If a Martian were to look at a map of the Earth’s religions, what he might find most surprising is the fact that such a map can be drawn at all. How strange--he might say to himself--that so many of the world’s Hindus are to be found in one place, namely India. And how odd that Muslims are so very numerous in the Middle East. With the disconcerting curiosity that is so typical of Martians, he might wonder what explains this geographical clustering. Do people move countries in order to be close to others of the same faith? Or do people simply tend to adopt the religion they grew up with?

The answer, of course, is the latter--on the whole. There are exceptions: Jews moving to Israel, for example, and there are many other cases of religious migration. Still, the huddling of the faithful is mainly explained by the fact that religion runs in families. If you have a religion, it is probably the same one as your parents. Earlier this year a survey by the Pew Forum on Religion and Public Life found that nearly three-quarters of American adults professed the religion in which they were raised. But instead of finding this glass to be three-quarters full, newspapers preferred to notice that it was one-quarter empty. It was the minority of Americans who either switched religions, or abandoned religion altogether, who were highlighted in reports of the survey (“Poll Finds a Fluid Religious Life in US”, ran a headline in the New York Times). Plainly it does not count as news that religion remains largely a family affair. Yet it should do, because of its largely unnoticed consequences. Some religious groups are dramatically outbreeding others, in ways that have an impact on America, Europe and elsewhere.

Consider the Mormons, who grew from six people in a log-cabin in upstate New York in 1830 to 13.1m adherents around the world in 2007. At the beginning of the 20th century, Mormons were a fringe sect in America, with decidedly unusual beliefs. (They officially hold that God once had a body; that people exist as spirits before they are physically conceived; and that Jesus will one day commute between somewhere in Israel and somewhere in the United States.) Today Mormons are about to overtake Jews in America; in fact, they may already have done so. And they almost had their own presidential candidate, in the person of Mitt Romney, a former governor of Massachusetts. The rapid rise of Mormons in America, growing by an average of 40% every decade in the 20th century, is mainly due to their large families. The American state with the highest birth rate is Utah, which is around 70% Mormon. In America, on average, Mormon women have nearly three times more children than Jewish women.

Ultra-Orthodox Jews, however, do have plenty of offspring. This fact is changing the face of Israel, where such families have three times more children than other Israelis. As a result, at least a quarter of Israel’s population of under-17s is expected to be ultra-Orthodox by 2025, according to Eric Kaufmann at Harvard. A similar but more gradual increase in the religious right has been taking place in America for decades, and not just because of Mormons. Conservative Protestant denominations as a whole grew much faster than liberal ones in 20th-century America, and it has been estimated that three-quarters of this growth is due simply to higher birth rates. Were it not for the fact that Evangelical Christians reproduce faster than other Protestants, George Bush--who attracted most of the Evangelical votes--probably could not have made it back to the White House in 2004.

Like other demographers, Eric Kaufmann expects western Europe to become markedly more religious in the course of the 21st century, as a result of the relatively low fertility of unbelievers and immigration from more pious places. Not only do denominations with traditionalist values tend to have higher birth rates than their more liberal co-religionists, but countries that are relatively secularised usually reproduce more slowly than countries that are more religious. According to the World Bank, the nations with the largest proportions of unbelievers had an average annual population growth rate of just 0.7% in the period 1975-97, while the populations of the most religious countries grew three times as fast.

If they want to spread their gospel, then, one might half-seriously conclude that atheists and agnostics ought to focus on having more children, to help overcome their demographic disadvantage. Unfortunately for secularists, this may not work even as a joke. Nobody knows exactly why religion and fertility tend to go together. Conventional wisdom says that female education, urbanisation, falling infant mortality, and the switch from agriculture to industry and services all tend to cause declines in both religiosity and birth rates. In other words, secularisation and smaller families are caused by the same things. Also, many religions enjoin believers to marry early, abjure abortion and sometimes even contraception, all of which leads to larger families. But there may be a quite different factor at work as well. Having a large family might itself sometimes make people more religious, or make them less likely to lose their religion. Perhaps religion and fertility are linked in several ways at the same time.

Mary Eberstadt, a research fellow at the Hoover Institution in Stanford, California, has suggested several ways in which the experience of forming a family might stimulate religious feelings among parents, at least some of the time. She notes that pregnancy and birth, the business of caring for children, and the horror of contemplating their death, can stimulate an intensity of purpose that might make parents more open to religious sentiments. Many common family events, she reasons, might encourage a broadly spiritual turn of mind, from selfless care for a sick relation to sacrifices for the sake of a child’s adulthood that one might never see.

Eberstadt argues that part of the reason why western European Christians have become more secular is that they have been forming fewer stable families, and having fewer children when they do. This, she suggests, may help to explain some puzzles about the timing of secularisation in certain places. In Ireland, for example, she notes that people started having smaller families before they stopped going to church. And, she argues, if something about having families can incline one to religion, this might shed some light on another mystery: why the sexes are not equally religious.

According to Rodney Stark, an American sociologist of religion, the generalisation that men are less religious than women “holds around the world and across the centuries”. In every country--both Christian and non-Christian--analysed by Dr Stark, based on data from the World Values Survey in the 1990s, more women than men said they would describe themselves as religious. There is no agreed explanation for this striking difference. Perhaps the fact that women play a rather larger role than men in the production and rearing of children has something to do with it. If family life does contribute to religiosity, then having larger families might backfire on unbelievers. It might make them more religious. And since faith is still largely a family affair, their children would then be more likely to be religious, too.