We all know that virality is the advertisers' Holy Grail, as marketers look for new ways to subvert the constraints of the traditional media model – because getting your idea across for nothing is always nice!

At its simplest, Virality is about sharing; it's about people sharing (talking about) an idea or object) which then becomes a social object. But what is it that transforms things and ideas into social or viral objects?

Memetics suggests that viral ideas are selfish replicators – units of cultural transmission ("memes") that are unconsciously imitated. Implausible? If so, why do people often talk (metaphorically) of being 'drawn towards' ideas they love, or of being 'repelled' by the things they hate? Conventional wisdom has it that people (rationally) choose ideas; but my company's work supports the converse notion: that ideas recruit people by operating below the conscious radar, firing up their emotions – either positively or negatively. In other words, emotions may be the viral Viagra that transform things and ideas into socially infectious objects.

But, which emotions give us viral Viagra? We have seen again and again that a wide range of emotions are associated with viral objects, but it is the intensity of emotional response that characterises and differentiates such objects. Two of the most talked about things we've measured recently are the Olympics and disgraced BBC media personality, Jimmy Savile. Both provoke strong emotional response, but it is the valence of those emotions that matters most – either strongly positive or negative. Why? For the simple reason that we share and talk about the things that we feel strongly about.

Many (most?) viral objects are infectious because of the positive emotions they engender, and the two feelings we find most commonly associated with them are energy and excitement. These emotions drive us to share ideas with others, and it's why we often speak (metaphorically) of viral ideas having 'a life of their own' - some inner energy that (seemingly) propels them through the population.

But there's a third powerful predictor of virality: empathy. And negative empathy (rejection) can be as powerful a driver of virality as positive identification. If I witnessed the following exchange between a brand lover and a detractor - "I love this brand." What? I can't stand it." - I'd guess that each of them is going to talk about it, but for very different reasons. Ideas we love draw us in (through the power of emotion) and inspire us to become their evangelists. Ideas (and brands) we hate repel us and encourage us to share our (strong) emotions with others.

Advertising too tends to go viral when intense emotions are triggered, but, because most advertising is made to a positive emotion model, it's the top end of measures like inspiration and excitement that are usually most predictive of a viral outcome. Happiness too is a strong predictor – particularly for amusing and engaging ads like CompareTheMeerkat – but sadness can work too in the appropriate context. For example, charity ads sometimes show disturbing or even shocking imagery - which creates feelings of sadness, yet leads to sharing. And the current Xmas ad for UK retailer John Lewis is a classic example of a 'tear-jerker' that's gone viral!

A version of this blog appeared previously on davidpennblog.wordpress.com