**Marketing Memetics**, by Michael Taylor

Humans don’t have good intuition for exponentials: we often underestimate them. Start with a group of 50 people, and grow membership 10% per week – you’ll have 7,100 at the end of the year. Keep up that rate, and next year you’ll hit 1 million. Viruses infect populations exponentially, explaining why governments take extraordinary measures to limit the spread. Memes, or ‘viruses of the mind’, follow the same math, as they jump from brain to brain via word of mouth.

Memes with a viral coefficient greater than 1 – each person spreads the idea to more than one other – grow exponentially. This is a function of participation rate (how many hosts share), branching factor (how many they share it to) and infection rate (how many exposed get infected). Also important is cycle time – how much time it takes for all this to happen – spreading in days not weeks has a dramatic effect on growth.

Exponential growth can’t continue indefinitely: 10% growth over 4 years is 20 billion members, more than Earth’s population. Viruses eventually run out of hosts to infect. Exponential ‘hockey-stick’ growth is just the first part of the logistic ‘S-curve’ shape, where growth flattens out as the population gets saturated. What follows is a long, slow decline as the meme is forgotten: memes that are memorable, or get documented by their hosts, will survive longer. Only technological change can reverse the decline: mature memes jump to a new medium, riding a higher S-curve.