A recent landmark study of Americans’ religious behavior confirmed what many observers of interfaith marriage have often suggested, but never proven: when Jews have interfaith marriages, they disproportionately marry Catholics.

According to the U.S. Religion Landscape Survey conducted by the Pew Forum on Religion & Public Life, 39% of interfaith married Jews are married to Catholics, even though Protestants outnumber Catholics in the U.S. by nearly two to one. Only 23% of interfaith married Jews are married to Protestants. Overall, slightly less than a third of all married Jews are in interfaith marriages.

“This is something that everyone has known for years,” says Rabbi Arthur Blecher, who noted the trend in last year’s The New American Judaism: The Way Forward on Challenging Issues from Intermarriage to Jewish Identity. “Social workers, rabbis, and certainly Jews. No one knows exactly why. It is not as though the Jews are saying ‘Gee, I would like to marry a Catholic…'”

While no one has formally studied the question, sociologist Steven Cohen, a professor at New York’s Hebrew Union College-Jewish Institute of Religion, boils the phenomenon down to one word: Geography.

“Jews are concentrated in the Northeast, and so are Catholics,” he says. “People tend to marry people they encounter. I can’t say that the Jews have any special affection for people who are Catholic.”

Melissa and Karl Simon of Reston, Va., are a case in point. Melissa, who is Jewish, and Karl, who is Catholic, met more than 20 years ago, when they lived near one another during high school near Providence, R.I. They married in 1990.

Even with different religious backgrounds, “I think our families had the same values,” says Melissa Simon.

Since every family has its own traditions, structure and quirks, it is hard to say what similarities certain couples find in their backgrounds, says psychologist Joel Crohn, author of the book Mixed Matches

However, in many cases, Jewish families and Catholic families have an emphasis on religious rituals such as attending services, hosting holiday dinners and saying prayers. So while the actual God may be different, the role of that God may provide a similar structure for both Jews and Catholics, says Crohn.

“I would be interested how to see how many Jews who marry Catholics attend synagogue,” says Crohn. “For some couples, it is that religiosity that some people see as a conflict, but it often it is a bridge.”

Traditional proximity between Jews and Catholics is a result of parallel immigration patterns, explains Rabbi Blecher. A majority of the Jews–as well as a good number of the Catholics–in the United States are descendants of European immigrants who came to the United States in the early part of the 20th Century.

“Socioeconomically, the groups were identical, even a few years down the line,” he says. “My grandparents lived in a neighborhood where you spoke Yiddish or Italian. The neighborhoods had blue-collar refugees from crowded European cities coming to port cities. They lived in the same tenements and went to the same schools.”

Even though the children, grandchildren and great-grandchildren have moved away from the immigrant neighborhoods, change happens slowly, Rabbi Blecher says.

“It takes more than a few generations to change patterns, whether it is voting or marriage,” he says. What has changed through the generations, though, is the general acceptance of interfaith marriage in most families, says Rabbi Blecher. “Fewer people want to alienate their children over interfaith marriage.”

Some of the same challenges remain, however. The big one is always going to be how to raise the children. Rabbi Blecher says that most of the couples he sees are deciding to elect a Jewish identity or a hybrid identity.

“When couples decide on a specific religion, it is more often that of the Jewish partner,” says Rabbi Blecher. “What is rare is to see children being entirely raised as non-Jews in this case. It doesn’t matter whether it is the father or the mother is Jewish.

“What I see a lot is Christian women raising Jewish kids,” says Rabbi Blecher. “Sometimes I see non-Jewish women pressing their Jewish spouses to provide a Jewish life.”

The Simons have bucked the trend. They are raising their two sons, ages 11 and 8, as Catholic, although Melissa Simon still considers herself Jewish.

“We still light the menorah and make latkes at Hanukkah,” says Melissa Simon, 44. “But the boys go to Sunday school at a Catholic Church. Before they were born, I read a bunch of books and decided that kids need to have one identity. I thought it was better to be Catholic with a Jewish mother, than to be a little of each.”

The Center for Applied Research in the Apostolate (CARA) at Georgetown University recently studied Catholics and marriage. In interviews with 1,000 Catholics of various ages, the group found that 72 percent of married Catholics have a Catholic spouse. However, only 31 percent of never-married Catholics say it is somewhat important or very important their spouse is Catholic.

Meanwhile, some experts warn not to read too much into the Pew study. Of the 35,000 interviewees, less than 700 were Jews.

Sylvia Barack Fishman, a professor of contemporary Jewish life at Brandeis University and author of the book Double or Nothing? Jewish Families and Mixed Marriage, says the study also did not separate ages of respondents. While the geography theory might have been true a generation ago, it’s not necessarily the case today, she says.

“The study said nothing about ages or even most recent marriages, such as fewer than 10 years,” she says. “If you look at younger people, I do think you would get a different stat concerning Jews marrying Catholics. For instance, the rate of Jews marrying Asian-Americans is much higher than we saw generations ago. They tend to meet in graduate school.”

Immigration patterns, technological connections, educational goals and changing demographics have certainly had an effect on whom people meet and marry. Consider this statistic from the Pew survey: nearly half the Hindus, one-third of the Jews and a quarter of the Buddhists have obtained post-graduate education, compared with only about 1-in-10 of all U.S. adults.

That considerably changes the makeup of places such as affluent communities, private schools and graduate schools.

In fact, if you study Jewish culture and Asian American culture, you will find similarities today similar to the European immigrants of the early half of the 20th Century, she says.

“Asian-Americans have an affinity for traditional, ancient religious cultures,” Barack Fishman says. “They place a lot on the wisdom of age and experience. I have never met someone who read the Joy Luck Club and didn’t think it was about Jews.”

No matter what the non-Jewish spouses’ cultural and religious background, there needs to be discussion and planning rather than disagreement, says Rabbi Blecher.

“Interfaith marriage is here to stay,” he says. “If you want to maximize our heritage and survival, we need to stop angsting over it.”