"'Hot is not the word I'd use," says Hannah of her 23-year to Barry.* "Slow simmer" is more like it. "One thing you learn over time," she says, "is that, no matter how long you live together, two people always inhabit separate worlds. Some part of your partner is deeply unknowable."

Although it is hard to coax any words out of her on a topic she considers, perhaps quaintly, so private, Hannah makes it clear that their life cleaves to the contours of their commitment. "There are nights, not often but indelible, when passion builds in molten intensity from an unremarkable start," she says. And there are nights—"almost more transcendent," she confides—when the two share the separateness, naked together, holding hands in rich silence. And there are many nights in between.

Hannah and Barry personify sex in America today. Contrary to conventional , married couples—and their cohabiting counterparts—have more sex than the nonmarried, a fact confirmed in a 2010 survey by the Kinsey Institute revealing who does not have sex. Three out of five singles had no sex in the previous year, versus one in five marrieds. In the prime years, ages 25 to 59, married individuals were five times more likely to have sex two to three times a week (25 percent) than singles (5 percent). Explains economist Heather Boushey, director of the Washington Center for Equitable Growth, who studies family patterns, "You don't have to go out and forage."

Evidence has long existed that couples have lots of sex early in the relationship and the frequency of sex declines over time. Aging and the dramas of raising a family and earning money change when and how people do it, but long-married couples still have an advantage: They enjoy it more.

Studies also show that long-term couples get better at sex and get more pleasure out of it. That is true of men as well as women, heterosexual and same-sex couples. As Vanderbilt University sex researcher Laura Carpenter explains, "While people get older and busier, as a relationship proceeds they also get more skillful—in and out of the bedroom."

The facts on the ground in no way preclude sex in long-lived relationships. Yet we seem to have trouble accepting that coexistence. We readily blame any loss of sexual desire on the domesticity of modern marriage—especially the sharing of household chores—or the constant proximity of familiar partners.

There seems to be a widespread aversion to the idea that sex is alive and well in long-term couples. Social scientists are not exempt. Very little research is dedicated to middle-age sex. "Not a lot of studies look at sex in established couples or sex in ," says Carpenter. Even "experts" have little clue what sex looks like in contemporary marriages: who initiates it and how, who does what to whom, how long it lasts.

If we have trouble fully grasping the compatibility between long-lasting love and sex, our own mental machinery must share blame. We have yet to erase those hoary icons of highly gendered bliss imprinted on our brains at the dawn of the media age—when men went off to work in suits and ties while women vacuumed the living room—and which have retained primacy in our minds ever since. In the absence of more updated models of how the sexes relate and share all aspects of their lives, including sex, we are prone to default to antique Mad Men beliefs about what's sexy. We may resort to outmoded ideas because we have yet to upgrade our sexual imagination, the movie that plays in our heads about the sex we want to have and dream of having. In this regard, our sex lives lag well behind our work lives.

A Shift to Shared Lives

Are household chores necessarily sex-killers?

, the longstanding dean of couples research and head of Seattle's Gottman Institute, contends that the path to sexual engagement runs directly through the willingness of partners to share in the running of their lives. Significantly, Gottman discovered that men who do housework have more sex than men who don't.

His studies are especially significant because they track couples' inner and outer experiences over time in multiple ways, rendering the results particularly reliable. The Seattle researcher pioneered minutely detailed measures of emotions and behaviors via heart rate, fidgetiness, and facial expressions. He scrutinizes the content and manner of partners' conversations, and he gathers self-reports of how they feel about their experiences, including sex.

Where men contribute to housework and child care, Gottman observes, their partners see them as sexy, and indeed they have more sex than couples in which the men are chore-free.

all readers: The finding establishes a correlation, not a cause. Housework does not cause sex, nor does it inhibit sex. Instead, Gottman says, the qualities of people who share in the one (housework) are the qualities of people who share in the other (sex). Most significantly, something is going on inside the woodwork of the relationship.

Sociologist Constance Gager has conducted her own studies and found that sharing chores actually helps couples stay sexually connected. Studying survey data from thousands of people who make up a representative sample of the population, the Montclair State University researcher also found that the more housework men do, the more sex they have. The same was true for women. The real variation in couples was not whether they performed one particular task or another but how much energy they had overall for everything.

