An understanding of how brains synchronize — or fail to do so — will be a game-changing scientific development.

Few behavioral forces are as strong as the delineation of in-groups and out-groups: 'us' and 'them'. Group affiliation requires alignment, coupling or synchronization of the brain states of members. Synchronization yields cooperative behavior, promotes group cohesion, and creates a sense of group agency greater than the sum of the individuals in the group. In the extreme, synchronization yields herding behavior. The absence of synchronization yields conflict.

People come under the grip of ideologies, emotions and moods are infectious, and memes spread rapidly through populations. Ethnic, religious, and political groups act as monolithic forces. Mobs, cults and militias are characterized by the melding of large numbers of individuals into larger units, such that the brains of individuals operate in lockstep – a single organism controlled by a single — distributed — nervous system.

Leaders who mobilize large followings have an intuitive ability to synchronize brains or to plug into systems that already are synchronized.

Herding behavior has received a great deal of attention in economics. In the recent financial bubble that eventually burst, investors and regulators were swept up by a wave of blinding optimism and over-confidence. Contrary information was discounted, and analysis from first principles ignored.

Herding behavior is prevalent in times of war. A group that perceives itself to be under attack binds together as a collective fighting unit, without questioning. When swift synchronization is critical and the stakes are high, psychological forces such as duty, loyalty, conformity, compliance – all of which promote group cohesion — come to the fore, overwhelming the rational faculties of individual brains.

Synchronization is found in many species, although the mechanisms may not be the same. Flocks of birds fly in tight formation. Fish swim in schools, and to a distant observer appear as one aggregated organism. Wolves hunt in packs. Some instances of synchronization are driven by environmental cues that regulate individual brains in the same way. For example, light cycles and seasonal cycles can entrain biorhythms of individuals who share the genetic predisposition to be regulated in this way. In other cases, the co-evolution of certain behaviors together with the perception of these behaviors holds individuals together, as in the ability to both produce and recognize species-specific vocalizations.

Synchronization is mediated by communication between brains. Communicative channels include language as well as non-verbal modes such as facial expressions, gestures, tone of voice, and music. Communication across regions of an individual brain is simply a special case of a system that includes communication between brains.

Elsewhere I have argued that music serves to synchronize brain states involved in emotion, movement, and the recognition of patterns — thereby promoting group cohesion. As with tradition or ritual, what's being synchronized needn't have intrinsic utility; it may not matter what's being synchronized. The very fact of synchronization can be a powerful source of group agency.

Just around the corner is an explosion of research that regards individual brains as nodes in a system bound together by multiple channels of communication. Information technology has provided novel ways for brains to align across great distances and over time. When a song becomes a hit, millions around the world are aligned, forming a virtual unit. In the future, brain prostheses and artificial interfaces for biological systems will add to the picture.

Some clues are emerging about how brains synchronize. The hot recent discovery is the existence of mirror neurons — brain cells that respond to the actions of other individuals as if one were making them oneself. Mirror systems are thought to generate simulations of the behavior of others in one's own brain, enabling mimicking and empathy. Other pieces of the puzzle have been accumulating for a while. Certain cases of frontal lobe damage result in asocial behavior.

Recent work on autism has drawn attention to the mechanisms whereby individuals connect with others. The brain facilitates (sometimes in unfortunate ways) the categorization of oneself and others into in-groups and out-groups. When white participants in an MRI machine view pictures of faces, the amygdala in the left hemisphere of the brain is more strongly activated when the faces are black than when they are white. The brain has circuits specialized for the perception of faces, which convey enormous amounts of information that enable us to recognize people and gauge their emotions and intentions.

Understanding how brains synchronize to form larger systems of behavior will have vast consequences for our grasp of group dynamics, interpersonal relations, education and politics. It will influence how we make sense of — and manage — the powerful unifying forces that constitute group behavior. For better and for worse, it will guide the development of technologies designed to interface with brains, spread knowledge, shape attitudes, elicit emotions, and stimulate action. As with all technological advances, leaders will seize on them to either improve the human condition or consolidate power.

Not all individuals are susceptible to synchronizing with others. Some reject the herd and lose out. Some chart a new course and become leaders. Being contrarian often requires enduring the psychological forces of stigma and ridicule. Understanding the conditions under which people resist will be part of the larger understanding of synchronization.

Understanding how brains synchronize – or fail to do so — will not emerge from a single new idea, but rather from a complex puzzle of scientific advances woven together. What is game-changing is that only recently have researchers begun to frame questions about brain function in terms not of individual brains but rather in terms of how individual brains are embedded in larger social and environmental systems that drive their evolution and development. This new way of framing brain and cognitive science — together with unforeseen technological developments — promises transformational integrations of current and future knowledge about how brains interact.