Then there is the question of whether evolution is the right framework for all the forms of cultural transmission? Are models for the spread of disease a better fit? You will find plenty of discussions of this type of question across the cultural evolution literature, but little convergence.
I think it's fair to say this issue has been widely discussed - with some folks promoting "viruses of the mind" and
"cultural virus theory" for example, and other folk criticizing them. However the issue is not very complicated and should not be that controversial. Here's a position summary that I think that most knowledgeable parties should be able to agree on:
Models of disease spread often need to be generalized before being applied to culture. One issue is the sign of the fitness correlation between symbiont and host. Diseases almost always have a negative fitness correlation with their hosts - i.e they reduce host fitness. By contrast, culture can have positive or negative effects on host fitness. The sign of the fitness impact matters, since negative fitness impacts on hosts are resisted and rejected, while positive fitness impacts on hosts are promoted and encouraged. Symbiology performs this generalization. However symbiology generally deals with close relationships. There's a larger field that deals with all kinds of relationships - known as biological interactions. Models of biological interactions are highly suitable for modeling cultural evolution.
Many models of epidemiology are explicitly evolutionary these days. It is widely recognized that parasites evolve in real time - including within host lifetimes. Less widely recognized, but still well-established science is the notion that host immune systems use evolution and natural selection to adapt to the parasites they face. Evolutionary epidemiology is clearly a thing. The main cases where non-evolutionary models of epidemiology are useful is when dealing with short time scales - or highly simplified models. For example, fleas might spread through an island community without either them or their hosts evolving very much. A similar cultural example of where a non-evolutionary epidemiological model might be appropriate is where Rubik cubes spread through a network of children in schools. Here, neither the cubes nor the children are doing much change or evolution. As a first approximation, models based on evolutionary theory can be ignored in favor of simple epidemiological models.
It is certainly true that in cultural evolution, epidemiological models are sometimes the most appropriate. The exact same thing is true in the organic realm. However evolutionary and epidemiological models are generally complementary and compatible - they are best used under different circumstances.
Until recently I had considered the obesity epidemic to be primarily a cultural epidemic -
an epidemic of memes. Fast food companies spread their obesity-promoting memes throughout
the population using advertising (which consists of more memes) - and so manipulate people into eating more of their food, thus making the companies richer and financing the production of more obesity-promoting memes.
Obesity isn't just down to memes. Genes in foods are also involved - and humans
are breeding fatter, tastier and sweeter produce to better cater to the human palate.
However, it is becoming clear that another source of genes is also involved - genes inside
microbes in our intestinal flora. A variety of microbes have been fingered as obesity-promoting
strains. One famous one is Candida albicans.
This suggests the disturbing picture of obese individuals as folks whose bodies have been hijacked by
malevolvent microbes and turned into production factories that leave a trail of infection behind them.
The microbes eat their hosts' food for them and in return pump hunger-promoting substances into their bloodstream.
This more complex picture of a swarm of obesity-promoting memes and genes makes it
harder to blame the obesity epidemic on memes. However, it creates an interesting
and unusual situation - where humans are being manipulated both by a swarm of memes
and by a swarm of genes inside microorganisms. Can scientists learn anything from this
simultaneous attempt to manipulate the same trait by both memes and microbial genes?
It may provide an interesting opportunity to compare organic and cultural evolution.
For example, consider the question of whether genes or memes adapt faster. Most
previous attempts to study this question have compared memes with human genes.
However human genes reproduce at a very slow rate - and this is obviously
an unfair comparison and a walk-over for the memes. Comparing memes and gut bacteria
seems like a fairer comparison between the organic and the cultural.
Another related topic is cultural
opportunistic infections
when a host is infected by one parasite, that sometimes opens the door to other parasites.
A classic example is Hepatitis D - which only attacks those already infected with Hepatitis B.
In general, it is common for an overworked immune system to divert resources from one area to concentrate
on an existing attacking force - leaving some areas less well protected. Learened immune deficiency can result if invaders attack the immune system.
