One of my friends’ neighbors recently told him a provocative story about cable repair. Apparently this neighbor—let’s call him Bill—had waited for hours at home for a repairman; finally, the cable company called to announce its representative’s impending arrival just as Bill was about to leave on an errand. Bill, by his own account, responded that there was no way a repairman was getting into his house when the only other person home was his wife. Bill’s tone, per my friend, conveyed indignation that such an insulting possibility had even been considered.

When my friend shared this story with me and several others via email, all agreed that there is nothing improper about a married person being home with a Comcast specialist when his/her husband/wife is away. Chill out, Bill! If we’re going to start making rules about when who is and isn’t allowed to fix the cable, the cable’s never going to get fixed. But my friend raised the stakes by proposing similar hypothetical situations that weren’t as easy to dismiss. Maybe it wouldn’t bother you if your spouse were home with a repair person—but what if he/she mentioned stopping by a single colleague’s house to finish up a project they were working on together? What if your life partner and an acquaintance, sports fans both, watched the Big Game by themselves? What if an attractive area parent brought his or her kid over for a play date while you were at work? What if you were out of town for the weekend and your spouse and a single friend went to see the buzzy movie du jour  It became clear on our email chain, and in my own conversations with a number of friends and colleagues about the subject, that for some (but not all) people (both men and women), the situations described just don’t seem quite right. Several said they’d be uncomfortable if their spouse or partner engaged in certain interactions without them and/or felt their spouses might not be happy with them if they did the same. “I would say ‘OK,’ but I will probably not really be OK with it,” said one. “I’d feel some unease. Not in any specific way. I’d just feel funny,” said another. Said one about the idea of a man he didn’t know well socializing with his wife: “My reaction would be to act like I’m cool with it then insult him constantly when he’s not around.”

None of my sources hold especially conservative beliefs about sexual relationships or roles. In many parts of the world, of course, ideas about what married people can and can’t do are enforced by religious and legal authorities (mostly to the detriment of women), but no one here was suggesting that they were morally uneasy about the mere idea of men and women (or gay men and women and other gay men and women, as it were) being alone together. None said that the hypothetical situations would make them seriously suspicious that their partners were being unfaithful. Some people that I put the question to, meanwhile, said none of the hypothetical situations described would bother them. Why was the idea of Nonspousal Alone Time so selectively unsettling? When is it OK to be by yourself with another person’s spouse?

For those who admitted concern, jealousy was a factor, albeit a nebulous one. One man said the possibility of a straight man thinking about his wife in a sexual way while alone with her made him uneasy even though he wasn’t worried about infidelity per se. A woman with two kids told me that the idea of her husband spending precious free time away from the family with anyone—female or male, sexually compatible or not—who wasn’t a longtime friend would make her jealous. A man with two kids of his own mused that the most upsetting aspect of his wife going on a business trip with someone else wouldn’t be the specter of infidelity—it would be the idea of her enjoying someplace luxurious and not covered with kids’ toys. “Room service and fresh linens and everybody all suited up and ****.” (Still, he said, he’d be OK with it.) Another man wondered if watching a sporting event with an unattached woman would somehow imply that his own wife was in some way undesirable because she didn’t like sports. And several people copped to knowing that they wouldn’t be worried about their partner’s time with another man/woman if the interloper in question was notably unattractive.

What might be happening is that our jealous reptile brains haven’t caught up with new social rules and gender roles. These hypotheticals are situations—men and women traveling together for business; a married man being a primary caregiver for children during the day; women identifying as sports fans—that to greater and lesser degrees didn’t exist until relatively recently. As historian Stephanie Coontz of Evergreen State College noted when I asked her about the subject, the Industrial Revolution–era doctrine of separate men’s and women’s spheres limited interaction between the sexes for a large chunk of American history: Men’s spheres were public life and the workplace, and women’s was the home. Marriages were often arranged through families and preceded by formal “courtship.” Over the course of the 20^th century, though, women entered previously male workplaces and outside-the-home dating became common. And roles that were pioneering in the baby boom era are now the norm: Since 1967 (to pick one year of noted upheaval), the share of women in the labor force has gone from 39 percent to 58 percent, the share of women 18 to 24 who are enrolled in college has gone from 19 percent to 45 percent, and the average age at which women marry for the first time has gone from 21 to 27. There are more opportunities for male-female socialization in the United States now than ever before.

At the same time, we’ve removed our government from its historic role as a regulator of sexual morality. Eric Berkowitz, author of a history of sexual laws called Sex and Punishment: Four Thousand Years of Judging Desire, pointed out to me that the American judicial system has historically taken great interest in private sexual behavior; until fairly recently adultery was considered a criminal offense and sex was often at issue in divorce proceedings because of rules that required one party in the breakup to be found at fault. (If you go all the way back to the era of the Assyrians, Berkowitz’s book says, “any man who ‘traveled’ with an unrelated woman had to pay money to her husband and prove—sometimes by jumping into a swiftly flowing river and surviving—that he had not taken the woman as a sexual partner.”) While some adultery laws are still technically on the books, the government is for the most part no longer concerned about what adults do with each other behind closed doors. No-fault divorce laws were widely adopted in the 1970s, which is also around the time that adultery prosecutions became increasingly unheard of. The Supreme Court’s 2003 Lawrence v. Texas decision legalized all consensual sex between adults. Younger (straight) married couples today have grown up in a world in which their sexual and relationship choices were never society’s business. (The situation is obviously much different for gay couples.)

And yet, as Berkowitz points out, “I don’t think that people’s desire to possess the body of their spouse is any less intense than it was 125 years ago.” A married man being alone with a woman besides his wife (and vice versa) was once a relatively rare occurrence that could even raise the specter of criminal activity. Now such interactions are common, and not suspicious in any legal sense—but can still carry the hint of wrongness. “We’re probably more suspicious now than we used to be because there aren’t the restrictions, the social restrictions, the whole broad range of habits that kept people separate 50 or 60 years ago,” Berkowitz says. “At that time there wouldn’t have been a business trip set up that two [married] colleagues would go on together. Now that happens, but it doesn’t mean that the spouses are not concerned.” As my unscientific survey indicates, jealousy can easily lead to hurt feelings and misgivings even if adultery isn’t committed or even contemplated.