For Mr. Godard, a former intelligence officer, it is the “nature” of conversions that has changed.

Conversions to marry have long been common enough in France, but a growing number of young people are now seen as converting to be better socially integrated in neighborhoods where Islam is dominant.

“In poor districts, it has become a reverse integration,” said Gilles Kepel, an expert on Islam and the banlieues, the poor, predominantly Muslim neighborhoods that ring Paris and other major cities.

Many converts are men younger than 40, experts say, often born in France’s former African colonies or overseas territories.

Charlie-Loup, 21, a student from nearby St.-Maur-des-Fossés, converted to Islam at 19, after a troubled adolescence and strained relations with his mother. He grew up Roman Catholic but had many Muslim friends at school. “Conversions have become a social phenomenon here,” he said, asking that his surname not be used because he considered his conversion a private initiative and did not want to draw attention to himself. Some convert simply “out of curiosity,” he said.

In some predominantly Muslim areas, even non-Muslims observe Ramadan, the Muslim holy month that requires fasting during the day, because they like “the group effect, the festive side of it,” said Samir Amghar, a sociologist and an expert on radical Islam in Europe.

In many banlieues, Islam has come to represent not only a sort of social norm but also a refuge, an alternative to the ambient misery, researchers and converts say.

For Mr. Amghar, Islam provides more structure and discipline than other religions. It is a way to “refuse modernism,” get back to a society with more family values and a clearer distinction between men and women. “Islam has a peaceful effect on the converts,” Mr. Amghar said. “The world looks clearer after they’ve converted.”