In addition to the general incapacities to marriage there are also prohibitions against marriage with certain people. The most important of these restrictions are those prohibiting marriages between relatives within certain degrees.

Incest is sexual intercourse (either with or without the assumption of the marriage relation) between persons within certain degrees of consanguinity.

Incestuous marriages are void. In Illinois marriages are incestuous when contracted between ancestors and descendants of any degree, between aunts and nephews, uncles and nieces and first cousins. The laws of the other states on this subject present some differences but are in the main very similar.18 At Common law marriage between first cousins were permitted, but on the other hand a man was forbidden to marry relations of his deceased wife within the same degrees that he was forbidden to marry his own relatives.

17 For a further treatment of Bigamy, see Subject of Criminal Law. Vol. X, Subj. 30.

In computing the degrees of relationship within which marriage is forbidden, bastards and relatives by the half-blood are considered the same as if they were legitimate relatives of the full-blood.

The validity of marriage between parties related in the degrees, as to the validity in which there is a considerable variance among the laws on incest of the different states, will be determined by the law of the place where the marriage was entered into, but marriages within the more closely connected degrees will not be recognized in other places, even if valid under the laws of the place where the marriage was contracted. This question is carefully discussed in the case of Sutton vs. Warren:19

"It is a well settled principle in our law, that marriages celebrated in other states or countries, if valid by the law of the country where they are celebrated, are of binding obligation within this Commonwealth, although the same might, by force of our laws be held invalid, if contracted here. This principle has been adopted, as best calculated to protect the highest welfare of the community in the preservation of the purity and happiness of the most domestic relations in life. There is an exception, however, to this principle, in those cases where the marriage is considered as incestuous by the law of Christianity, and as against natural law. And these exceptions relate to marriages in the direct lineal line of consanquinity, and to those contracted between brothers and sisters; and the exceptions rest on the ground, that such marriages are against the laws of God, are immoral, and destructive of the purity and happiness of domestic life. But I am not aware that these exceptions by any general consent among writers upon natural law, have been extended further, or embraced other cases prohibited by the Levitical law. This subject has been carefully discussed by Chancellor Kent, in the case of Wightman vs. Wightman, 4 Johns, Ch. 343; and while he is clear as to the exceptions before stated, he thinks, beyond them there is a diversity of opinion among commentators. 2 Kent Com., Lect. 26. See also Story's Conflict of Laws, Secs. 113, 114. There is also a provision in our statute, making marriages void in this state, where persons resident in the state, whose marriage, if solemnized here, would be void, in order to evade our law, and with the intention of returning to reside here again, go into another State or country and there have their marriage solemnized. Rev. Sts., Ch. 75, Sec. 6. The only object of this provision is, as stated by the commissioners in their report, to enforce the observance of our own laws upon our own citizens, and not to suffer them to violate regulations founded in a just regard to good morals and sound policy. As to the wisdom of this provision it is unnecessary here to speak. But the provision is noticed to show that it has not been overlooked in the consideration of the case at bar, which presents no such state of facts. In view of the whole matter, considering it as a part of the jus gentium, we do not feel called upon to extend the exceptions further. By our statutes, the marriage contracted between Samuel Sutton, the plaintiff, and Ann Hills, his mother's sister, if celebrated in this State, would have been absolutely void. But by the law of England, this marriage, at the time it was contracted, viz., in November, 1834, was voidable only, and could not be avoided until a sentence of nullity could be obtained in the spiritual court, in a suit instituted for that purpose. See Poynter on Marriage and Divorce, 86, 120; 2 Stephen's Com., 280. In The Queen vs. Inhabitants of Wye, 7 Adolph & Ellis, 771, and 3 Nev. & P., 13, the Court of Kings' Bench affirmed this doctrine, and held such a marriage voidable only, and that, till avoided, it was valid for all civil purposes. Rose. Crim. Ev. (2nd Ed.), 286. Since this marriage was contracted, the St. of 6 Wm. 4, c. 54 has been passed, making such marriages which should afterwards be celebrated, absolutely void. In the present case, the marriage of these parties was not void by the laws of England, and though absolutely prohibited by our laws, yet not being within the exception, as against natural law, we do not feel warranted in saying the parties are not husband and wife. The plaintiff, Samuel Sutton, sues on a promissory note given to the said Ann Sutton, and, as her husband, he can maintain an action thereon, in his own name alone, there being no other cause of objection raised than the one stated in regard to the legality of their marriage."

18 Consult, statutes of particular state.

19 10 Metc. (Mass.), 451.