In the case of obesity, it is easy to imagine fast food memes promoting Candida infections. Probably hungry Candida sufferers are also more susceptible to fast food memes. Cross-domain opportunistic infections seem likely to be a real thing - but memeticists need to study this topic to better understand it.
Numerous students of cultural evolution have got a lot of mileage over the years out of viewing animal brains as the central locus of cultural evolution. Brains are envisaged as repositories of cultural information that selectively adopt it and selectively transmit the cultural information to other creatures.
It's always been a central thesis of memetics that this is only one way of looking at the situation. As well as focusing on the cultural hosts, one can alternatively model the cultural symbionts themselves. The cultural symbionts have their own life cycles and lineages - which are independent of those of their hosts. From this perspective the hosts are one part of the environment. The environment includes multiple other copying devices (such as printing presses) and multiple other sources of selection (such as fires). Animal brains are just one part of this bigger and better picture.
It is worth noting that biology has gone through this conceptual revolution once before. It was once not widely recognized that many diseases were caused by microscopic pathogens - such as bacteria and viruses. The germ theory of disease was developed in the 19th century. Before that time, many diseases were modeled in a variety of ways using many inaccurate theories. Miasma theory held that some contagious diseases were the result of airborne pollutants. Sometimes pathologies were considered to be the results of hauntings, curses, karma, witchcraft or the work of the gods. Some of the theories came close to the right answer. The ideas of demonology and possession attributed some maladies to the actions of external agents that lived inside the host's body. These theories were partly validated by the germ theory of disease - though the causitive agents turned out to be rather different.
I think we are now facing a situation which is similar to that faced by 19th century physicians - with the modern study of culture. The cultural equivalent of the germ theory of disease is the the germ theory of culture. This was proposed by Cloak (1975) and Dawkins (1976). As with the germ theory of disease, the idea faced resistance, and took a considerable quantity of time to be widely adopted - 50 years approximately. Memetics further supports the basic model present in demonology and possession - that pathology is caused by invasion of hostile agents from outside the body - which can potentially be evicted again.
Today people routinely talk about culture "going viral". Cultural epidemiology has been enshrined in the dictionary - in the form of "memes" and "contagions" - which dictionaries explicitly acknowledge take cultural forms. However, the whole the idea still faces resistance within academia. Many still just don't understand that "viruses of the mind" are real things that can cause or contribute to pathologies. The concept is widely denigrated as being an imperfect "analogy". Others describe fear of manipulation by microscopic entities as "Darwinian Paranoia". Others opine that cultures are complex wholes that can't be meaningfully split up into germ-like pieces - and so on.
It is rather tragic to watch this situation unfolding - in the light of all those who died during the development of the germ theory of disease. You might think we had learned that small, information-carrying entities that can be transmitted between hosts can cause real problems. A wider recognition of the germ theory of culture seems likely to lead to more and better diagnosis and treatments of transmissible pathologies - just as happened with the the germ theory of disease.
I have some comments on that paper as well. I think it is a bad paper. The authors claim to have found:
key differences between the two domains that compromise, we think, the attempt to understand cultural evolution on par with genetic evolution.
The paper concludes:
This suggests that in each domain, specific cognitive mechanisms lead to the emergence of domain-specific cultural dynamics. There is therefore no particular reason to build models of cultural evolution based on an analogy with population genetics (Daly 1982).
It is true that there are domain-specific cultural dynamics. However, there are also domain-specific dynamics in the organic realm. That doesn't prevent models from population genetics being useful there. It seems to me that the merit of models based on techniques from population genetics in cultural evolution has been well established empirically over the last four decades. The authors don't really present a case which counters this large body of work.
Instead they focus on the issue of "Transmission Modes" - which they define as follows:
A central feature of population genetics is the reliance on the concept of transmission modes (TMs). A TM is a way in which genetic material is transmitted between individuals.
The authors claim in the abstract:
Modes of cultural transmission are, by analogy with modes of genetic transmission, ways in which cultural information is transmitted between individuals. Despite its importance across the behavioral sciences and for theories of cultural evolution, no attempts have been made, to our knowledge, to critically analyze this analogy. We here aim at such detailed comparison and show that the fundamental role of modes of transmission in biology results mainly from two properties of genetic transmission: (1) what is transmitted generally does not influence the way in which it is transmitted; (2) there is a limited number of simple and stable modes.
