Interbreeding and marriage

I’m assuming men are not allowed to marry their mothers or sisters; what about first cousins, aunts, etc.? And if that is disallowed, what was the reason for this? Were ancient Catholics aware of the biological dangers of inbreeding?

I’m asking because I’m curious about the medieval monarchs that inbred to dangerous degrees, and what the Church said about them.

Here’s an article on consanguinity which can give you historical information:

newadvent.org/cathen/04264a.htm

The current canons are:

Can. 1091 §1. In the direct line of consanguinity marriage is invalid between all ancestors and descendants, both legitimate and natural.

§2. In the collateral line marriage is invalid up to and including the fourth degree.

§3. The impediment of consanguinity is not multiplied.

§4. A marriage is never permitted if doubt exists whether the partners are related by consanguinity in any degree of the direct line or in the second degree of the collateral line.

Can. 1092 Affinity in the direct line in any degree invalidates a marriage.

Can. 1093 The impediment of public propriety arises from an invalid marriage after the establishment of common life or from notorious or public concubinage. It nullifies marriage in the first degree of the direct line between the man and the blood relatives of the woman, and vice versa.

Can. 1094 Those who are related in the direct line or in the second degree of the collateral line by a legal relationship arising from adoption cannot contract marriage together validly.

In the collateral line, cousins for example, the impediment can be dispensed. In the direct line, it cannot.

Canon Law…and this probably requires further definition!

catholicdoors.com/misc/marriage/canonlaw.htm

Canon 1091.1 Marriage is invalid between those related by consanguinity in all degrees of the direct line, whether ascending or descending, legitimate or natural.

Canon 1091.2 In the collateral line, it is invalid up to the fourth degree inclusive.

Canon 1091.3 The impediment of consanguinity is not multiplied.

Canon 1091.4 A marriage is never to be permitted if a doubt exists as to whether the parties are related by consanguinity in any degree of the direct line, or in the second degree of the collateral line.

Canon 1092 Affinity in any degree of the direct line invalidates marriage.

Further explanation in the Catholic Encyclopedia

newadvent.org/cathen/04264a.htm

This site explains consanguinity more explicitly

albrojournal.com/consang.asp

Okay, thank you for the help. God bless.

The rule presently contained in the canon law is:
Can. 1091 §1. In the direct line of consanguinity marriage is invalid between all ancestors and descendants, both legitimate and natural.

§2. In the collateral line marriage is invalid up to and including the fourth degree.

§3. The impediment of consanguinity is not multiplied.
A degree of consanguinity (for purposes of the 1983 Code, though I think it was reckoned differently before that) is each step up or down the family tree that is needed to connect two people through a common ancestor. Thus a man and his aunt are related in the third degree of consanguinity: one step from the man up to his parent (the aunt’s sibling), one step up to the grandparent (the common ancestor), and one step back down to the aunt.

Now, the impediment can be dispensed (this happens sometimes for cousins, and I think almost never for uncle/niece or aunt/nephew) under Canon 1078:
Can. 1078 §1. The local ordinary can dispense his own subjects residing anywhere and all actually present in his own territory from all impediments of ecclesiastical law except those whose dispensation is reserved to the Apostolic See.
However:
§3. A dispensation is never given from the impediment of consanguinity in the direct line or in the second degree of the collateral line.
So people in the direct line (mother/son, grandfather/granddaughter) or in the second collateral degree (brother/sister) can never marry.

Those are the current rules. You may benefit from reading this old Catholic Encyclopedia article on the history of consanguinity in canon law.

I will be quite frank in this post, I am being utterly lazy, it’s been one hell of a week! Can anyone quickly post an answer, for my benefit:D, to the OP’s question, save me going through all the links in previous posts. Why the prohibitions before the biological dangers were known? I’ll hazard a guess - morality; because even today that’s what would shock people, not oh dear are you aware of the genetic consequences.

I think there’s a natural replusion to marriage/sexual relations within close family relations because it deforms the family unit. Intuitively we know that this is harmful to a healthy, functioning society. A strong society is built from strong family bonds with intact relationships.

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