After a sharp increase in terrorist threats in recent weeks, a number of prominent German politicians suggested a controversial response: banning any type of full-face covering worn by some Muslim women.

"A ban on the full-face veil, such as the niqab and burqa, is overdue, and it would be a signal to the world," Jens Spahn, a rising figure in German Chancellor Angela Merkel's Christian Democrat party and self-described "burqaphobe," told Die Welt newspaper last month.

Yet when Germany's Interior Ministry announced several new security measures Thursday, no such prohibition was included. “You can’t ban everything you’re against,” Interior Minister Thomas de Maizière told reporters. “And I’m against wearing a burqa.”

Germany seems to have bucked a trend. Bans on Muslim face veils or head scarves have been implemented in countries around the world in recent years — generally on the grounds that burqas and niqabs serve as a barrier to better integration for Muslim women.

Some researchers, however, say that the burqa bans increase isolation for many women and in fact may push an alienated minority further away. In some cases, the bans may contribute to security problems rather than help solve them.

Consider the case of France, one of the first European countries to pursue a burqa ban. Since 2011, full-face coverings have been prohibited in public places there, meaning that Muslim women who wear niqabs or burqas can face a fine of 150 euros ($167) or be required to take a citizenship course. Any man who forces a woman to wear a face covering can be hit with a heftier fine of 15,000 euros ($16,700).

There was widespread support for the French ban when it was announced: Parliament passed the legislation with near-unanimity, and polls suggested that more than 80 percent of the public agreed with it. In a country where secularism is held dear and religion is regarded as a private matter, there's relatively little debate over the merits of the ban even now, five years later. Just this week, the French resort town of Cannes went further by banning the "burqini," a full-body swimsuit worn by some Muslim women.

The law was accorded international approval in 2014, when the European Court of Human Rights upheld the ban, saying it encouraged citizens to "live together."

But Agnes De Feo, a sociologist and documentary filmmaker who has studied the impact of the French law on Muslim women, says it has had some unintentional effects. De Feo says that since 2009 she has interviewed about 150 women who wear a veil and has seen how the law changed their perception of French society. "Almost all the people wearing the niqab in France today started after the law [was implemented]," she says. "The women wearing the niqab before the law now stay at home and now they never go outside."

Even before the ban took effect, France's Interior Ministry admitted that niqabs were worn by fewer than 2,000 women in the country — hardly a significant portion of the country's Muslim population of more than 5 million — and that almost none wore the burqa, the more concealing one-piece veil. Some have suggested that this estimate was flawed, however, and that perhaps the real number was even lower.

Figures released by the Interior Ministry in 2015 showed that 1,546 fines had been imposed under the law, though they often involved repeat offenders — one woman had been fined 33 times. French officials have admitted that the ban is not always enforced, in part because there have been a number of incidents of violence directly or indirectly related to it. In 2013 there were three days of riots in the Parisian suburb of Trappes after police told a woman to remove her veil. According to reports from the time, the woman's husband had been arrested after allegedly attempting to strangle a police officer.

At the same time, several Muslim women wearing niqabs have been attacked over the years in alleged hate crimes. One pregnant woman miscarried after she was attacked by two men in 2013. France's National Observatory Against Islamophobia has collected information that suggests the vast majority of anti-Muslim harassment and assault is directed against women.

There have been no publicized cases of a man being fined for making a woman wear a full-face veil. De Feo says that in her research she did not find evidence that this practice was widespread. "I've never met a woman who was forced to wear a niqab," she says. Instead, women who wear the banned face coverings now do so as an act of personal rebellion against the French state, she says.

The women she interviews tend to be young converts who are single or divorced. One, a young convert named Emilie Konig, eventually fled to Syria to join the Islamic State. The U.S. State Department said in 2015 that videos have shown Konig training with weapons in Syria and that she has instructed individuals in France to attack French institutions. De Feo says she doubts she is the only one to travel to Syria — other women she has spoken to have become radicalized, too, and broke off contact with her. "These women became like that because of the French context," she said.

Haroro Ingram, a research fellow at the Australian National University who studies extremist literature, notes that extremist groups have specifically used France's face-veil ban as a recruitment tool: The very first issue of al-Qaeda in the Arabian Peninsula's Inspire magazine included an article titled "The West Should Ban the Niqab Covering Its Real Face." He doubts, however, that any one issue drives recruitment for these groups.

"Rather, it is the cumulative effect of a range of issues and factors — of which any one is indicative of the broader trend — that is most important here," Ingram says. While France's full-face-veil prohibition directly affects only a small fraction of French Muslims, more general discrimination is widespread: One study conducted in 2013 and 2014 found that Muslim candidates experienced considerable discrimination in the French job market.

There seems to have been surprisingly little research into the effect the burqa ban has had on Muslim communities. "I don't think policymakers would pay such studies any mind since these laws are never about integration effects," said Jonathan Laurence, a professor at Boston College who has done extensive research on Muslim communities in Europe. Instead, these laws are often more about a society's own wedge issues, Laurence says.

This may well be the case in Germany, too. While polls have shown support for a ban on face veils, some German journalists have noted that there may be only a few hundred women who wear niqabs in the entire country and even fewer who wear burqas. Perhaps the proposed burqa ban's biggest problem would have been finding a burqa to ban.

More on WorldViews