However to my mind they fail to establish either of these points.
There are an enormous number of methods of transmission in the organic realm. Genes may be transmitted between individuals by sneezes, by sexual intercourse, by contaminated water, by blood transfusions, by biting insects - and so on and so forth. If you are classifying transmission methods into discrete "modes" and arguing that these differ in number in cultural and organic evolution, the you should present a classification scheme, and argue for its utility in both domains. However, the authors present no methodology for doing that. Also, few other authors do that: "transmission method" is a much more common phrase in genetics than "transmission mode" is - as can easily be seen by performing some searches.
Saying that a transmission mode is "a way in which genetic material is transmitted between individuals seems vague. Are mosquito bites and tick bites different "ways" of transmitting genetic material between individuals? Or is it one "way" - on the grounds that both are biting insects? Are coughing and sneezing different transmission modes? - or one mode? - on the grounds that both involve airborne particles. The authors don't say - and don't provide any way of answering such questions.
Since they don't defend a classification scheme for transmission methods, it seems to me that they have failed to substantiate their case that the organic realm features a limited number of simple and stable transmission modes. Looking at the range and diversity of transmission methods in the organic realm, this claim seems implausible to me.
Also in the organic realm, what is transmitted very often does influence the way in which it is transmitted. When a cold virus is transmitted, it makes the host sneeze. When a warts virus is transmitted it makes the host itch. When a stomach bug is transmitted, the host gets diarrhoea - and so on and so forth. Where there are multiple tranmission methods, mutations may alter the balance between them - showing that genetic control over the transmission method exists.
For me, these simple observations cause the whole thesis of the paper to collapse.
Interestingly, the paper describes itself in grandiose terms, saying:
This article therefore specifies the fundamental properties upon which the analogy between cultural and genetic transmission modes rest, suggests different interpretations of previous data, raises challenging modeling opportunities and develops a new hypothesis regarding the origin of the difference between biological and cultural transmission.
I think this captures some of the excitement of those who seek to characterize the differences between the cultural and organic realms. These explorers are seeking out new laws and new principles. However, step one for scientists is to familiarize yourself with the existing literature. Often, once you have done that, not everything looks quite so new and shiny. To me, many of the supposed innovations associated with cultural evolution fit into the category of phenomena that have already been characterized by evolutionists.
Memetics takes the relationship between the organic and cultural realms more seriously than any other theory of cultural evolution I am aware of. However, the close parallels seen by memeticists are not always shared by cultural evolution enthusiasts in academia. Alberto Acerbi helps us to highlight this difference in perspective today. To him, the close parallel seems wrong - and he explains why.
He recently posted about the differences between organic and cultural evolution on his blog. Here's one of his comments:
Moreover, a part from having effects in cultural dynamics, regulatory traits represent a difference between cultural and biological evolution. This is an “hot” topic in modern cultural evolutionary theory so I do not want to go in depth here (let me just say at least that I think it is interesting to study also the differences between the two evolutionary processes). “Rules” of genetic transmission tend to not be under genetic control, and models of cultural evolution inspired by evolutionary biology tend to consider the rules of cultural transmission (for example the propensity to learn from others) in the same way not under “cultural control” (they are usually considered under genetic control).
...and here's my reply - in which I explain why I regard the parallel as being closer:
Surely, everyone believes that the rules of genetic transmission are under genetic control! Probably, you are talking about how easy the rules are to change by making genetic modifications.
Note that, if you are comparing genetic with cultural evolution, you should really be looking at whether the capacity of organisms to transmit DNA-based symbiotes (e.g. gut bacteria, and parasites) is easy to change by making genetic modifications. In which case, you will see that the way in which such genes are transmitted is relatively malleable. It is easy enough for genetic changes to make an organism more or less likely to sneeze, for instance.
