Esmein, "Le mariage en droit canonique", Paris, 1891, I, 337, sqq.; see also Wernz, op. cit. IV, 686-37, and the Encyclical of Gregory XVI, 22 Nov., 1836).

In calculating the degree of consanguinity special attention must be paid to three things, the line, the degree, and the stock or root. The stock, or root, is the common ancestor, or the person, male or female, from whom descend as from the nearest common bond the persons whose blood-relationship is to be determined. The degree is the distance of one person from the other in regard to blood-relationship. The line is the classified series of persons descending from the common stock through one or more generations. The line is direct when the series of persons descend one from the other, as father and son, grandfather and grandchild. The line is transverse, or collateral, when the blood-relations spring from a common stock, yet do not descend one from the other but form different branches side by side, as two brothers, two nephews. This collateral line is equal or unequal according as these persons derive equally or unequally from the same stock or root. The blood-relationship is computed according to the distance from the stock whence it is derived, and this is the rule by which the degrees or steps of consanguinity are determined.

In the direct line the Roman civil and the canon law agree on the principle that there are as many degrees as generations; hence as many degrees as there are persons, omitting the stock or root. A son is one degree from his father, a grandchild two degrees from the grandfather. In the computation of the degrees of the transverse or collateral line there is a serious difference between the Roman civil and the canon law. The civil law founded its degrees upon the number of generations, the number of degrees being equal to the number of generations; thus between brothers there are two degrees as there are two generations; between first cousins four degrees, corresponding to the four generations. The degrees are calculated easily in the civil law by summing up the number of persons in each line, omitting the common ancestor. Except for marriage, the canon law follows regularly the computation of the civil law, e.g. in the question of inheritance. But the Canon law, in the collateral line of consanguinity, computes for marriage one series only of generations, and if the series are unequal, only the longer one. Hence the principle of canon law that in the transverse or collateral line there are as many degrees of consanguinity as there are persons in the longer series, omitting the common stock or root. If the two series are equal, the distance is the number of degrees of either from the common stock. Thus brother and sister are in the first degree, first cousins in the second degree; uncle and niece in the second degree because the niece is two degrees from the grandfather who is the common stock. Thus if Caius has two sons, Titius and Sempronius, and Sempronius has a son and grandchild, the relationship of the grandchild of Sempronius to Titius is in the third degree, because this grandchild is distant three degrees from the common stock, Caius. This rule holds if the common stock should only be one person; thus half-brothers and half-sisters, that is from either father or mother, are in the first degree. Children of the same father and mother are called german, as from the common germ; those of the same mother and not of the same father are called uterine, as from the same womb; and children of the same father and different mother are called blood-children. The legitimacy or illegitimacy of any member of the series does not modify the relationship as a bar to marriage.

For civil effects the civil law's computation of degrees must be known. In most European countries the law follows mainly the computation of the Roman civil law. In England, since the Reformation, the Levitical law has been recognized as the standard by which to determine the prohibitions of marriage. For Catholics everywhere, as Alexander II decreed (c. 2, C. 35, q. 5), the ecclesiastical calculation (computatio canonica) must be followed for the direct question of the lawfulness of marriage. Clement V, in the Council of Vienne (1311), decreed that any one who knowingly contracted marriage within the forbidden degrees should by the fact incur excommunication, though not reserved; this penalty has ceased since the Bull "Apostolicæ Sedis" of Pius IX (1869). The Council of Trent (1563) required the absolute separation of those who knowingly contracted marriage within the prohibited degrees, and denied all hope of obtaining a dispensation, especially if the attempted marriage had been consummated. But in this regard the practice of the Church, probably on account of the recognition of such marriages by the State, and the consequent difficulty of enforcing the dissolution of illicit unions, has tended towards greater leniency. The Council of Trent, it is true (Sess. XXIV, c. v, De ref., matr.), made no changes in the existing legislation, despite the wishes of many for a reduction of the limits of the impediment (Theiner, Acta Conc. Trid., Leipzig, 1874, 336, 342). Such reduction would in all probability have been discussed at the Vatican Council (1870), had it not been interrupted (Lämmer, Zur Codification des can. Rechts, Freiburg, 1899, 137, sqq., and Martin, Coll. docum. Conc. Vat., p. 162 sqq.).

In the Eastern Churches, the marriage of blood-relations is forbidden in the collateral line to the seventh civil degree, 1. e. second cousins touching third, but in that degree is only preventive, not diriment (Wernz, IV, 627). Among the Italo-Greeks, however, the Maronites, and the Syrians the legislation of the Roman Church obtains (Benedict XIV, Etsi Pastorails, 26 May, 1742; Synod of Mount Lebanon, 1736; Synod. Sciarf. Syror., 1888). In the schismatic Churches of the East all marriages of relations in the direct line are prohibited; in the collateral line the seventh (civil) degree is the limit of prohibition; the remotest degree, however, is only a preventive impediment. In the National Greek Church, since 1873, marriage is forbidden within the sixth (civil) degree, i.e. second cousins; in Russia, since 1870, within the fourth (civil) degree, i.e. first cousins (cf. Zhishman, Eherecht d. oriental. Kirche, Vienna, 1864, and Milas, Das Kirchenrecht der morgenländ. Kirche, Mostar, 1897).