Sociologist Julie Brines at the University of Washington has something to say on the matter. Using survey data on sexual frequency and the division of housework among couples that was collected in 1993 and 1994—that is, 20-year-old data on relationship dynamics that are rapidly evolving—she found, like Gager, who used the same survey data, that more housework hours equals greater sexual frequency.

But she went one step further and looked at what kinds of chores partners did. Sexual frequency was higher in couples where the men spent more time doing such traditionally "masculine" chores as car maintenance and yard work, as opposed to couples in which the men did such "feminine" chores as cooking and cleaning. Is it possible that women are turned off by their men doing household chores most traditionally deemed to be in women's domain?

Significantly, in the Brines study, neither sexual satisfaction nor relationship satisfaction was affected by the kinds of chores partners did. "We found that egalitarian couples have less frequent sex but report happy marriages and have levels of sexual satisfaction that are comparable to 'old-school' husbands and wives," she says. Nor was sexual frequency influenced by which partner earned the greater share of the household income.

Although men are doing an increasing share of housework, studies show, their participation inside the house lags well behind women's contributions outside the house. And sharing responsibility for such household imperatives as income has helped families keep their heads above water in an increasingly challenging economy.

According to the Center for Economic and Policy Research (CEPR), in 2012, the majority of women (67.8 percent) and mothers (72.0 percent) between the ages of 16 and 64 worked, most full time throughout the year. Since 1979, the typical woman has increased her time of paid work per year by 739 hours (to 1,664 in 2012).

Sociologist Paula England studies changes in family life and follows the money. For a report to the Council on Contemporary Families, she took a close look at Brines's research. Couples where a wife works more hours for pay than a husband and makes a lot more money "have sex the same amount as those couples in which the man is the sole breadwinner and the woman stays home," she observes. If husband and wife earned more or less the same, there was no difference in sexual frequency either.

Fewer and fewer relationships are organized on the notion that men must be the breadwinners and decision makers while women stick to vacuuming and the emotional heavy lifting. Indeed, American couples who believe in sharing all responsibilities report the highest relationship satisfaction, observes Oriel Sullivan, a sociologist at the University of Cambridge. She points to a 2007 national opinion poll conducted by the Pew Research Center: 62 percent of respondents ranked sharing household tasks as very important for a successful marriage, up from 47 percent in 1990. In a 2010 Pew poll of young people 18 to 29, a full 72 percent agreed that the best marriage is one in which husband and wife both work and both take care of the house.

Diminished Lust or Expanded Love?

What do the results of Brines's study mean for lust and love? To some experts, the belief that some tasks have more sex appeal than others is questionable. "Our current notion of what's a feminine and what's a masculine task is very recent," says Evergreen State University historian Stephanie Coontz. "Two hundred years ago, women fed the pigs and wrung the necks of chickens. Men did the shopping."

Even where there were statistically significant differences in sexual fre-quency, the differences were, in practice, not huge. Men who did more "feminine" housework had sex about four times a month, versus five times a month for men who did "manly" chores.

Yet sexual frequency is not even a good measure of relationship quality, Gottman maintains. Moreover, chores are only one, albeit very visible, way to define the equality of participation most modern couples are seeking. "Far more important are mutual respect and whose needs take priority in a pinch. Housework is a very remote variable. To understand relationships you need to look at proximal variables, namely emotions, including interest, affection, , sadness, contempt."

Housework has nothing essential to do with evolved sex drives, points out Coontz. It's not inherently lethal to lust. There's no "natural" division of chores so deeply embedded in our makeup that they are linked to our sex drives. The homemaker-breadwinner model of marriage, she explains, was a 19th-century invention. Even so, it was practiced widely only for a few decades in the twentieth century, and then mainly among white families and only those of the upper classes. Yet it's deeply embedded in our psyches as a template of his-and-her behavior.

Relationships are not a zero-sum game in which more housework ends in less sex, says Gager. Housework doesn't rob lives of love; it can enlarge love. She takes an expansionist view of couple life, one pioneered by famed sociologist Emile Durkheim. "Time spent on important and worthwhile shared activities can enrich couples and energize them to have more sex," she says. Couples feel more of a shared purpose. Partners don't experience an opposition between housework and sex, not because housework is sexy but because they are in it together.

