Mass action (sociology)

Mass action in sociology refers to the situations where a large number of people behave simultaneously in a similar way but individually and without coordination.

For example, at any given moment, many thousands of people are shopping - without any coordination between themselves, they are nonetheless performing the same mass action. Another, more complicated example would be one based on a work of 19th century German sociologist Max Weber, The Protestant Ethic and the Spirit of Capitalism: Weber wrote that capitalism evolved when the Protestant ethic influenced large number of people to create their own enterprises and engage in trade and gathering of wealth. In other words, the Protestant ethic was a force behind an unplanned and uncoordinated mass action that led to the development of capitalism.

A bank run is mass action with sweeping implications. Upon hearing news of a bank's anticipated insolvency, hundreds or thousands of bank depositors simultaneously rush down to a bank branch to withdraw their deposits, and protect their savings.

More developed forms of mass actions are group behavior and group action.

Epidemiology meaning
In epidemiological (disease) models, assuming the "law of mass action" means assuming that individuals are homogeneously mixed and every individual is about as likely to interact with every other individual. This is a common assumption in models such as the SIR model.