If you adopt this perspective on evolution in the organic realm, you will see that the case of DNA-based symbionts turns out to be closely comparable to the case of cultural symbionts - with respect to how easy it is for DNA to modulate the transmission pathways involved.
If anyone has ever reasoned from organic evolution to cultural evolution - arguing that since genetic transmission is not easy for genes to regulate, it will not be easy for culture to regulate cultural transmission - then probably their understanding of the dynamics of genetic transmission is faulty. There's no need to question their understanding of the relationship between organic evolution to cultural evolution on these grounds - since this is an example of where these processes are very similar. There are, in fact, plenty of known cases where genes can easily regulate the transmission pathways of other genes residing in symbionts.
Although more flexible modalities of gene transfer exist [34, 35], genes typically propagate to offspring from just two (sexual reproduction of chromosomal DNA) or one parent (asexual reproduction or sexual reproduction of mithocondrial DNA). Cultural information instead can be transmitted in many different ways and, potentially, from any individual to any other individual, which creates the opportunity to regulate the flow of information in a more fine–grained and context–dependent way.
Cultural information typically propagates from one (or sometimes more) parents to offspring as well. However, the parents aren't human, they are cultural. For example, the parent of a bible is typically another bible. To conceptually mix together human hosts with cultural descendants would be to muddle together cultural and organic evolution. If you do that, you typically do get into a muddle about these issues - but the solution is to not do that. Humans aren't the "parents" of cultural information. They are the parents of their own children. Cultural information is unrelated to them. It's in a completely different lineage, which almost never recombines with human genes.
In both the organic and cultural realms, symbionts may potentially be transferred from any individual to any other individual. Organic symbionts - such as flu viruses may be passed between any two individuals. Similarly, in cultural evolution, bibles may be passed between any two individuals. The situation is a pretty close parallel in this regard. That's because both cultural and organic evolution are implementations of universal Darwinian rules.
Of course, there are differences between cultural and organic evolution - but they are more subtle than this. A failure to recognize the similarities results in existing work not being reused - and reinvention-of-the-wheel syndrome. Also, exaggeration of the differences hampers the development of a unified theory that covers both the cultural and organic realms. We have known about the importance of symbiosis for a long time. Culture is well modeled within a biological framework using the idea of cultural symbionts - and the existing theory of symbiosis - as was recognized long ago by the cultural evolution pioneers Ted Cloak and Richard Dawkins.
It is frustrating for me to see modern students of cultural evolution struggling to understand the topic without using the concept of symbiosis. In 2011, Alex Mesoudi managed to write a whole book on the topic without event mentioning concepts from symbiosis at all. To me, it all seems a bit like time-traveling back to the 1950s - before the idea of symbiosis was widely understood.
It reports on the successes of meme therapy targeting violence. It also discusses the difficulties in getting people to accept an epidemiological approach to violence. Of course, violence as a mind virus is pretty much part of memetics 101.
Treating violence as an infectious epidemic is effective
Three main strategies are used in reversing infectious epidemic processes. These are:
detecting and interrupting potential infectious events;
determining who are most likely to cause another infectious event and reducing their likelihood of developing disease and subsequently transmitting; and
changing the underlying social and behavioral norms, or environmental conditions, that directly relate to this infection.
These methods have resulted in reductions in shootings and killings of 16% to 34%.
The Cure Violence method is designed around these principles. This method begins with epidemiological analysis of the clusters involved and transmission dynamics, and uses several new categories of disease control workers – including violence interrupters, outreach behavior change agents, and community coordinators – to interrupt transmission to stop the spread and to change norms around the use of violence. Workers are trained as disease control workers, similar to tuberculosis workers or those looking for first cases of bird flu or SARS.
Meme inoculations, therapy and engineering have potential for treating other disorders besides violence. Drug abuse, road rage and many other mental health disorders could likely be treated as infectious diseases. However, first academics need to understand the idea of cultural epidemiology before they can properly investigate hypotheses based on it.
The symbiosis between humans and their gut bacteria is usually a happy one. The humans feed the gut bacteria and the gut bacteria feed the humans. It's a classic win-win relationship - an example of a mutualsim.