A Sexual Mystery

Sexual feelings are typically not enacted raw. Rather, in every society, they are filtered through learned cultural guidelines that shape desire, arousal, , and our most intimate interpersonal behavior. As a result, we each carry a set of "sexual scripts" that more or less dictate what we allow ourselves to do, and what those behaviors mean. Because they are heavily cultural, sexual scripts change over time, and, along with them, behavior in the bedroom. Fifty years ago, for example, straight couples rarely said they had oral sex, whether they did or not. Today, couples are more likely to avoid admitting that they don't have oral sex. It is much more a sexual staple than in the past.

Desire needs distance, not security and definitely not the shared life favored by modern couples, argues Esther Perel, a psychotherapist who authored in Captivity: Unlocking Erotic . By inhabiting separate worlds—retaining sexual scripts more reflective of attitudes popular in the 1960s—men and women maintain a sexual mystique that enhances appeal. The path to sexual connection is through mystery.

Using help-seeking couples as a reference point, Perel reports that "couples who describe themselves as loving, trusting, and caring complain that their sex lives have become dull and devoid of eroticism." She teaches how to "reconcile our fundamental need for safety and security with our equally strong need for adventure and novelty." We need more play, she says. Sexual desire is chaotic, something couples who feel sexually adrift need to tap into, rather than tamp down. Her diagnosis hit a nerve: A 2013 TED talk she gave on sex in long-term relationships has garnered 5 million views.

At a practical level, far more couples share the load of earning and housekeeping today than ever before. Parel suggests that such sharing is anti-sexy, like a flannel nightgown or worn-out boxer shorts. Economist Boushey is puzzled by Perel's prescription for more distance between partners. "Couples already inhabit two different work cultures all day; they're already separate most of the time."

To Brines, the results of her own studies indicate that even as roles in and out of the house are generally being redefined, there has been a stall-out. Heterosexual couples are still holding on, in a measurable if small way, to traditional his-and-her scripts. Some sexiness is still attached to old-school gendered activities (gardening for her, yard work for him). And it's most notable in the bedroom.

If desire is blunted in couples, says Brines, it's because couples are scriptless, or between scripts, themselves struggling with what sexiness is today. "I don't think we have newer alternatives to traditional sexual scripts in marriage," says Brines—what sex is supposed to feel like, who is supposed to initiate it and how, what fantasies are turn-ons. Sexual scripts have not changed to reflect the new ways couples relate outside the bedroom. Our sexual imagination has yet to come through with a rewrite.

Zero-Sum, Zero Sex

For John Gottman, desire hinges primarily on how couples relate in the routines of everyday life. It's not the form of a marriage that kills sex but the processes within it. In his famous housework finding—couples in which men shared housework had more sex—what sustained the sexual connection was that the wife felt respected and understood.

Respect and understanding are communicated by accepting influence from one another; that is, partners are responsive to each other's feelings. In a study conducted with psychologists Laura Carstensen of Stanford and Robert Levenson of UC Berkeley, Gottman followed couples over 20 years—one group starting at age 40, another starting at age 60. Barring health problems, the couples had sex with each other on a regular basis, but only under one important condition: that they didn't resort to stonewalling, pre-empting, or defensiveness to shut out the other when discussing problems.

In the 16 percent of marriages that were sexless, says Gottman, there was no "give." The relationship was adversarial, shut down, zero-sum. And that kills desire.

Exactly how does the dynamic of influence and connection support a vibrant sex life? "We learned that saying 'no' to sex isn't what leads to the end of sex. It is the cost imposed by the other person." Specifically, Gottman analyzed interactions when one partner came on and the other partner wasn't interested. It happens in the best of couples. The next move in the interchange—what the "pursuing" partner says or does in response to the turndown—is the significant one. Showing annoyance is a cost, a form of .

After being refused sex, the pursuing partner may say "fine." But tone is everything. There's "Fine!" said with an air of anger. (Read: "Fine. I didn't really want to do it anyway." "Fine. I don't need you.") "'Fine,'" Gottman explains, "is one of those things that doesn't even have to be spelled out. It is an injured, hurt, little bit indignant response. So the initiator turns away, implying 'I don't need you, I will be fine.'"