Whatever dispensing power is available resides principally in the supreme authority of the Church, namely the Apostolic See. The pope generally exercises his power of dispensing through the Roman Congregations. For public dispensations (in foro externo) the Dataria (see ROMAN CURIA) is the ordinary medium for so-called Catholic countries; the Sacra Penitentiaria for cases of conscience (occult impediments) and of late for the cases of the poor. The Congregation of Propaganda is the medium for countries dependent on it, e.g. Great Britain and its dependencies and the United States. This power of dispensation with the right to subdelegate is often delegated to bishops, vicars Apostolic, and others having pastoral authority over souls. In whatever is forbidden by the law of nature there is no dispensation. In the direct line of consanguinity Nicholas I supposes that there is no room for dispensation. However, in cases of infidels when one or both are converted, while it is to be held that marriages within the first degree of the direct line are invalid, in all others the Holy See has to be consulted. The Holy See has the supreme right in doubtful cases to determine what may or may not be forbidden by the law of nature or by the Divine positive law. Benedict XIV, as already said, emphasized the fact that the popes had never granted a dispensation for a marriage between brother and sister, even where the union might have occurred without a knowledge of the relationship on the part of the contracting persons.

Consanguinity may be duplicated as arising from two sources: first, from two roots, e.g. two brothers marrying two women who are cousins; the children of each brother will be related to those of the other in the second degree on the father's side, and in the third degree on the mother's side; second, from one root, but when the descendants intermarry. hence, where there is a double consanguinity, there is a double impediment which must be expressed in the petition for dispensation; and should there be a more extensive duplication by still further intermarriages, all the forbidden degrees resulting from the blood-relationship should be mentioned in seeking dispensation. In the petition for dispensation, both series in the collateral consanguinity must be mentioned, though this is not necessary for validity of the dispensation. A special proviso is made when dispensation is sought from collateral consanguinity. It must be mentioned, even for validity, if the one part is next of kin to the root or common ancestor and the other within the forbidden degrees; the sex of the next of kin should also be mentioned, because of the greater difficulty of the dispensation for a nephew to marry his aunt. If the farthest should be in the fifth degree, there is even in that case no prohibition of marriage. The impediment of marriage arises also from any carnal intercourse, even outside of marriage, to the fourth degree of consanguinity. To consanguinity within the prohibited degrees may be added the gravamen of the crime of incest. If the incest were committed in the hope of facilitating the grant of a dispensation, this circumstance must be mentioned in the petition for dispensation; mention is also required if an attempt at marriage had been made, even if not consummated.

In the Eastern Church the Quinisext Council (692) forbade, as we have seen, marriages between first cousins. In the eighth century Emperors Leo and Constantine confirmed this decree and forbade alliances between persons in the sixth degree of consanguinity according to the computation of the Roman civil law, i.e. between the grandchildren of brothers and sisters, and still later in the seventh degree of the same computation. This holds today in the Greek Church. The question of consanguinity is important in determining civil rights, which are mainly under control of the State, though illegitimacy often produces ecclesiastical disbarments (see BIRTH). The hindrances to marriage based on consanguinity vary considerably in different States. In Germany consanguinity is a bar only in the direct line, and between brothers and sisters. In France uncle and niece, aunt and nephew, are forbidden to intermarry, but dispensation may be granted by the head of the State. The prohibition does not extend to this relationship arising from an illegitimate union. Even in the most conservative Catholic countries, there is a tendency to limit the impediment of consanguinity. In England the statutes of Henry VIII, repealed in part by Edward VI and wholly by Phillip and Mary, were revived in Elizabeth's first year, the provision being that "no prohibition, God's law except, shall trouble or impeach any marriage outside Levitical law". The ecclesiastical interpretation was that consanguinity was an impediment to marriage as far as the third degree of civil computation. A man might not marry his aunt, or his niece, but might marry his first cousin. Relationship by the half-blood was put on the same footing as the full-blood, and illegitimate consanguinity was treated as equivalent to legitimate blood relationship. The courts regarded marriages within the forbidden degree as voidable rather than void, but such marriages were declared void by an act of 5 and 6 William IV (1835). In the United States all the States prohibit marriage between lineal descendants; most of them prohibit marriages between uncle and niece, nephew and aunt, and between first cousins (Desmond, The Church and the Law, Chicago, 1898, C. X).

We subjoin a genealogical table which exhibits the various degrees of consanguinity according to a custom in use in the Western Church since the seventh century (Isidore of Seville). This will be a useful guide in determining the extent of the impediment of affinity. Affinity from a true marriage is a diriment impediment to the fourth degree of consanguinity of the deceased spouse; according to the ecclesiastical law a widower may not marry any of his deceased wife's blood-relations as far as the fourth degree inclusively, nor a widow her deceased husband's blood-relations. There is a modification if the affinity be one arising from illicit intercourse.