RIYADH, SAUDI ARABIA - Nader al-Mutairi stiffened his shoulders, clenched his fists and said, "Let's do our mission." Then the young man stepped into the cool, empty lobby of a dental clinic, intent on getting the phone number of one of the young women working as a receptionist.

Asking a woman for her number can cause a young man anxiety anywhere. But in Saudi Arabia, getting caught with an unrelated woman can mean arrest, a possible flogging and dishonor, the worst penalty of all in a society where preserving a family's reputation depends on faithful adherence to a strict code of separation between the sexes.

Above all, Nader feared that his cousin Enad al-Mutairi would find out that he was breaking the rules. Nader is engaged to Enad's 17-year-old sister, Sarah. "Please don't talk to Enad about this," he said. "He will kill me."

The sun was already low in the sky as Nader entered the clinic. Almost instantly, his resolve faded. His shoulders drooped, his hands unclenched and his voice began to quiver. "I am not lucky today; let's leave," he said.

It was a flash of rebellion, almost instantly quelled. In the West, youth is typically a time to challenge authority. But what stood out in dozens of interviews with young men and women here was how completely they have accepted the religious and cultural demands of the Muslim world's most conservative society. They may chafe against the rules, even at times try to evade them, but they can be merciless in their condemnation of those who flout them too brazenly. And they are committed to perpetuating the rules with their own children.

That suggests that Saudi Arabia's strict interpretation of Islam, largely uncontested at home by the next generation and spread abroad by Saudi money in a time of religious revival, will increasingly shape how Muslims around the world will live their faith. Young men like Nader and Enad are taught that they are the guardians of the family's reputation, expected to shield their female relatives from shame and avoid dishonoring their families by their own behavior. It is a classic example of how the Saudis have melded their faith with their desert tribal traditions.

Enad is the alpha male, a 20-year-old police officer with an explosive temper and a fondness for teasing. Nader, 22, is soft-spoken, with a gentle smile and an inclination to follow rather than lead.

They are more than cousins; they are lifelong friends and confidants. That is often the case in Saudi Arabia, where families are frequently large and insular. Enad and Nader are among several dozen Mutairi cousins who since childhood have spent virtually all their free time together: boys learning to be boys, and now men, together.

They are average, young Saudi men, residents of the nation's conservative heartland, Riyadh. It is a flat, clean city of 5 million that gleams with oil wealth, two glass skyscrapers and roads clogged with oversized SUVs. It offers young men little entertainment, with no movie theaters and few sports facilities. If they are unmarried, they cannot even enter the malls where women shop.

Deriding a woman alone

Nader sank deep into a cushioned chair fiddling with his cell phone. If there is one accessory that allows a bit of self-expression for Saudi men, it is their cell phones. Nader's is filled with pictures of pretty women, tight face shots of singers and actresses. His ring tone is a love song.

Three days later, in a nearby restaurant, Nader and Enad were eating out. Suddenly, the young men stopped focusing on their food. A woman had entered the restaurant, alone. She was completely draped in a black abaya, her face covered by a black veil, her hair and ears covered by a black cloth pulled tight. "Look at the batman," Nader said derisively, snickering.

Enad pretended to toss his burning cigarette at the woman, who by now had been seated at a table. The glaring young men unnerved her, as though her parents had caught her doing something wrong.

"She is alone, without a man," Enad said, explaining why they were disgusted, not just with her, but with her male relatives, too.

When a man joined her at the table -- someone they assumed was her husband -- she removed her face veil, which fueled Enad and Nader's hostility. They continued to make mocking hand gestures and comments until the couple changed tables. Even then, the woman was so flustered she held the cloth self-consciously over her face throughout her meal.

"Thank God our women are at home," Enad said.