It's not often that marketers turn to the Bible for brand insight. However, this quote from Matthew reflects an interesting occurrence. When consumers know that a product, or even a behaviour, is popular its appeal tends to increase.

Whoever has will be given more, and they will have an abundance. Whoever does not have, even what they have will be taken from them. Matthew 13 : 12

The leading academic exploring this bias, known as "social proof", is Professor Cialdini of Arizona State University. In a classic experiment he worked with an American hotel chain to understand what would motivate guests to re-use their towels. He persuaded the chain to randomise the messages in their rooms encouraging re-use. His control message, which reminded guests of the environmental benefits, encouraged 35% of people to re-use their towels. The social proof message in contrast simply stated that most people re-used their towels. This version, shorn of any rational message, boosted up-take to 44%. This is not a one-off finding. The results from this experiment, have been duplicated in environments ranging from restaurants to iTunes, from weight gain to tax returns.

Whilst the evidence for social proof is over-whelming, there are disputed explanations. Some point to the evolutionary advantage of following the herd. Slightly flippantly, your chances of surviving to reproductive age increased if you ran when your tribe ran, or avoided eating berries that others shunned. Conformity paid.

Others prefer a modern, rational explanation. They suggest that consumers have learned from bitter experience that brand claims are partial and prone to exaggeration. Therefore, consumers give greater weight to the behaviour of others when choosing brands.

Why don't more brands rely on social proof?

All these experiments raise an interesting question. If social proof is so effective why don't more brands make it a central part of their messaging? There are two reasons that brands feel loathe to do this. The first is that consumers claim that social proof has no impact on them. This is certainly what Cialdini found. In an interesting twist to his towel experiment he asked another group to predict their own behaviour. Consumers overwhelmingly claimed that they preferred the environmental message. Since their actual behaviour tells a different story, it's crucial that brands don't just rely on claimed data to guide their decisions. After all as David Ogilvy memorably stated: "Consumers don't think how they feel, they don't say what they think and they don't do what they say".

The second reason is market leaders assume that their status is already well known. However, our research, amongst 1,003 consumers, suggests that this is a mistake. We asked who the leading brand was in each of four categories: cars, instant coffee, draught lager and coffee shops. For three of the four categories only a minority answered correctly. This was most extreme for draught lager where only 24% knew that Carling was Britain's best-selling pub pint.

Social proof provides an easy opportunity for brands to boost their sales by referencing their scale. Some brands already employ this approach. Most famously Whiskas declared they were preferred by 8 out of 10 cats. However, Cialdini suggests that social proof messages are more powerful when the claims are tailored. We tend to mimic the behaviour of those we have a link with more than the population as a whole. In his towel experiment he ran a third message asking people to re-use their towel because most people in their room had done so. Even this tenuous link boosted re-use rates up to 49%.

The outtake for brands is to tailor their social proof messages. This tailoring could be by media brand – so running messages saying that you're the Daily Mail's or Cosmopolitan's favourite brand to the relevant readers. Or geographically – stating that you're the fastest selling brand in London rather than the whole of the UK.

Brands need to ignore customers' misleading claims and incorporate social proof into their messaging. Those brands brave enough to do so are likely to significantly boost the ROI of their marketing.