Social contagion involves behaviour, emotions, or conditions spreading spontaneously through a group or Social network. The phenomenon has been discussed by Social sciences since the late 19th century, although much work on the subject was based on unclear or even contradictory conceptions of what social contagion is, so exact definitions vary. Some scholars include the unplanned spread of ideas through a population as social contagion, though others prefer to class that as memetics. Generally social contagion is understood to be separate from the collective behaviour which results from a direct attempt to exert social influence.
Two broad divisions of social contagion are behavioural contagion and emotional contagion. The study of social contagion has intensified in the 21st century. Much recent work involves academics from social psychology, sociology, and network science investigating online social networks. Studies in the 20th century typically focused on negative effects such as violent mob behaviour, whereas those of the 21st century, while sometimes looking at harmful effects, have often focused on relatively neutral or positive effects like the tendency for people to take action on climate change once a sufficient number of their neighbours do.
From the 1950s, studies of social contagion began to investigate the phenomena empirically, and became more frequent. There was no widely shared definition of social contagion in the 20th century, so many of the studies had little in common. In 1993, David A. Levy and Paul R. Nail published a review where they stated that social contagion captures the broadest sense of the phenomena, as opposed to subtypes like behavioural or emotional contagion.
In a 1998 review, Paul Marsden suggested that social contagion is a similar phenomena to memetics, a field of study inspired by Richard Dawkins' 1976 book The Selfish Gene. Marsden suggested that the two fields could be complementary, in the sense that work on social contagion largely lacked a coherent theory, but contained much evidence-based analyses, whereas memetics was rich in theory but lacking on the empirical side.
From the 1990s and into the 21st century, interest in social contagion grew rapidly, based in part on cross fertilisation with the then emerging field of network science, especially its application to the Internet. With respect to positive social contagions, a series of experiments and field trials since 2009 (by Nicholas Christakis and diverse collaborators) have shown that cascades of desirable behaviors can be induced in social groups, in settings as diverse as Honduran villages, Indian slums, online,Rand D, Arbesman S, and Christakis NA,"Dynamic Social Networks Promote Cooperation in Experiments with Humans," PNAS:Proceedings of the National Academy of Sciences 2011; 108: 19193-19198 or in the lab. Diverse other experiments have documented the social contagion of voting behavior, emotions, risk perception, and diverse other phenomena.
Echo contagion represents the spontaneous imitation of a behaviour, or transition into conformance with an emotional state shared by others. Hysterical contagion represents the unwanted transmission of a behaviour, emotion or affect among a group by unknown means. Unlike with echo or disinhibitory contagion, what is being transmitted may in no way be desirable or attractive, yet it transmits anyway.
In simple contagion, an individual only needs a single exposure to the new behaviour. For instance, cars travel in groups on a two-lane highway since the car in each cluster travels at a slower speed than the car behind it. This relative speed spreads through other cars who slow down to match the speed of the car in front.
In complex contagion, the individual needs to be in contact with two or more sources exhibiting the novel behaviour. This is when copying behaviours needs reinforcement or encouragement from multiple sources. Multiple sources, especially close friends, can make imitation legitimate, credible and worthwhile due to collective effort put in. Examples of complex contagions include a New York University School of Business study in California which found that households were more likely to install solar panels in neighborhoods that already had them, and that the rate of installation increases with more and more installations, creating a chain reaction that added up to a significant increase in solar adoption. Other examples can be copying risky behaviour or joining social movements and riots.
Work has been done to understand social contagion as a way to encourage positive behaviour, as a possible complement to nudge theory. It has been suggested as a way to assist in the rehabilitation of criminals and drug addicts, and as something that can encourage the adoption of climate friendly behaviour. Such as the increased tendency to install solar panels on one's personal home once a portion of one's neighbours have already done so.
In social network analysis and related network science fields, the contagion metaphor has been described as potentially misleading in various ways. For example, an actual virus can affect someone after a single exposure, whereas typically with social contagion, people need several exposures before adopting the new behavior or emotion.
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