A possible future where the number of people pre-disposed to religiousness dwarfs the numbers of the non-religious is hinged on a "fertility differential between religious and secular people". The bigger this differential, the faster the ranks of the religious swell relative to the non-religious.
The model’s assumptions are based on data from previous research. Studies have shown that, even controlling for income and education, people who are more religious have more children, on average, than people who are secular (defined here as having a religious indifference). According to the World Values Survey for 82 countries, adults attending religious services more than once per week averaged 2.5 children, those attending once per month averaged 2.01 children, and those never attending averaged 1.67 children. The more orthodox the religious sect, the higher the fertility rate, with sects such as the Amish, the Hutterites, and Haredi having up to four times as many children as the secular average.
This is not to say, people with the "religiosity gene" will necessarily be religious in practice...
If both parents have the religiosity allele, their children are also more likely to have the religiosity allele than if one or both parents did not have it. However, children born to religious parents may have the nonreligiosity allele, while children born to secular parents may have the religiosity allele. Having the religiosity allele does not make a person religious, but it makes a person more likely to have characteristics that make them religiously inclined; the converse is also true.
The study coincides with information making headline news in recent days highlighting an on-going boom in the world's Muslim population...
[...] in Europe, [researchers at the Pew Forum on Religion and Public Life] predicts the Muslim population will grow by nearly a third in 20 years, from 44.1 million people, or six per cent of the region's inhabitants in 2010, to 58.2 million or eight per cent of the projected total population by 2030.
Some European Union (EU) countries will see double-digit percentages of Muslims in their population by 2030: Belgium's Muslim population is projected to rise from six per cent to 10.2 per cent over the next 20 years, while France's is expected to hit 10.3 per cent in 2030, up from 7.5 per cent today.
In Sweden, Pew predicts Muslims will comprise nearly 10 per cent of the population compared to less than five per cent today.
Britain's Muslim population is predicted to rise from 4.6 per cent to 8.2 per cent by 2030, and 9.3 per cent of the population of Austria is forecast to be Muslim by then, compared to less than six per cent of residents of the alpine country now.
There you go.