The phrase "herd behaviour" has acquired a certain currency in popular psychology, where it is offered as an explanation of such phenomena. In reality, however, it is highly unlikely that these behaviours have anything in common other than the superficial fact that they all involve a number of individuals doing more or less the same thing. Attributing such collective behaviour to a "pack mentality" or "group mind" explains nothing, and diverts attention from the true explanation of the group's actions.
In the case of animals evading a predator, it can be shown that each individual can minimise the danger to itself by choosing the location and behaviour that is as close to the centre of the group as possible; this was the subject of a famous paper by evolutionary biologist W. C. Hamilton called Geometry for the selfish herd. In this case, clearly the "herd behaviour" is as far from a pack mentality as possible, since it emerges from the unco-ordinated behaviour of self-seeking individuals. In the case of market bubbles, the optimal behaviour for an individual may again be to do what everyone else is doing, because even though everyone knows that they are in a bubble, until it bursts, most profit is to be made by staying in the market; again the irrational "collective" behaviour emerges from unco-ordinated individual choices. These phenomena are now much better understood as a result of investigations in experimental economics, particularly by Nobel laureate Vernon Smith. In the case of behaviour in demonstrations, the idea of a "group mind" or "mob behaviour" was put forward by the French social psychologistss Gabriel Tarde and Gustav Le Bon, and was widely adopted by right-wing politicians, particularly in the inter-war years, to justify the repression of demonstrations and the neglect of the causes of protest.