Complex Contagion: The Science of Human Networks

Complex contagion explains how culture is generated, how behaviors are adopted, and how a community influences you more than you think.

Globalisation as complex contagion of human processes.

What is Complex Contagion?

For complex contagion to occur, it must spread; but what does it mean for it to spread complexly?

Most familiarly, contagions are usually characterised as simple. They are spreading events that are passed on through once-off contact between individuals, like a virus. Whereas complex contagions are spreading events that are passed on through multi-input contact.

To put it simply, the individual who is not activated with the “spreading entity” will not have it spread to them unless they have multiple exposures from different people who have adopted it.

Examples of Complex Contagion

Examples of complex contagion are:

1. Emotion amongst colleagues

2. Culture

3. Knowledge/ ideologies

4. Software adoption

One can see from the above list that complex contagions are more sociologically based. This makes sense considering the above definition, which requires a ‘society’ of people to spread the entity through multiple exposures to an individual.

The idea can be further illustrated as follows:

Figure 1: The process of complex contagion from three activated individuals to person (A).

One can see that in Figure 1, the person (A) requires exposure from three activated individuals to be activated.

Influences of Complex Contagion

Complex contagion is primarily influenced by:

1. Legitimacy

2. Credibility

3. Complementarity

4. Emotion


If the complex thing to be adopted (like behaviour or knowledge) does not enjoy a broad consensus among multiple people, it is socially illegitimate and will not be adopted.


If the complex entity does not allow the individual to gain more social credit, praise, or power, it is unlikely to be adopted. Adopting a behaviour that diminishes social credibility is risky.


The more convenient it is to adopt a behaviour, the more likely it will be adopted. A good example would be email – many people have adopted it by the mere fact that not doing so would inconvenience them.


Emotion is also a form of complex contagion. However, it too can be a strong driving force in the adoption of other complex entities. For instance, the excitement around a product launch can motivate its adoption as the product of choice among a tech community.

Requirements for Complex Contagions

Close Communities

Complex contagion requires a strong community, which is analogous to tightly knitted wool

Complex contagion require close social interactions. Therefore, a key requirement for complex contagion is a tight-knit community of connections (clustered network).

This can be seen in Figure 1, where any individual is closely connected to two other individuals and their connections know each other.

Therefore, the way a network is composed (multiple connections) and the nature of that connection (strong) is important. An individual would not adopt behaviour from a stranger.

Influence of Weak Ties

A weak tie is characterised as an associative tie through acquaintances. These individuals associate with part of the social network not usually reached by individuals in the clustered network in which they associate.

Weak ties hinder complex contagion because they cannot spread them to other parts of the social network and because they weaken the clustering in the networks they associate with.

Multi-lateral Input Thresholds

We have already mentioned that we require multiple inputs from different activated individuals for complex contagion. This can be further broken down to numerical-based input and percentage-based input.

Numerical based input occurs when there are roughly a certain number of people exposing the individual to the behaviour to be adopted. For instance, after seeing 4 people adopt software, individual (A) also does so.

Percentage based inputs occur when a certain percentage of connections have already adopted a behaviour. For instance, after seeing approximately 30% of their connections adopt software, individual (A) also does so.

The type of threshold is dependent of the type of complex contagion; it is situation and domain-specific.

The type of threshold has a massive implication with respect to the type of nodes that occur in a network. For instance, hubs, which are highly connected individuals in a network, will require many people to adopt a behaviour if the contagion is percentage-based. This usually makes them impervious to change and rather the vectors through which change occurs.


Understanding the basis of behavioural social reproduction through complex contagion can be profoundly useful in analysing why an entity was adopted or not.

Complex contagion requires multiple points of contact in a clustered network with strong connections to be effective.

Complex contagion also requires:

1. Legitimacy

2. Credibility

3. Complementarity

4. Emotion

Complex contagion is truly profound and can explain the undercurrent of culture and identity.

For more information, check out Damon Centola’s book “How Behaviour Spreads”:

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