The Dictionary of Human Geography

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forces push residential, business and other

users away from the congested, polluted,

high-density and expensive inner-city areas

towards thesuburbsand beyond (cf. decon-

centration: see counter-urbanization;

decentralization;sprawl), whereas centri-

petal forces attract them towards the centre

for the benefits ofaccessibilityandagglom-

eration(cf.gentrification). The balance of

these two forces at any time determines the

changing urbanmorphology. rj

chain migration A term used to describe

migrationthat occurs in a sequence, when

the movement of one person causes others to

follow. It is a major component ofnetwork-

based theories of migration. Chain migration

typically begins at the family scale, with a sin-

gle person moving to a new place in search of

better opportunities. Once settled, that person

facilitates the migration of other members of

the nuclear or extended family (Hugo, 1994).

In time, the migration network extends to

friends and acquaintances as well. As this

occurs, migration becomes self-perpetuating.

New arrivals are assisted by those who have

already learned to cope in the receiving society

(Castles and Miller, 2003). This is particularly

effective in the process of finding shelter and

work, and newcomers generally live in close

proximity, and take similar jobs to those who

help them. This process leads to the develop-

ment of immigrantenclavesin housing and

labour markets, as Banerjee (1983) has shown

in the case of internal migration in India (cf.

segregationand segmentedlabour market).

Chain migration also fosters financial trans-

fers, or remittances, between immigrants and

family members who are still residents of the

source country, as well as other forms oftrans-

nationalism(Levitt and Glick Schiller, 2004).

From the point of view of receiving coun-

tries, the chain migration process can be

thought of as an ‘immigrationmultiplier’, in

the sense that each individual immigrant is

likely to generate the entry of several others

over time (Jasso and Rosenzweig, 1986). Once

established, these networks develop routinized

systems of movement that can even circum-

ventstate authority (Bocker, 1994). It is

widely believed, for example, that chain

migration has been an essential ingredient in

the entry of the approximately 12 million

undocumented immigrants residing in the

USA. This insight has often been used by

critics of immigration, who argue that the

combination of the self-perpetuating nature

of chain migration and the multiplier effect

will cause a geometric increase in immigration

(cf. Goering, 1989). However, it is worth

remembering that social networks are just

one contributing factor in migration, and that

other factors, notably economic differentials

between countries and various forms ofvio-

lence, are structural causes of migration. dh

Suggested reading

Castles and Miller (2003); Hugo (1994).

chaos theory A branch of non-linear math-

ematical theory dealing with dynamic systems

which exhibit aperiodic behaviour that is sen-

sitive to initial conditions and is unpredictable

in detail. Such sensitivity to initial conditions

is often referred to as the ‘butterfly effect’, with

the illustration that the flapping of a butterfly’s

wings in one part of the world might, through

tiny impacts on the atmosphere, have major

impacts elsewhere in the world. The behaviour

of systems that exhibit chaos may appear to

be random, even though the system is deter-

ministic in the sense that it is well defined and

contains no random parameters. This tech-

nical use of the word ‘chaos’ is at odds with

everyday language, which suggests complete

disorder. Chaotic systems are ‘orderly’ in

being deterministic, and also usually have

well-defined system structure and statistics.

A very simple example of such a system is

provided by May (1973) in his standard logis-

tic model of population growth:

Xtþ 1 ¼aXt[ 1 bXt=a],

where next year’s populationXtþ 1 is depen-

dent on the current populationXtand par-

ameters aandb. The system exhibits very

different behaviours depending on the values

ofa:if1<a<3, thenXttends towards a

stable equilibrium population. Ifa>4, then

there is a divergence to minus infinity (a total

collapse of the system), but for values ofa

between 3 and 4, there are interesting dynam-

ics: if 3<a<3.8495, thenXtoscillates with

a regular, periodic frequency, but if 3.8495<

a<4, then the oscillations are chaotic, with

no regular frequency.

Chaos theory was a popular and much-

vaunted term for a time, but, like the related

mathematical systems ofcatastrophe theory,

it is usually now treated as part of the wider

arena ofcomplexity theory,andfurtherread-

ing will be found under this term. lwh

Chicago School The first sociology depart-

ment in the USA (founded in 1892), located

Gregory / The Dictionary of Human Geography 9781405132879_4_C Final Proof page 78 31.3.2009 9:45pm

CHAIN MIGRATION