It is not clear whether willingness to indulge in ritual is an inherited trait. Whitehouse suspects it is, and is planning studies with children to find out. Clearly, though, ritual is not the exclusive preserve of religion. Obsessive hand-washing, drinking tea in a certain way and crossing oneself with holy water all have one thing in common: "Rituals are by their very nature puzzling activities that invite interpretation," says Whitehouse. Rituals also have an emotional aspect - ranging from a comforting feeling of security or togetherness to extreme terror. And rituals can be repetitive - although the frequency of repetition varies enormously. These three traits are what make religion and ritual such good bedfellows. They provide the all-important elements that allow a religion to flourish: meaning, motivation and memory.
A complex web of interactions link rituals to religion, but for Whitehouse, any attempt to tease out a thread must start with memory. "The reason why there are only two types of religion is that there are only two basic systems of memory that matter," he argues. The first is semantic memory, which deals with things we are conscious of remembering and stores what we have learned about the world. Then there is episodic memory, which hangs onto memorable events from our own lives. Whitehouse argues that to persist and spread, a religion must elicit the help of rituals that reinforce memories in both these systems.
Consider some of the most extreme rituals. According to Whitehouse, they are all classic examples of rituals that invoke episodic memory -- creating personally significant events that are imprinted into the initiate's mind. They are characteristically infrequent, often once-in-a-lifetime experiences, and all are highly traumatic. Being personally consequential and shocking, they are likely to evoke intensely vivid memories, known as "flashbulb memory" of the kind experienced by people with post-traumatic stress disorder. They also leave participants struggling to make sense of the experience, and so constructing elaborate personal meanings for what has happened. Such low-frequency, high-arousal rituals are the lifeblood of "imagistic" religions -- cults and other religions based on iconography, analogy and intense cohesion.
The second type of ritual is exemplified by Muslim prayer, called salat. Muslims pray at least five times a day, following a highly prescribed routine that includes facing Mecca, bowing to Allah and reciting extracts of the Koran. Like Catholic mass and the Jewish Sabbath observance, salat is a high-frequency, low-arousal ritual - the sort characteristic of doctrinal religions. Repetition fixes the information in semantic memory so that believers acquire a deep knowledge of their particular religion's liturgy, no matter how complex or counterintuitive. This way of doing things has advantages over the imagistic mode in that it keeps the ideas at the core of a particular religion cohesive and stable. As a result, doctrinal religions spread easily - so all the world's major religions fit this pattern. By contrast, imagistic religions, with their more creative, fluid and idiosyncratic rituals, tend to be small-scale and localized.