However, the relationship is not always a happy one. Sometimes "bad" bacteria get inside people - and cause them to commit anti-social acts - such as leaving a trail of liquid feces behind them whereever they go. This is good for the bacteria, but not so good
for the humans. We say that such people have diarhoea.
Memes are not always good for their host humans either. One of the manifestations of some types of bad memes is verbal diarhoea. You've probably encountered the type: pyramid marketing enthusiasts, conspriacy theorists, apocalyptic missionaries, people trying to recruit you for their preferred cause or religion. Of course a certain amount of talking can be good for you - but too much talking is probably good for the associated memes, but not so good for their hosts. Those with verbal diarhoea are taking their talking too far.
The expression verbal diarhoea is an interesting case of folk cultural epidemiology.
A recent article in The Economist criticises approaches to cultural evolution based on epidemiology.
The article is called: Social contagion - Conflicting ideas
The article boils down to saying that disease organisms are bad, and that people want to avoid them - whereas many ideas are good - so people want to acquire them. On that basis it apparently advocates ditching models based on epidemiology.
The article concludes with:
But it suggests that ideas are sufficiently different from diseases that it might not be wise naively to apply models designed for one to probe the other. High time, then, for social psychologists to stop piggybacking on epidemiologists and work harder on their own models.
My assessment is that we need generalised epidemiology - or infodemiology - that deals with both deleterious and beneficial interactions with cultural symbionts. A generalised epidemiology is needed in both the organic and cultural realms - since both feature parasitism and mutualism.
The article's suggestion - that we should ditch epidemiological models and start again from scratch is misguided. It violates the basic principle of building on what you already have. Switching from parasitism to mutualism in basic models of contagion is just a case of switching a sign around. It is true that adaptations for avoiding symbiotes can look a bit different from adaptations for acquiring them - but that does not entail discarding all our existing models, terminology and framework. We do have some three decades of work in cultural evolution built on epidemiological foundations. Existing models don't make the half-baked predictions that author claims arise from an epidemiological perspective.
Copverage in Physorg puts a similar spin on the paper - saying:
Now however, researchers from Cornell University have shown that users adopting Facebook, tend to do so more predictably when receiving invitations from multiple sources, rather than a lot of requests from members of the same group, which implies that Facebook and its growth, does not actually compare with biological contagion at all.
It has thirty one references, practicallly none of which are to the primary literature on cultural evolution. Three cites support their assertion that "traditional" models predict that ideas necessarily spread according to exposure to them - rather than to the extent that recipients want to be exposed to them.
Of course, the actual literature on cultural evolution has long been aware that contagiousness of a cultural entity depends on the properties of the entity, the properties of its hosts, and the structure of the host population (e.g. see Laland and Odling-Smee, 2000) - and it has long recognised both cultural parasitism and mutualism (e.g. see chapters 4 and 5 of Boyd and Richerson, 2005).
Epidemiology dealls with the spread of health-related phenomena through populations. These could be pathogen-borne illnesses, poisonings, work-related illnesses, or a range of other health-related events.
Many traits that spread are the result of some kind symbiosis. Pathogens spread through populations - but so do mutualists - such as the relationship between humans and various kinds of fruit and vegetable. Humans also have mutualistic relationships with a variety of decorative and medicinal plants.
Other traits that spread through populations are cultural traits. These can usefully be seen as being cultural symbionts. Fads, fashions, and trends spread through populations much like plagues and viruses do. They too spread like epidemics and plagues - and have their own epidemic threshold.
Other types of phenomena spread as well - for example radioactive fallout, sunburn and frostbite.
However, not all of the phenomena that spread are particularly health related. Currently the fields of mutualisms and memetics borrow heavily from the terminology of epidemiology to describe the dynamics of the systems they study. It seems desirable to generalise epidemiology to cover all traits.
So, here we propose generalised epidemiology - the science that studies how traits spread through populations - irrespective of whether the traits are related to health.
This field differs from demography through its focus on change - and through not being confined to humans.
