For his part, McCarthy never fails to be impressed by his wife's combination of a high-powered intellect with a gift for putting others at ease.

"When people find out that Tracy is a surgeon, they're always surprised," he says. "She's not pretentious. That's the last thing she is. She's very gracious and welcoming."

The authors say that this example demonstrates how couples friendships differ from similarly strong connections between individuals.

"Individual friendships can be more self-centered experiences," the authors write.

"Although they hold enormous value, they are different than what happens when one couple shares an experience with another couple. In the sharing, the partners in a couple experience not only the other couple, but also each other."

Deal and Greif divided their couples into three categories: "seekers," like Ishman and McCarthy, who are eager to establish new couple relationships; "keepers" who nurture their existing couples friendships but aren't looking to form new quartets; and "nesters," introverts who are content to remain mostly within their home circle.

The authors say it's not unusual for a seeker to be wed to a keeper or even a nester, and vice-versa. When that happens, complicated negotiations can ensue.

For instance, McCarthy identifies himself as a nester, while Ishman, who works between 60 and 70 hours a week, is a seeker. But McCarthy has more time and opportunities than his wife to initiate new friendships.

"People come up and introduce themselves when I'm walking in the neighborhood with Ben and David ," he says."So, I'm usually the one who gets to know the family first and sets up a date for us all to get together."

But even when couples do get acquainted, they may have different expectations about how to spend their time together.

Greif and Deal say that couples friendships tend to be characterized primarily by either "fun-sharing" or "emotion-sharing."

About two-thirds of their research subjects prefer the former pattern of interaction. For these couples, group get-togethers are a chance to play cards or golf or to attend a concert.

The remaining third also enjoy group outings, but they will also bounce problems off their mutual friends.

The authors point out that even those who prefer a fun-sharing style with other couples will have individual friends with whom they engage in regular heart-to-hearts. And among even the most tightly knit foursomes, some topics are easier to discuss than others.

"We live in a society where people can be very close with another couple, but they still don't talk about sex and money," Greif says. "These topics are off the table, and continue to be taboo with even your best friends."

But as "Two By Two" makes clear, different couples declare different conversational areas off-limits.

For example, Ishman and McCarthy met when both were living in Chicago. Both brought long-standing friendships with couples into their marriage. Ishman vividly recalls a beach vacation that involved four or five pairs of her and her husband's mutual friends, and which included a no-holds-barred discussion about family planning and sex.

In addition, as a physician who counsels residents as they are embarking on job searches, she's also comfortable conversing about money.

But Ishman draws the line at discussing family relationships with friends. If she were ever to have a serious disagreement with her parents, for example, she doubts she'd talk about it in a couple setting.

"To me, that's more personal," she says. "I don't think I would talk about that with very many people."