A professor of religious studies at Stanford University, Arnold M. Eisen has studied Judaism for 25 years. But he worked in an academic capacity, "without ever getting inside the heads of ordinary Jews."

However, with his new book, "The Jew Within — Self, Family, and Community," Eisen was able to do just that. "Now I think I understand them a lot better than I did before," he said.

Co-authored with Steven M. Cohen, associate professor at the Melton Centre for Jewish Education at the Hebrew University of Jerusalem, "The Jew Within" offers a sociological look at the how and why of observance among moderately affiliated American Jews.

By "moderately affiliated," the authors mean Jews who belong to a synagogue or Jewish organization, but are not among its most active members. The majority of American Jews fall into this category, Eisen noted, adding that 80 percent of them will affiliate with some Jewish organization in their lifetime.

To collect the data, researchers conducted two lengthy interviews with 50 Jews from around the country.

"Multiple choice and 'yes or no' questions don't get you very far in understanding what people are about," Eisen said. "We wanted to find out what's going on inside the heads of people. We didn't want to know 'Do you fast on Yom Kippur,' but 'Why?'"

One major finding illustrated the ways Jews tend to express their Judaism. Whereas in prior generations it used to be mostly in the public sphere, including in synagogue or organizational affiliation, today that emphasis has shifted more to the home.

This explains why Passover is the most widely observed holiday. "If you have a successful ritual at home which transmits identity, the Jews will go for it," Eisen said.

Eisen noted that there has been an increase in Shabbat observance. "In this generation, people lead harried lives and don't often eat together," he said. "So the idea of eating together as a family has more significance."

In another family-related finding, Eisen said his sources likely received most of their formative Jewish memories from their grandparents, saying that they passed down more of a sense of Jewish tradition than their own parents.

Among Jews, as among other Americans, religious observance today is very much a matter of personal choice, Eisen discovered.

"Jews are much more protective of their autonomy and options," he said. "They are more individualistic than people were just a generation ago."

Even those Jews who go to synagogue every week, Eisen said, make that decision each time whether to go.

"The self is sovereign," he said. "They reserve the right to decide every day or week or year what they're going to do, if they're going to be Jewish and how."

Eisen was not surprised that those surveyed felt less attached to Israel than in prior generations, with only 30 percent saying they felt "very or extremely attached."

While most of those interviewed tended to feel positive about their synagogues, and the majority reported believing in some kind of God, Eisen said they don't necessarily associate God with going to synagogue.

He found it surprising that Jews went to synagogue mostly for a sense of tradition, for experiencing community, and for quiet time and introspection.

Eisen said the findings could be useful for Jewish professionals who work in outreach, because it shows what kinds of things Jews are likely to get involved in.

People respond to things like adult education, he said, because it "relates to them as smart, thinking, autonomous people. What they don't like is people making them feel guilty, and telling them they're bad Jews."

What surprised Eisen most while doing this book was that sources were willing to speak in great depth about their Jewishness.

"Sometimes, people didn't want the interview to end," Eisen said. When it comes to things like belief in God, "people will talk to an interviewer sometimes more than they'll talk to their spouse."

He found this one of the more positive aspects of his research.

"People are not neutral about being Jewish; there's no indifference about it," he said. "They care very much. Sometimes it leads them to reject it, but at least they care."