A similar coinage is infodemiology. One paper claims:
Infodemiology can be defined as the science of distribution and determinants of information in an electronic medium, specifically the Internet, or in a population, with the ultimate aim to inform public health and public policy.
That's a pretty duff definition, but the word could work. Generalised epidemiology is intended to cover infodemiology and epidemiology. However, "infodemiology" may be more of a rival term - and it is a bit of a shame that the first stab at it didn't cover the general case.
It is proposed that use of the term "epidemiology" to refer only to health traits should eventually be deprecated, allowing the term "epidemiology" be reassigned to refer to the concept of generalised epidemiology described here. This proposal is supported by the etymology of the word "epidemiology" - which says that it means, roughly speaking: "what is on the people".
Memetics is deeply based on the science of symbiosis. However, sometimes the study of symbiosis seems to be in almost as much of a muddle as the evolution of culture.
I think one of the more obvious proposals for fixing its terminology involves the term "epidemiology".
Epidemiology is the study of health-events, health-characteristics or health-determinant patterns in a population.
Epidemic is worse - that says epidemics are concerned with disease spread.
At the moment, the word occupies a position which could potentially be useful to the science of symbiosis. What the science of symbiosis really needs is a term to refer to the spread of symbionts through a population of hosts.
However, epidemiology today seems to be all about health. Health varies due to host genes and inorganic environmental factors as well as being affected by symbionts.
At the moment, epidemiological terminology is used ubiquitously to describe symbiosis. However it is pretty bad at dealing with mutualisms. Students of symbiosis really need to decide on their terminology. The most obvious options are:
Hijack epidemiology and attempt to repurpose it;
Develop a "generalised epidemiology" variant that covers any trait - not just health;
Develop a "symbiotic epidemiology" variant;
Deploy some entirely new terminology to describe the spread of symbionts;
I think there's a good case to be made for the first option. The second option has some merits too. I expect that the fourth option is not going anywhere.
The "rationale" for the first option is: mutualist symbionts actually have a positive effect on health. Exactly neutral symbionts are too rare to be worth mentioning. As for the fact that health varies due to host genes and inorganic environmental factors: that is acceptable.
This is pretty-much how I treated epidemiology and its associated concepts in my "Memetics" book.
I think the etymology is supportive of widespread deployment in the context of symbiosis.
An alternative to repurposing epidemiology would be to develop a science of symbiology - with its own terminology. This would deal with the spread of symbionts - but not inorganic environmental factors such as chemical spills or nuclear fall-out. However, symbiology is currently underdeveloped and doesn't yet have good terminology in this area.
Perhaps in time, "epidemics" and "epidemiology" will lose their explicitly medical associations as well.
The terms "pandemic" and "plague" should also probably be taken away from medicine and repurposed for the greater scientific good.
Social media marketing departments must decide how to allocate their budget between making content that will spread - and distributing that content.
The initial distribution is sometimes called "seeding", and focusing on that distribution is sometimes called using a "big seed".
Big seeding was popularised in 2007 by an article entitled Viral Marketing for the Real World - by Duncan J. Watts, Jonah Peretti, and Michael Frumin.
So, in the light of their article, the question naturally arises: does big seeding actually work?
For my epidemic threshold article I made a simple computer
simulation of epidemics that produced graphs showing how the number of infected individuals could increase or decrease over time.
An epidemic - showing the number on infected hosts plotted against time.
The model that produced the above diagram is extremely simple. Individuals are modelled as being either infected or not infected. They have a constant probability of dying in each generation. Senescence and pathology are not modelled. Infected agents infect more agents (randomly) in each generation. Then some individuals (chosen randomly) die, and are replaced by healthy newly-born individuals. The population size is a fixed constant - so the death rate and birth rate are equal. The plot was made by varying two parameters: the infection rate and the death/birth rate. These variables are sampled from a uniform bounded random distribution to create the plot.
The seed population is fixed at the same value for each run and is shown as the y-intercept on the left hand side of the diagram.
The diagram illustrates the concept of an epidemic threshold - if content is insufficiently infectious, it dies off, and goes extinct.
