"...first introduced in 1998 and, in the intervening 15 years, widely tested and supported (Fredrickson, 1998, 2001, 2013).
The theory posits that the function of positive emotions, as shaped over millennia by the processes of natural selection, was to build an individual's resources for survival.
The means by which this build function was achieved was through the accumulated effects of moments marked by an affect-induced broadened scope of awareness, creating a temporary form of consciousness within individuals that included a wider array of thoughts, actions, and percepts than typical.
One implication of the broaden-and-build theory is that negative and positive emotions alike came to be part of our universal human nature through selective pressures related to survival, albeit on vastly different time scales.
Negative emotions carried adaptive significance in the moment that our human ancestors' experienced them, as their associated action urges - for example, to fight, flee or spit - drove behaviors that saved life and limb in dire circumstances.
Positive emotions, by contrast, carried adaptive significance for our human ancestors over longer time scales. Having a momentarily broadened mindset, after all, is not a key ingredient in the recipe for any quick survival maneuver. It is, however, in the recipe for discovery: discovery of new knowledge, new alliances, and new skills. In short, broadened awareness led to the accrual of new resources that might later make the difference between surviving or succumbing to various threats. Resources built through positive emotions also increased the odds that our ancestors would experience subsequent positive emotions, with their attendant broaden-and-build benefits, thus creating an upward spiral toward improved odds for survival, health and fulfillment.
In sum, the broaden-and-build theory states that positive emotions have been useful and preserved over human evolution because having recurrent, yet unbidden, moments of expanded awareness proved useful for developing resources for survival.
Little by little, micro-movements of positive emotional experience, although fleeting, reshape who people are by setting them on trajectories of growth and building their enduring resources for survival.
The broaden-and-build theory describes the form of positive emotions as to broaden awareness and their function to build resources."
Fredrickson, B.L. (2013). Updated Thinking on Positivity Ratios. American Psychologist. Advance
online publication. doi:10.1037/a0033584