'Conversion’ is all around us these days. Whether running a brand campaign, sending targeted direct marketing, or managing a website, the aim seems always to be to ‘convert’ people – from believing x to believing y, from rejectors to prospects, or from customers into loyalists. You'd think we were missionaries, not marketers.

But this religious conversion language is dangerous when applied to marketing. The word ‘conversion’ implicitly assumes a world where people have clear beliefs, which lead to stable, well-defined patterns of behaviour. You are a customer or a non-customer, promiscuous or a loyalist, a Pepsi buyer or a Coke buyer.

And market research data superficially seems to back this up. Ask people whether they believe Pepsi tastes better than Coke, and most people will give a definite answer. Track their responses over time, and behaviour and beliefs seem to move together. Armed with this view of the world, marketing has an evangelical role. You spread the gospel of Pepsi's taste, and though most people will ignore it, a few will hear the call and switch from Coke. Trouble is, the real world ain't like that.

People's ‘beliefs’ about brands are nowhere near as stable and consistent as we like to think. As Byron Sharp points out in his excellent book, How Brands Grow (reviewed in this month's Speed Read on page 47), re-contact surveys reveal that individual opinions about brands are much more volatile than the top-line tracking data suggest. The overall percentage of people who agree that ‘Pepsi tastes better than Coke’ might stay the same from survey to survey, but that does not mean individual respondents are answering the same way each time. Look at the data closely, and it seems people answer questions like this in a 'probabilistic’ way. They may lean slightly in favour of one brand or another, but they certainly don't have fixed beliefs.

Behaviour patterns are inconveniently fluid and messy. We like to think that people divide into distinct buying groups, but look at long enough runs of data, and it becomes clear that real-life buying behaviour is much more ‘agnostic'. Buyers of premium brands also buy own-label. Low-fat buyers also buy full-fat. Coke buyers also buy Pepsi. It seems that, just as our opinions about brands fluctuate, depending on mood and occasion, so do our brand choices. Think of your own life – in the morning, you may be feeling healthy and go for a low-fat, organic product. In the afternoon, you may feel like a treat, and go for something full-fat and indulgent.

Conscious beliefs have much less influence on consumer behaviour than most marketers think. Yes, when brands grow, image scores on tracking studies tend to improve. But, as Sharp points out in his book, the causality mainly goes the other way: that is, sales drive image, not vice versa.

So talk of 'conversion’ is wide of the mark. Most people don't have many strong beliefs about any brands. In fact, most people don't think much about brands at all, at least not consciously. That's why rational messages have so little influence on them (see Mythbuster in June 2010 issue of Admap). Following on from this, people rarely convert from one brand to another in a Damascene way. The best we can hope for is a weak loyalty of habit – a slight tendency to choose your brand over its rivals. Infrequent purchases, such as cars or bank accounts, might seem different, but the slow rate of switching here is just a question of inertia. The kind of true commitment implied by the term ‘conversion’ is rare.

This is another of those situations where we all need to step outside the tidy, idealised marketing world into the infinitely messier world of real life. So let's abandon all the quasi-religious talk. Marketing is not religion. A little agnosticism is a much more useful mindset when it comes to the day job.