Marketing does like to believe in its own power. Persuasion, conversion, fans, advocates, loyalty. The language reveals the fundamental assumption that marketing is a strong force.

Yet when one looks at actual purchase data, there is little evidence that advertising works by converting or persuading.  The notion of creating passionate, committed and unwavering consumer devotion is nothing more than a marketing fiction.

As Professor Ehrenburg pointed out many years ago, if consumers had been strongly persuaded in the past, “there would be many near-100% loyal customers for each successful brand, and they would be important to its sales.”

But few people remain exclusively loyal to any one brand.  Brands are largely composed not of loyalists, but of light buyers who brand the brand  - as well as competing brands - occasionally. Which means there’s no such thing as ‘[insert brand] consumer.’ Just buyers of another brand who sometimes buy you.

These divided loyalties follow a predictable pattern. Brands share their customers with other brands, and do so more or less in line with market shares of those brands.  Buyers of any brand in a category are most likely to also buy the biggest brands in a given category. And they’re much less likely to also buy the smallest brands in the category.

Marketers might like to imagine the consumer in permanent orbit around their brand. But the fact of the matter is that people are happily polygamous in their brand ‘relationships’ for the simple reason that the choice between one brand and another is not that important.

The understanding that customers regularly buy from a repertoire of brands highlights the fact that advertising isn’t a ‘strong’ force that persuades and converts people.

It doesn’t have to.

Instead, advertising works as a weak force.

It works through nudging, not through bringing about Damascean experiences of conversion.

So in place of advertising operating as a ‘strong’ force that converts consumers, we should be thinking of it as ‘weak’ one, nudging people’s purchase behaviours, keeping our brand in their repertoires, or nudging it into people’s repertoires to add it as an extra or substitute brand.

Obviously all this doesn’t happen in blissful isolation. Competitor brands also exercise this nudging effect. And that means there will be a need for sustained and active brand maintenance.  People after all know that brands in any given category are broadly similar.

When so much of marketing is short-term in its horizons, understanding that the role of advertising is to keep nudging people, rather than convert them in a one-off act, reminds us that the real economic power of advertising is felt over the longer term. Advertising is not a one-off cost, as some would treat it. It’s a long-term investment in the health of a brand. For brands to survive and prosper, they require long-term commitment. They require long-term financial investment. And crucially, they require a sustained commitment to creative excellence.

While none of the above is at all new thinking (Ehrenberg was challenging the strong force theory of marketing decades ago), regarding advertising as weak force perhaps also serves to help us through some of the rhetoric that’s out there.

For we’ve been told by some that the alternative to ‘big idea’ marketing is ‘small idea’ marketing. That rather than the froth and the bluster of advertising that publicly seeks to arouse (allegedly) fervent devotion, we can deploy the humbler, and less public nudging effects of for example, behavioural economics. Or the harvesting of established consumer intent.

It’s a perspective that (perhaps unwittingly) is born of regarding advertising as a strong force.  But the perspective that advertising’s primary role is as a weak force erases the distinction between strong and weak effects. And between so-called ‘big’ and ‘small’ ideas.

Whether it’s shelf space, coupons, pricing, advertising, sampling, product design... what marketing does is nudge consumer behaviour.

Seen from this perspective, the question becomes - what along the consumer’s path to purchase can marketing do to nudge people’s behaviour?

In other words rather than indulge in some largely self-serving ideological battle between advertising and ‘other stuff’, we should be building effective ecosystems of weak effects.

Those whose egos cannot abide the notion that advertising and marketing more broadly work through exercising a weak rather than a strong influence might find some solace in considering the workings of gravity.

Gravity is the weakest force in nature - just jump into the air. With your few kilograms of muscle you’ve overcome  all 6 × 1024 kilograms of the Earth’s gravitational pull.

And yet this puny force is responsible for the Moon orbiting around the Earth, the Earth around the Sun, and the motion of the galaxies.

Weak does not mean without consequence.