PARIS — Why do young people always meet at cafés? That's the question frequently posed after the Nov. 13 terror attacks in Paris. "Either we live in a 10-meter studio apartment, or we're still with our parents," 22-year-old Jules Boissard answers.

A university student in his third year, Boissard lives with his girlfriend at the home of her parents. He's grateful for their hospitality, but he dreams of one day being able to invite his friends over to share dinner without having a couple of fifty-somethings joining in. "It was fun ... at first,"he says.

This intergenerational co-habitation is definitely a thing. In France, 4.5 million young adults live with their parents or friends, according to a recent study by the Abbé Pierre Foundation. The average age for young people to leave the nest is 23.5, six months older than in 2008. But the national average tends to hide the records being set in the biggest cities, starting with Paris.

In Paris and its surrounding region, fully half of young adults under 25 still live with their parents. But often these arrangements come with not just one, but two, young people.

"My mother found it weird at first that my boyfriend slept with me in the same bed under her roof," says 21-year-old Mathilde Régnier, a university student and Jules Boissard's sweetheart. "Things were pretty tense around the breakfast table. But when she saw that our relationship was stable, it didn't bother her anymore."

The suburban house of Régnier's architect parents then "naturally" became the young couple's first home. "Since we can only ever see each other after classes, Jules started to come every evening," she says. Solutions are scarce when you have no scholarship, no space in a university dorm, and too many classes to have enough time for a job that would pay a rent.

If they're always either at his or her parents' — they spend the weeks at hers, the weekends and holidays at his — it's because they can't yet stand on their own two feet. Since Tanguy — a very famous 2001 French comedy about a 28-year-old named Tanguy, who's still living with his parents (and badly getting on their nerves) — the parallel trends of longer studies and the economic crisis have conspired to bring generations together.

"Generational relationships have changed," says sociologist Anne Dujin. "We talk, we exchange, and today's greater emotional closeness makes it possible for young couples to live happily together with parents."

Pascale Brette, a teacher close to her sixties, occasionally accommodates her daughter and her partner, and her 25-year-old son still lives at home. "That was unthinkable for us, with parents of the De Gaulle generation," she says. "There's no cultural gap between us. We listen to the same music, we dress almost the same." Her husband was unemployed for a long time. So she understands why her son, whose job is unstable and pays very little, returned to the family home after a long year in London.

This is the boomerang effect, or the "accordion family," as Americans put it. The process of young adults moving away from their parents happens later than it used to, and it's also reversible. Families have become the fallback solution "whatever the social background," explains Sandra Gaviria, a professor at the University of Le Havre. Any number of situations can lead young people back to their parents' home, often with a partner: a hard blow professionally, a wrong turn in studies, even apartment renovations.

Of course, it's delicate a move for a young couple to move in with one of the families. Space and layout are obviously very important to limit the impact of household habits and to keep the shock of intimacy to a minimum. Laurie Pinaud, who is 24 and about to become a teacher, lives with her boyfriend at his mother's home. "I stay in the room most of the time," she says. "Thank God for TV and computer. I tell myself that even though his mother is very welcoming, she probably wants to be alone."

Pinaud never procrastinates when it comes to doing the dishes, and she refrains from hanging around in her pajamas. She's also been taking very quick showers "since that day when she knocked on the door asking me to hurry up. She thought she was talking to her son."

Jules Boissard never enters his girlfriend's parents' house if she's not with him. "I try to take as little space as possible," he says. "I help her younger brother with his homework, and I address her parents formally so as not to become the fourth child of the house."

Meanwhile, his girlfriend is looking for the right strategy to convince her parents to subsidize their moving out. She's tired of being questioned about her comings and goings, of the infantile remarks in front of her boyfriend, of the absence of a lock on her bedroom door. "We're being careful all the time," Mathilde says. "Thankfully, my younger brother listens to music quite loudly with headphones, and my sister is a serious student who goes to bed early."

As for parents, these arrangements mean no more TV dinners in pajamas. It's not easy for them either to be such close witnesses to their offspring's love lives, during good times or bad. When this debate first emerged in the 1970s, many psychologists subscribed to the principle that "no more than one sexually active couple shall live under the same roof."

But youth sociologist Cécile Van de Velde says the financial crisis has forced families together. "At 25, if the young couple has shown signs of stability, it's difficult for parents to say no," she says, adding that France has always had a moderate position within Europe, between the all-accepting protestants in the north and the prudish Catholics in the south. "But today, faced with the reality of the crisis, things have changed even in Spain!"

People no longer mock these young adults trying to find their way. Everybody knows times are hard for young people. Now, it seems, it's the adults who must justify themselves when they choose not to open their door to young lovers.

"I know I sound outdated," admits Anne Thomas, a practicing Catholic from northern France who told her son he can't bring his girlfriend to live there with them. "But that's how it is under my roof. I don't want to get up and run into a girlfriend in a bathrobe."

Marie Brard, who owns a guesthouse in Normandy, has taken the same position with her own son. "I'm not old-fashioned," she says. "I do think that the era of "nothing before the wedding" is over. If they want to live as a couple, that's fine, but then they accept that responsibility."