On countless weekend nights growing up I chose between taking the M79 home to my parents' apartment and asking Jeff's mom if I could sleep on the trundle bed. The M79 bus, which connects the Upper West Side of Manhattan, where Jeff lived, to the Upper East, where I did, rarely comes after midnight, and in the manner of all public transportation, is a little depressing at odd hours. The trundle came with its own set of hazards.

Jeff was my closest friend. Because he had a curfew and I did not, we'd often end up at his place, watching movies to the point of exhaustion. There was nothing between us; it was a strictly platonic relationship. Still, it was embarrassing to ask his mom if I could spend the night, and despite the Platonism, a little uncomfortable to stare up at the ceiling side-by-side, him on the bed, me on the trundle—to say nothing of needing to borrow an old pair of shorts so that I wouldn't have to sleep in jeans. I ended up staying over only twice.

Beneath the fluorescent lighting of the M79, surrounded by drunk people, it sometimes occurred to me that, if I had a female best friend, there'd be no trundle/bus dilemma. Nor would I have to contend with the suspicions occasionally leveled against us.  We were a physical match: both short, curly-haired Jews, and there were some who wondered if we were secretly dating—which is part of why it seemed safer to take the M79.

We were sure that we would never become romantic partners, that our relationship would always be placidly sexless. This has so far borne out: Excluding the summer when we first met and shared an awkward, pubescent kiss on Independence Day—and another, even more awkward moment on a trampoline shortly thereafter—there's been no romance. Jeff and I have been friends for more than 14 years, without interruption. In our mid-twenties, we lived together for more than three years, during which period we'd watch movies late into the night and then go our separate ways, much like when we were kids. I find all this, at the personal level, unremarkable and unsurprising; the skepticism of outsiders strikes me as funny and narrow-minded. Yet from a historical perspective, my blasé attitude is all wrong: We are remarkable, in a way, and our relationship is not only surprising but radical.

That Jeff and I are friends and nothing more has as much to do with the era in which we live as with our personalities. Summarizing the scholarly consensus, the critic William Deresiewicz has written that "friendship between the sexes appears to have been nonexistent before the 19^th century" and did not become a "widely accepted social possibility" until the 20^th. Just two generations ago, the institutions that incubate friendship—universities, workplaces, even summer camps (where Jeff and I met)—were still segregated by sex. The college I attended, for example, began admitting women in 1969. And only in the 1970s did employment open up at a rapid clip.   If Jeff and I had been born in, say, 1923 instead of 1983, we might never have had the chance to develop a nonromantic attachment.

There's evidence that cross-sex friendships, once extremely rare, are increasing along with structural opportunities. In 1981, sociologist Rebecca G. Adams surveyed 70 female senior citizens and found that all but three defined cross-sex friendships as courting relationships. (They were confused by the very concept of a platonic relationship between a man and a woman.) Just over two decades later, in 2002, the magazine American Demographics commissioned a far more sweeping, national survey on friendship and got dramatically different results. It found that more than 1 in 10 adults aged 25 to 34 have a best friend—not just a pal or an acquaintance—of the opposite sex.

My own survey of nearly 600 Slate readers, conducted last winter and spring, also suggests an upward trajectory. I asked respondents to approximate what percentage of their friends had a close but platonic cross-sex relationship. The average for teens was 73 percent; for twentysomethings, 59 percent; for thirtysomethings through fiftysomethings, about half; and for sixtysomethings, 24 percent. The trend could be taken to mean that younger (unmarried, unattached) people are more likely to form cross-sex friendships in any era. But it could also reveal something about the time in which we live: Maybe it's much easier to make cross-sex friends now than it's ever been.

And yet, the academic literature on cross-sex friendship is spare, dwarfed by studies of same-sex pairs—there has been a significant lag between the emergence of this relationship as a widespread phenomenon and substantive analysis of it. The culture, highbrow and pop, is just as dismissive. Mr. Duffy's assertion in James Joyce's A Painful Case, that "friendship between man and woman is impossible because there must be sexual intercourse," is almost an axiom. Hollywood's take, immortalized by Harry in When Harry Met Sally …, echoes Joyce: "men and women can't be friends because the sex part always gets in the way." Why am I bombarded with the idea that what Jeff and I have cannot exist?

In recent years, the gay-male/straight-girl pair has taken off in the popular imagination—think of Will and Grace, or Stanford Blatch and Carrie Bradshaw—and become the paradigmatic platonic friendship, maybe because it's a way of taming a subject that we find uncomfortable. (It's a less confusing arrangement than straight-male/straight-girl friendships. The gay man's universal lack of desire for women makes his lack of desire for a particular woman—the "fag hag"—graspable.) But the closest thing that Jeff and I, who are both straight, ever had to role models were Seinfeld's Jerry and Elaine—friends, we're constantly reminded, who once dated.

Before I started researching this topic, I knew what cross-sex friendship was like for me (for me and for Jeff), but not what it was like for others. In the coming days, I'll try to fill the gap, telling the story of the contemporary men and women who share their thoughts, but not their beds. In so doing, I'll focus exclusively on heterosexual pairs—on relationships that could theoretically become romantic but don't.As much as not having sex, or at least not focusing on it, is what distinguishes male-female friendship from romance, this relationship is also much more than a negatively defined state. It's inextricably bound up with modern ideas of equality, and it could very well reshape gender relations in the century to come.

Read Jeff's account of his platonic friendship with Juliet.

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