So how come doctrinal religions haven't overrun imagistic ones? Religions driven by frequent, unexciting rituals face their own problems, Whitehouse says. The most crushing of these is what he has dubbed the "tedium effect". An obvious solution would be to make the rituals more emotional, but he says this risks inspiring participants to generate personal interpretations and so fracturing the religion. They would undermine the doctrinal orthodoxy that is crucial for these sorts of religions to flourish. One strategy is to employ charismatic preachers to offset the tedium and use more committed members of the religion to police it and keep the flock in check, which might explain the hierarchical nature of organized religion.
Whitehouse's theory is compelling because it integrates many aspects of religion, from the psychological to the socio-political, but how reliable is it? One advantage is that it makes testable predictions. For example, religious rituals are unlikely to be both low frequency and low arousal - because such rituals would not be easily remembered - or high frequency, high arousal - because most people will not willingly undergo too much torment even in the name of religion. It also predicts that doctrinal religions will tend not to have low-frequency, high-arousal rituals because they undermine orthodoxy, and imagistic religions will tend not to have high-frequency, low-arousal rituals because these undermine exclusivity.
Whitehouse and his colleagues are currently compiling the world's largest database of religious rituals to test such predictions. It is part of an ongoing project, EXREL (Explaining Religion), but preliminary indications suggest that religious rituals do indeed cluster around two forms that associate respectively with the two types of religion. He describes the observed pattern as "bunching with some outliers", and points out that you would expect the occasional anomaly because there is nothing stopping the emergence of an exciting, high-frequency ritual or a boring, low-frequency one. However, he expects to find that such rituals are a flash in the pan because they don't have what it takes to glue together compelling religion.
Not everyone is convinced by Whitehouse's argument. Robin Dunbar, also at the University of Oxford, says it fails to address the key question of why we have religion at all. He believes religion evolved as a survival mechanism and that the main purpose of rituals is social cohesion, which is at the core of our success as a species. And Whitehouse's theory doesn't explain why even within major religions there are pockets of extreme behaviour, such as the self-flagellation rituals practiced by some Shia muslims during the holy day of Ashura, the practice of anaesthetic-free infant circumcision in conservative Jewish circles, or the re-enactments of crucifixion by some Christians in the Phillipines.
Christmas itself is probably a little too racy to fit neatly into Whitehouse's theory. But Whitehouse has an explanation. Feel-good, annual events tend to be a feature of doctrinal religions, he says. Happy or euphoric experiences are not remembered in the way traumatic ones are, so you can have the occasional knees-up to boost motivation without undermining doctrine. "You just need to find the sweet spot," he says. Which at this time of year is probably good advice for anyone of any religious persuasion, or indeed none at all.