Original article Table 1 Natural properties and features of complex systems Properties of complex systems Healthcare manifestations Natural networks Natural hubs and scale-free behaviour Natural pathways, connectivity and small worlds Natural appeal and stickiness Natural propagation and tipping points Natural categories and natural mapping Natural interest and self-selection Groups of clinicians who interact professionally to share information, support, consult, refer and jointly manage patients Opinion leaders in networks who disproportionately influence policies, events or practices Communication channels facilitating the rapid dissemination of information via ‘‘grapevines’’ and communities of practice Messages and communications that are convincing and are absorbed among clinical cohorts The point at which a message, idea or practice whose time has come is readily adopted by a critical mass of clinicians The identification of clinically relevant problems grouped as accessible data, to facilitate decision-making and solutions to healthcare problems Clinicians with common concerns and complementary expertise voluntarily grouped together to collectively resolve coal-face clinical problems solved or ends to be achieved. They occur wherever humans cluster in the pursuit of a common purpose or activity, such as adolescent friendships in schools (see fig 1),28 the sharing of wisdom across boundaries29 30 or the improvement of patient care.31 Medical examples range from geographically dispersed clin- icians weakly related by discipline or common interests (for example, in stroke, trauma or ophthalmology), collaborating to improve care in Australia,31 to teams in America reducing catheter-related infections in 103 ICUs,32 to action-orientated researchers promoting high levels of cooperation among paediatric clinicians in Finland.33 Natural hubs and scale-free behaviour Nodes in a network might be thought to be randomly related, each with a similar number of connections. However, most networks are actually ‘‘scale-free,’’ with unequally distributed nodes with varying numbers of connections.34–36 A few nodes with many connections form ‘‘hubs’’ which emerge naturally and become the distributed force field of the network (for example, the Google search engine, King’s Cross station or a prominent, influential clinician). Scale-free networks are less likely to become congested because they concentrate effort efficiently.37 The internet has such a structure, with a limited Figure 1 Friendship clusters in a US school. Reprinted with permission from Moody J. Peer influence groups: identifying dense clusters in large networks. Soc Netw 2001;23:261–83. 38 number of hubs, an inner layer of strongly peer-connected components and an outer layer with many relatively isolated nodes (fig 2).38 Analogous patterns emerge in healthcare, where hubs (prestigious services or clinicians) can have pervasive influences on practices and attitudes among their well-linked inner layer colleagues, and progressively less on the outer layer of poorly connected isolates. Natural pathways, connectivity and small worlds Scholars have sought recently to understand the sociology of different types of networks through an analysis of another natural characteristic, that of ‘‘small worlds.’’39 Regardless of the size of any particular network, there are fast routes through them, creating small worlds within large networks.40 Even among the six billion people on Earth, for example, we can usually map the ties of any two through substantially less than the famous six degrees of separation.41 Making a small world work using natural pathways does not require personal or corporate knowledge of everyone in a chain or explicit representation of connections, pathways or hierarchies. Natural appeal and stickiness Administrators are always trying to spread messages or influence people through directives, emails and meetings. However, most communications are ignored, and meetings avoided or attended only by a few. This brings us to another concept—not a natural feature of networks, but a phenomenon related to group behaviour. It comes from marketing, and is known as stickiness. Any message to be both remembered and acted upon needs to be sticky.42 43 Stickiness is a function of the intrinsic nature of a message, how it is presented and the effect it has on the recipient. Sticky messages have natural appeal. Awareness of the importance of stickiness challenges commu- nicators to communicate well. Novel or effective communica- tion, smooth transmission modes, embedded cues in the environment and workplace to remind people of the message, forcing functions to facilitate compliance with the message and a critical mass of champions or opinion leaders can all be important in getting a message to stick. Natural propagation and tipping points The stage at which a critical mass for sustained or escalating momentum in any system is reached is what Gladwell calls a ‘‘tipping point,’’18 an idea used in classic epidemiology. It is the pivotal juncture at which a concept, social movement or epidemic takes hold.44 45 One or more triggers may be needed for a state change. Although there are many potential candidates, several stand out: persuasive, catalytic individuals,46 memorable, even irresistible messages47 and conducive contexts.45 A tipping point is not easy to evoke. An example in medicine that has taxed the best Qual Saf Health Care 2009;18:37–41. doi:10.1136/qshc.2007.023317