Must first cousins be forbidden to marry? In the Bible, and in many parts of the world, the answer is no. But the answer is yes in much of church law and in half the United States.

This issue became news when the April issue of the Journal of Genetic Counseling said risks have been exaggerated for serious birth defects, retardation or genetic diseases among children of first-cousin marriages.

Generally, an unrelated couple has a 3 percent to 4 percent risk of having a child with such problems, while marriages of close cousins add 1.7 percent to 2.8 percent to the risk. Genetic problems are considerably higher with the forms of close inbreeding that the Bible forbids and secular culture abhors as "incest."

First cousins cannot marry under the age-old laws of the Roman Catholic and Eastern Orthodox churches, covering much of world Christendom.

But in the Reformation, the Church of England followed Protestantism's "sola scripture" (Scripture alone) principle and returned to biblical law, which also binds traditional Jews.

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Under Queen Elizabeth I, Anglicanism decreed that "no prohibition, God's law except, shall trouble or impeach any marriage outside Levitical law." Two dozen biblical prohibitions were listed in the "Table of Kindred and Affinity" in the Book of Common Prayer.

This "Levitical law" is found in Leviticus 18:6-18, supplemented by Leviticus 20:17-21 and Deuteronomy 27:20-23. Among the forbidden couples are parent-child, sister-brother, grandparent-grandchild, uncle-niece, aunt-nephew, and between half siblings and certain close in-laws.

A noteworthy biblical exception was "levirate marriage." A man was forbidden to marry his brother's wife except when the brother died without leaving a child, in which case the surviving brother was urged to marry the widow although he had the option to refuse (Deuteronomy 25:5-10).

There were early examples in Genesis of couples that would later be forbidden under the Law of Leviticus, for instance Abraham's marriage to his half sister Sarah.

The New Testament's single passage on incest (1 Corinthians 5:1-2) condemned a man "living with his father's wife," apparently meaning his stepmother, which came under the Leviticus bans and was penalized with excommunication.

Why limit marriage partners? The Bible simply says marriage with a "near kinsman" or "near kinswoman" is wrong, expressing a near-universal aversion in human cultures.

St. Augustine, in the early fifth century, and the later St. Thomas Aquinas developed a sociological explanation: The welfare of society requires the widest possible bonds of friendship and love, making marriage outside the family desirable. The resulting unification of tribes reduces warfare and fosters social peace.

These writers also said that prohibition of close kin marriages strengthened a healthy feeling of respect in the family circle and helped prevent sexual corruption of children by those living within households.

Later theologians added scientific arguments, saying natural law appeared to abhor close intermarriage due to the increased physical and mental defects that result from repeated inbreeding, as the new genetic study indicates.