It's an emotionally delicate situation. "If there is any cost—even a small one—it leads to not having sex."

A noncostly response to "no" is entirely possible. Far better than "fine" is something like "I really appreciate your telling me you're not in the mood. I don't want to make love to someone who isn't in the mood. What would you like to do?" The best outcomes—for your sex life—occur when you reward a "no" and treat it positively.

If the request for sex always has a cost, the sum of the disappointments is toxic. Shutting out the voice and needs of a mate damages the mar-riage; it also harms the partners them-selves. In a separate analysis of Gottman's original data, neurosci-entist Tara Madhyasta discovered that over the 20 years of the study, men who were in zero-sum relationships were seven to 11 times more likely to die than men in influence-sharing relationships.

Zero-sum men felt good—or merely neutral—when their partners felt bad about the outcome of a disagreement: Her loss was his victory. Zero-sum women also suffered; they were sicker than others, although their life spans were not truncated. Madhyasta notes that the number of zero-sum couples was small, since they are less likely to volunteer for a long-term study of marriage.

"Couples who are going to have a lot of sex end up somehow being able to communicate to one another that it's a priority," Gottman reports. "It is not going to be the last item on the infinite to-do list." And they have an attitude of flexibility. "A woman or man who feels somewhat uninterested in sex may say, 'I'll help you ' or 'I'll help you with a hand job or a blow job.' Or agree to a quickie." This, Gottman explains, is how couples work it out. They emotionally reassure each other.

Distance vs. Connection: The Brain's POV

Gottman believes that sexual imagination has one necessary ingredient: the freedom to play. Play requires a feeling of safety. That's something University of Virginia James Coan knows about.

Director of the Virginia Affective Neuroscience Laboratory, Coan uses imaging techniques to observe how our brains find safety in the face of fearful situations, like the anticipation of a mild shock. Coan monitors regions of the brain that manage and other emotions—and the prefrontal cortex, which manages planning and social behavior and is also key to calming the emotional brain.

A fear response is expensive to the brain and usurps many of its processes. The brain gets busy solving problems, including how to escape from the situation. With fMRI, Coan can see the brain get worked up in response to the threat of electric shock—and see what calms it down. He has applied this paradigm to married individuals who are either holding their spouse's hand, holding a stranger's hand, or holding no hand at all. "When you are holding a hand, the attenuates. We have seen this with over 100 people. Your body is not preparing to work as hard in response to a threat." You can think about other things.

The familiarity of a partner is soothing; it creates the least neural activation. Conventional wisdom suggests it might actually be too calming for couples to get it on. But Coan made some surprising discoveries. In the alone situation, with no touching, the prefrontal cortex—predictably—turned on to help individuals calm down. That effort is costly, like a tough day at work. When there was touch—especially familiar touch—the emotional brain more readily quieted. "The calming is cheaper," says Coan. It takes less energy.

Surprisingly, when partners held hands, "everything went quiet," Coan reports. "There was no prefrontal mediation. There was just a massive decrease in emotional responding." The prefrontal cortex helps you regulate yourself—when you're by yourself. It also keeps you self-involved, feeling exhausted.

Coan speculates that when it comes to sex, self-focus can decrease and inhibit full focus on the task at hand—having sex. It doesn't operate that way when someone else is soothing you. When someone familiar touches you, he says, "It's like magic. Calm washes over the whole brain." The potential is much greater for leaving yourself behind and being in the moment, ready to play, with fewer distractions.

The touch of a longtime partner, Coan explains, "takes away whatever might be interfering with what you are doing right now. It offloads things that are not relevant. It allows us to get more aroused, not less, with someone we are comfortable with."

That's why Bill Harrison, a Baltimore financier married 24 years, sees no war between love and lust.

"For me, touch and closeness have always been as important as sex is. We hold hands, we snuggle. Nine times out of 10, 19 times out of 20, that is as good—or better—for our relationship. Our society conditions us to believe we can achieve and maintain a peak sexual relationship for decades. That isn't the way it is. There are valleys and plateaus, and they involve other things in life, including careers and children."