The next issue I wanted to explore was to see how the seed population size
influenced the extinction rate.
This is a plot of survival against seed population size.
Here, "survival" refers to having a population size of at least
1 at the end of the run - i.e. it refers to not going extinct.
This graph illustrates two main things:
Having a seed population too small is often fatal - random fluctuations in population size too easily
cause your seed population to execute a random walk into
extinction.
Big seeding rapidly runs into diminishing returns - provided you seed on a reasonable scale, success depends quite a bit on how much you exceed the epidemic threshold by - and not so much on the size of your seed population.
How to manage the tradeoff between the seeding budget and the contagiousness budget is beyond the scope of this article - but hopefully these graphs will help people to understand the basics of the dynamics involved.
Memetics has access to a plethora of negative words to describe the spread of ideas.
"Viral", "contagion" and "epidemic" are some of the most commonly-used terms.
These draw on the language of epidemiology. There are also words to describe rapid growth sometimes exhibited by these systems: "explosion", "boom", "ignition" and "wildfire". These terms are associated with fires and explosions.
Unfortunately, a lot of these terms are pretty negative. This is unfortunate - since we know that ideas were - on average - positive among our ancestors - since humans have idea-collecting and spreading adaptations.
Marketers would probably prefer not to use such negative terms. After all,
they are typically trying to hook consumers up with producers in win-win deals
- and not trying to infect them with some kind of deadly plague.
So: what positive terms are there out there? Not so many, alas. After
surveying the positive terms for growth: "branching", "budding", "sprouting",
the most appropriate positive term I managed to find was "bloom" - as in
"algal bloom".
So, perhaps in the future, positive marketing campaigns will bloom - and then bear fruit.
The epidemic threshold refers to a threshold above which agents can spread explosively and cause epidemics. It is usually expressed in terms of the ratio between birth rate and death rate. If the ratio is larger than one, then an epidemic may result. If it is less than one then an epidemic becomes extremely unlikely.
The epidemic threshold is an important concept in epidemiology. It has previously been applied to cultural evolution - for example by Seth Godin in Unleashing the Ideavirus (p.77) - though he called it the magic-number.
Many factors influence the birth rates - including their level of virulence, their lifespan, whether they are defeated by attacks from the host immune system - and so on.
Being above the epidemic threshold does not guarantee that an epidemic will result. Statistical fluctuations may result in extinction before an epidemic forms. Being below the epidemic threshold makes an epidemic extremely unlikely, though.
The epidemic threshold is a pretty important idea in viral marketing. If your shareable item is below the threshold, and you get exponential decay - with negligibe viral effect. If it is above it, you could see exponential growth - as your marketing work is done for you by the masses.
Diagram
Epidemic failure and success.
This diagram shows the output from a very simple model of an epidemic. Infected hosts are plotted against time, from a fixed initial "seed" population. Different plot lines reflect different birth and death rates for the parasite.
Economic significance
The result of this is typically a non-linear R.O.I. on the marketing budget:
Below the epidemic threshold the effects are so small that you might as well not have bothered.
Above the epidemic threshold you see explosive growth - then it is more likely that the marketing budget has been well spent.
Much the same model applies equally well to mutualists as well as pathogens.
Big seed marketing
A related idea is big seeding - which is another strategy for avoiding extinction.
Tipping points
Malcom Gladwell once wrote a book called The Tipping Point - which referred to essentially the same concept. For more about that, see our post on that topic.
Note that the term epidemic threshold is also - rather confusingly - used to describe a number of hosts which need to be infected before an outbreak is classified as an epidemic. This post is not about that concept.
The idea of a tipping point is one that comes from physics. Adding a small amount of weight to a balanced object can cause it to suddenly and completely topple. The point at which it does so is known as the tipping point. References to the idea of a tipping point with reference to water buckets tipping over date back to the 1800s.
Since chaos theory is so well established it would make a lot of sense to refer to "tipping points" as "catastrophe points" or "critical points". However, the term "catastrophe" has powerful and not always appropriate negative implications.
