I went last night to a marvellous talk by an American anthropologist who has been studying Californian charismatic Christians. Tanya Luhrmann's enquiry into how these people construct their idea of God will result in a book eventually, but in the meantime her talk on her work with the Vineyard churches was full of insight, sympathy, and deadpan humour.

The Vineyard churches are a loose international network of mostly white, mostly middle class, very charismatic churches. They aren't exactly fundamentalist but they see the Holy Spirit everywhere and talk to God every day. They were the source of the "Toronto Blessing" - a craze which swept through the English charismatic network in the 90s where people fell on the floor and made animal noises. Luhrmann is interested in how you get to talk to God like this. After all, most churches for most of history, haven't done anything like that.

Her answer is that you need a certain kind of temperament, one which makes you good at make-believe, and then you need to work at it. The personality traits which make it easiest to talk to God are those measured on the Tellegen absorption scale, which she summarises as the ability to focus attention on a non-instrumental subject: in other words, some thought interesting for its own sake, whether or not it is obviously useful. It's the facility you need to construct compelling daydreams.

If you have this talent, or temperament, in the first place, these churches will nourish it. By treating God as real, you come to detect his presence more easily; and the God for whom the are searching is one just like another person. "People learn about God by mapping onto Him what they know about persons; then they map back what they suppose about God onto the world around them."

All this activity is the subject of tremendous social reinforcement. These are not Sunday only churches. Members can fill their lives with meetings with other members – and with God. "They pay constant attention to what's going on in their minds. They are constantly looking at their thoughts and images. It's a social shaping of what you would imagine to be a private space in their minds.

"It is striking", she said "Just how explicit is the invitation to suspend belief". Some churches urge people to pour a second cup of coffee for God at the breakfast table; some members would invite God for dinner, and lay a place for him. One woman would have "Date nights" with God, where she would go into the park and sit on a bench with him, eating a sandwich.

All this takes time, and effort, and it doesn't work for everyone. She said that in her own participation in a prayer circle ("I hoped to get a book out of it; they hoped to get a soul") her own, agnostic experience was not that different from that of the believers around her. "These things are very powerful, and very real, but it is very very hard to have god come close. Still, there is no question that if you can walk this walk it will make you happier and more cheerful."

Related to this is the interesting point that their faith in God, and their experience of him, is often actually strengthened by the failure of prayer. The members of these churches believe two things about God: that he can give you anything he wants, and that he stands beside you in your disappointments. So the failure of prayer can lead to a deeper experience of God, and this is more valuable than the success of the haircut you prayed for.

All this sounds like reasons entirely to dismiss the experience as obvious make-believe or hallucination which anyone could be reasoned out of. There are two parts to her story which make that more difficult. She herself says it's not her job, as an anthropologist, to comment on the truth of the beliefs she studies, but that she is perfectly comfortable with the possible reality of the supernatural. A friend you can see only in your imagination need not be an imaginary friend.

But what is clear is first that the voices people hear, and the sights they sometimes see from God are very different to the delusions of psychotics. The sudden quiet voice, interpreted as that of God, is very common – about half of the students in her classes have heard them at least once (as I have, for what it's worth). But they are brief, rare, and, though startling, not disturbing. The voices that people here in psychosis are none of those things. They talk a lot, and often, and their content is often upsetting and hostile.

The second point is that "This kind of god that seems so primitive is in fact the product of a plural, sceptical, society." For many people the liberal god who works no miracles has failed. These practices make the personal god of the Vineyard churches immediately present and available. The concentration on experience invites derision but disarms it at the same time: "God is special and more real than real, so that their experience is in some sense protected from the scepticism which members of these churches encounter every day."