Are religions viruses of the mind? I would have replied with an unequivocal "yes" until a few days ago when some shocking data suggested I am wrong.

This happened at a conference in Bristol on "Explaining religion". About a dozen speakers presented research and philosophical arguments, mostly falling into two camps: one arguing that religions are biologically adaptive, the other that they are by-products of cognitive mechanisms that evolved for other reasons. I spoke first, presenting the view from memetics that religions begin as by-products but then evolve and spread, like viruses, using humans to propagate themselves for their own benefit and to the detriment of the people they infect.

This idea began with Richard Dawkins's The Selfish Gene, was developed in his later article "Viruses of the mind" and taken up by others, including myself in The Meme Machine and other works. It is one version of "dual-inheritance" theory in which genes and culture are both seen as evolving systems.

The idea is that religions, like viruses, are costly to those infected with them. They demand large amounts of money and time, impose health risks and make people believe things that are demonstrably false or contradictory. Like viruses, they contain instructions to "copy me", and they succeed by using threats, promises and nasty meme tricks that not only make people accept them but also want to pass them on.

This was all in my mind when Michael Blume got up to speak on "The reproductive advantage of religion". With graph after convincing graph he showed that all over the world and in many different ages, religious people have had far more children than nonreligious people.

The exponential increase in the Amish population might be a one off, as might Catholics having lots of children, but a comparison of religious and nonaffiliated groups in the USA, China, Sweden, France and other European countries showed that the number of children per woman in religious groups ranged from close to zero (for the Shakers) to between six and seven for the Hutterites, Amish and Haredim, while the nonaffiliated averaged less than two per woman – below replacement rate.

Data from 82 countries showed almost a straight line plot of the number of children against the frequency of religious worship, with those who worship more than once a week averaging 2.5 children and those who never worship only 1.7 – again below replacement rate. In a Swiss census of 2000 the nonaffiliated had the lowest number of births at 1.1 per woman compared with over two among Hindus, Muslims and Jews.

Another striking comparison came from Eric Kaufmann's book Shall the Religious Inherit the Earth?, to which responses differ on whether secularists should be terrified of an impending world dominated by religion or not. When European Jews were classified as orthodox, nonreligious and atheist, the atheists averaged around 1.5 children per woman and the religious Jews nearly three, with the Haredim in Israel averaging six to eight children per woman over many generations.

All this suggests that religious memes are adaptive rather than viral from the point of view of human genes, but could they still be viral from our individual or societal point of view? Apparently not, given data suggesting that religious people are happier and possibly even healthier than secularists. And at the conference, Ryan McKay presented experimental data showing that religious people can be more generous, cheat less and co-operate more in games such as the prisoner's dilemma, and that priming with religious concepts and belief in a "supernatural watcher" increase the effects.

So it seems I was wrong and the idea of religions as "viruses of the mind" may have had its day. Religions still provide a superb example of memeplexes at work, with different religions using their horrible threats, promises and tricks to out-compete other religions, and popular versions of religions outperforming the more subtle teachings of the mystical traditions. But unless we twist the concept of a "virus" to include something helpful and adaptive to its host as well as something harmful, it simply does not apply. Bacteria can be helpful as well as harmful; they can be symbiotic as well as parasitic, but somehow the phrase "bacterium of the mind" or "symbiont of the mind" doesn't have quite the same ring.

This is how science (unlike religion) works: in the end it's the data that counts. Being shown you are wrong is horrid, but this has happened to me often enough before (yes, you may make jokes if you like) and one gets used to it. This shock may not be as bad as when I discovered I was wrong about the paranormal, but it's still a shock. The good side is that it has thrown me into new thoughts, new lines of inquiry, and set me wondering again just how religions can have such power over us.