Sociology
The "tipping point" phrase was first applied in a sociological context by Morton Grodzins who studied American neighborhoods in the 1950s and 1960s. He discovered that most of the white families remained in the neighborhood as long as the comparative number of black families remained very small. But, at a certain point, when "one too many" black families arrived, the remaining white families would move out en masse in a process known as white flight. Inspired by the physics term, he referred to that moment the "tipping point".
Memetics
The term is a fairly general, but it appears to have an important application to memetics and cultural evolution.
The term is frequently applied with memes that "go critical". For viral content to explosively spread virally, people need to share it more frequently than they forget about it. For most wannabe-memes, they don't reach the required critical threshold - and are quickly forgotten about. However, other memes find themselves on the other side of the threshold - and spread virally - increasing in an exponential fashion. This effect is similar to what happens when a nuclear reaction goes critical. The critical threshold is now referred to as the epidemic threshold - a topic which I have a post about.
There are several possibilities here for memes to cross the threshold:
Mutation - One is that your meme needs to mutate. An example of this would be the geddan meme - which had to cross the divide between video-game glitch and popular dance before making the big time.
Hitchhiking - Another possibility is that it needs to find the right hitchhiking partner. An example of that is Keyboard Cat. The original keyboard cat video languished in obscurity - until it was used in a mashup by Brad O’Farrell - and then it exploded onto the internet - with thousands of copies being made, all of which used memetic hitchhiking with other viral content.
Memes can lie dormant for other reasons as well. Sometimes, they have not wandered into the right niche yet. Sometimes, they just need to find the right vector, who will get a proper infection started. Lastly, they may need environmental changes before they can thrive. Legal changes, changes in moral behaviour or the extinction of competing forms can all help a meme go over the tipping point.
One example of an environmental change is when there is a competitive shakeout in a contested market, usually leading to one winner. Initially the market is small, and several contenders aim to reap the rewards of dominating it. As time passes, one competitor draws ahead, and then users switch to their product or service in a rapid cascade.
We see this with VHS vs Betamax and HD-DVD vs Blu-ray, Facebook vs MySpace - and so on.
Malcom Gladwell's The Tipping Point
The idea of tipping points was popularised by Malcom Gladwell in his popular book, The Tipping Point - How Little Things Can Make a Big Difference.
Videos
Here's Meme Molly, explaining the The Tipping Point concept.
Here's a Guinness advert - illustrating Malcom Gladwell's point.
Jason Collins recently raised the issue of cultural evolution vs cultural epidemiology, thus:
Until recently I had considered the obesity epidemic to be primarily a cultural epidemic -
an epidemic of memes. Fast food companies spread their obesity-promoting memes throughout
the population using advertising (which consists of more memes) - and so manipulate people into eating more of their food, thus making the companies richer and financing the production of more obesity-promoting memes.
Numerous students of cultural evolution have got a lot of mileage over the years out of viewing animal brains as the central locus of cultural evolution. Brains are envisaged as repositories of cultural information that selectively adopt it and selectively transmit the cultural information to other creatures.
Memetics takes the relationship between the organic and cultural realms more seriously than any other theory of cultural evolution I am aware of. However, the close parallels seen by memeticists are not always shared by cultural evolution enthusiasts in academia. Alberto Acerbi helps us to highlight this difference in perspective today. To him, the close parallel seems wrong - and he explains why.
The symbiosis between humans and their gut bacteria is usually a happy one. The humans feed the gut bacteria and the gut bacteria feed the humans. It's a classic win-win relationship - an example of a mutualsim.
A recent article in 
Memetics is deeply based on the science of symbiosis. However, sometimes the study of symbiosis seems to be in almost as much of a muddle as the evolution of culture.
Social media marketing departments must decide how to allocate their budget between making content that will spread - and distributing that content.
Memetics has access to a plethora of negative words to describe the spread of ideas.
The 
The idea of a tipping point is one that comes from physics. Adding a small amount of weight to a balanced object can cause it to suddenly and completely topple. The point at which it does so is known as the tipping point. References to the idea of a tipping point with reference to water buckets tipping over date back to the 1800s.
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