Like a Flock of Birds

Imag­ine, if you will, a flock of birds. There are thou­sands of them scat­tered in a field rest­ing, until, sud­denly in one glo­ri­ous motion they lift off at once. The entire flock rises into the sky, flips back and forth, as if it were a sin­gle organ­ism with a sin­gle pur­pose, and comes to rest at once on some power lines. It’s an amaz­ing sight, espe­cially with larger flocks of birds, and it’s an exam­ple of emer­gent behavior.

Flock­ing birds do not delib­er­ately coor­di­nate their actions. The do not plan out ahead of time who goes where or in what direc­tion they will go, yet they reg­u­larly and spon­ta­neously pull off com­plex areal maneu­vers that dar­ing human pilots would only attempt after years of prac­tice. The trick is that each bird fol­lows the bird next to it. If that bird starts off in a new direc­tion, then the first one fol­lows. They con­tin­u­ally main­tain a safe dis­tance from each oth­er, each going with the flow so that they all end up going in the same direc­tion at once. When a few birds start off in a dif­fer­ent direc­tion, the oth­ers fol­low. This is called flock­ing (or herd­ing when sheep do it,) and it’s the result of sim­ple behav­iors on the part of indi­vid­u­als in a group result­ing in more com­plex behav­iors on the part of the whole of the group. This is emer­gent behav­ior.

Emer­gent behav­ior is present through­out nature. Ants are another good exam­ple. Ants build enor­mous nests, coop­er­a­tively raise food for colonies with mil­lions of indi­vid­u­als, ford rivers using their own bod­ies as bridge parts, and fight elab­o­rate wars with other colonies and even other species. Some species of ants are even capa­ble of migrat­ing an entire colony at once. Yet no one ant plans out the actions of the colony. No group of ants either. In fact, ants are not clever enough to under­stand the entirety of the colonies endeav­ors any­way. Instead, instinct guides the ant. One ant dis­cov­ers food and leaves a scent trail all the way back to the nest; other ants fol­low the scent trail and soon enough there is an entire col­umn of ants rapidly dis­as­sem­bling a piece of food and trans­port­ing it back to the nest. The har­vest is so orderly and coop­er­a­tive that the ants don’t even have traf­fic jams.

One inter­est­ing thing about emer­gent behav­ior is that the behav­ior is often more com­plex that what would have been pos­si­ble through a planned approach. The rules that an indi­vid­ual mem­ber of a flock fol­low are sim­ple com­pared to a hypo­thet­i­cal attempt to plan the loca­tion of every sin­gle bird as part of a large move­ment. In such sys­tems you not only have to con­sider the loca­tion, speed, direc­tion, and accel­er­a­tion of each indi­vid­ual bird, but you have to con­sider the same for each bird, in relation to every sin­gle other bird as well. When you have prob­lems like that, solu­tions don’t “scale,” that is, the solu­tion stops work­ing as the prob­lem size grows larg­er. In com­puter sci­ence, the com­plex­ity of a prob­lem is often expressed in terms of some­thing called “Big-O” nota­tion, or for example: O(n^2). “n” being the num­ber of birds, this means that the com­plex­ity of this prob­lem is the num­ber of birds squared, because each bird needs to be con­sid­ered in terms of each of the other birds. So when there are only five birds, we need only con­sider 25 rela­tion­ships, but with there are 50 birds, 2500 rela­tion­ships need to be han­dled. So, as the num­ber of birds grows, the dif­fi­culty of plan­ning their move­ments grows at an unman­age­able rate.

Yet, if we use the emer­gent behav­ior approach, each bird need only con­sider those imme­di­ately around it, shrink­ing the prob­lem to at most O(n), or more often to O(k), where n is the num­ber of birds in a flock and k is the num­ber of birds able to be near each other bird. So if there are at most 10 birds near each other bird, the prob­lem will always 10 in size no mat­ter how many birds their are. In this approach, the prob­lem scales triv­ial­ly, which shows the power of emer­gent behav­ior.

My favorite exam­ple of emer­gent behav­ior is that of the human econ­o­my. The task of suc­cess­fully pre­dict­ing and ful­fill­ing the needs and wants of bil­lions of humans every day is a mon­u­men­tal one. Not only do com­pletely unpre­dictable events, nat­ural dis­as­ters and wars com­pli­cate the task, but peo­ple’s wants and needs change and inter­act in com­pletely unpre­dictable ways. As peo­ple move into an area for exam­ple, the num­ber of auto­mo­biles sold there will increase, but after a cer­tain thresh­old, this will change because increas­ing traf­fic and pop­u­la­tion den­sity makes cars less use­ful and more of a has­sle. As cer­tain needs are met, other needs rear their heads and become promi­nent. Plan­ning, pro­duc­ing, and dis­trib­ut­ing the worlds goods is a nearly impos­si­ble task, yet it mostly gets done con­sis­tently year after year.

This prin­ci­ple of indi­vid­ual action lead­ing to results which are ben­e­fi­cial to the whole is some­thing on which our species depends for sur­vival. If it did not exist, human­ity would not have been able to form the economies on which early civ­i­liza­tions and their learn­ing and gov­er­nance depend­ed. In all of human his­to­ry, there has never been a body capa­ble of plan­ning, or even enforc­ing a plan for a whole civ­i­liza­tion.

His­tor­i­cal­ly, attempts at cen­trally planned economies have gen­er­ally been fail­ures. Soviet Rus­sia had to aban­don it’s orig­i­nal col­lec­tivism for the New Eco­nomic Pol­icy for exam­ple. But, remove the task of arrang­ing soci­ety from a select few and give it to the mul­ti­tude and the prob­lem sud­denly becomes tractable. Indi­vid­u­als act­ing only in response to their own cir­cum­stances inad­ver­tently con­tribute to the feed­ing and cloth­ing of the plan­et. Mar­kets are an intrin­sic part of human nature that arise when­ever a large enough civ­i­liza­tion forms and they remain in place regard­less of the offi­cial eco­nomic sys­tem put in place by any gov­ern­ment. Even North Korea has an under­ground mar­ket which staves of star­va­tion for many in that econ­o­my.

Mar­kets are an amaz­ing prin­ci­ple that under­pins much of human behav­ior. The are an excel­lent exam­ple of emer­gent behav­ior and it’s a shame that they are some­times under­-ap­pre­ci­at­ed.

    Last update: 10/07/2013

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