Within 40 years, given current demographic trends, the white population in France and the rest of old Europe will recede, creating a Muslim majority, a French researcher says.

Charles Gave, an economist, fund manager and political commentator, published his conclusions this month on the webpage of his think tank, Institute des Libertes. He writes of the “disappearance of the European populations” as native populations shrink and Muslims continue to exhibit a robust fertility rate.

Mr. Gave, president of Gavekal Research, acknowledges that his decidedly unpolitically correct view may bring him scorn and possibly censorship. The political left generally protects Islam from criticism. In the U.S., President Obama consistently defended Islam and mocked Christians for their criticism.

The paper, titled “The White Plague,” is dangerous, Mr. Gave said, “for my personal respectability and my chance to be heard in our beautiful democracy.”

The financier draws his conclusion from demographics. He assesses France’s white, or native, birthrate at 1.4 children per woman, compared with a Muslim rate of 3.4 to 4 children. France’s population today is 67 million. Unlike the U.S., France does not conduct a census on ethnic origin, but based on outside polling, some researchers, including Mr. Gave, believe the French population is already 10 percent Muslim, with 6.7 million people.

France’s official birthrate is 1.9 per woman, but Mr. Gave’s calculations put the native rate at 1.4. Overall, the European birthrate is a low 1.6 per woman.

Mr. Gave extrapolates those numbers, a declining white population and a growing Muslim population, and concludes that France will have a Muslim majority by 2057.

“And so, within 40 years at the latest, it is almost certain that the majority of the population will be Muslim in Austria, Germany, Spain, Italy, Belgium and Holland,” he writes. “Again, these are not predictions but calculations, and I do not even call for new immigrants.”

The pace could be accelerated given continuing Muslim migration into France and other Western European states, either through regular legal processes or through refugees escaping conflicts in the Middle East and North Africa.

“Our summer will be really over the day when demographics will change, simply because we will have become a minority in our own countries and the majority will no longer pay attention to 68-year-old jeremiads, all of whose authors will be retired or dead,” he wrote.

“The immense news of the next 30 or 40 years will thus be the disappearance of the European populations, whose ancestors have created the modern world. And with these populations will disappear the diverse and complementary European nations that have made an immense success of the old continent for at least five centuries.”

Mr. Gave takes an agnostic view on what an Islamic Europe would mean for liberal democracy and free speech.

“I do not say it will be wrong, or it will be good,” he writes. “I am simply saying that this will be very different and that this will necessarily have an influence on the political system.”

Uncertainty

The Gatestone Institute, a conservative foreign policy think tank, analyzed Mr. Gave’s paper and did not agree with all of it. Analyst Drieu Godefridi forecasts that the native French population will not disappear or lose its prominence in the space of four decades.

“It will take more than 40 years for them to vanish from the surface of the earth,” Mr. Godefridi said.

Another problem with Mr. Gave’s projections is uncertainty: What Islam in Europe will look like in 40 years is difficult to predict.

“Only two or three generations ago, tens of millions of Europeans knelt several times a week in churches to show their adoration of Jesus Christ,” Mr. Godefridi said. “Forty years after this religious fervor, almost nothing remains. What we have instead is the well-known phenomenon of ‘dechristianization,’ which has engulfed the whole of Europe.”

Abortions in France have surged in recent decades. Government statistics show that 204,000 abortions were performed in 2015 compared with 760,421 live births.

“Bluntly put, Europeans are not making babies anymore,” he said. “And this has nothing whatsoever to do with Islam; this ‘malady’ is entirely self-inflicted.”

Across Europe, there are signs of the coming Muslim majority. Muslims make up nearly 50 percent of primary school children in the Belgian port city of Antwerp. A quarter of Brussels’ 1 million population is of Muslim origin. The Daily Mail, citing the Office of National Statistics, reported that the most popular boy’s name in Britain in 2015 was Mohammed or its variant spellings.

The Christian Broadcasting Network’s Dale Hurd in 2012 took his cameras inside Belgium’s Muslim strongholds to interview the leader of a small but growing militant group, Sharia4Belgium.

“Democracy is the opposite of Shariah and Islam,” said Belgium-born Fouad Belkacem, referring to his hometown of Antwerp as a “dirty perverted community.”

“Even disbelievers themselves, they say in 2030 there will be majority Muslims here. It’s just a matter of time.”

Three years after the interview, Belkacem was convicted on charges of grooming terrorists to travel to the Islamic State in Syria. A court sentenced him to 12 years in prison.

France is in the throes of a particularly dicey time for the government and its Muslim population. The Nov. 13, 2015, Paris attacks led to a suspension of some individual rights under a national state of emergency. Subsequently, authorities raided suspected radical Muslim enclaves to break up budding attacks.

The emergency decree is scheduled to end in November.

Public officials and scholars talk of “no-go zones,” especially the “banlieues,” the band of predominately Muslim neighborhoods around Paris that stand isolated from traditional French life. French officials deny there are any such zones where police do not patrol.

Mr. Gave’s numbers also conflict with Pew Research polling in 2010. It put the Muslim birthrate at 2.8 children per woman, not 3.4 to 4 children, and did not project an increase.

In 2010, seven years ago, Pew pegged France’s Muslim population at 7.5 percent. Researchers take that number, look at the Muslim birthrate and constant migration, and conclude that the population now must be at least 10 percent.

Pew does not project a Muslim majority in the European Union anytime soon.