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A unified theory of urban living

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Abstract

It is time for a science of how city growth affects society and environment, say Luis Bettencourt and Geoffrey West.
A
t the start of the twenty-first century,
cities emerged as the source of the
greatest challenges that the planet
has faced since humans became social.
Although they have proven to be human-
ity’s engines of creativity, wealth creation
and economic growth, cities have also been
the source of much pollution and disease.
Rapid urbanization and accelerating socio-
economic development have generated
global problems from climate change and
its environmental impacts to incipient
crises in food, energy and water availabil-
ity, public health, financial markets and the
global economy1,2.
Urbanization is a relatively new global
issue. As recently as 1950, only 30% of the
world’s population was urbanized. Today,
more than half live in urban centres. The
developed world is now about 80% urban
and this is expected to be true for the entire
planet by around 2050, with some 2 billion
people moving to cities, especially in China,
India, southeast Asia and Africa2.
Cities are complex systems whose infra-
structural, economic and social components
are strongly interrelated and therefore dif-
ficult to understand in isolation3. The many
problems associated with urban growth and
global sustainability, however, are typically
treated as independent issues. This frequently
results in ineffective policy and often leads to
unfortunate and sometimes disastrous unin-
tended consequences. Policies meant to con-
trol population movements and the spread of
slums in megacities, or to reverse urban decay,
have largely proven ineffective or counterpro-
ductive, despite huge expenditure.
In New York City in the 1970s, for example,
a strategy of ‘planned shrinkageintentionally
removed essential services from some urban
areas — notably the Bronx — to prompt peo-
ple to move away and allow for redevelopment.
Instead, this strategy led to increases in crime
and general socio-economic degradation.
In North America in the 1950s to 1970s (and
earlier in Europe), policies of urban renewal
intended to reduce high urban densities,
by razing poorer old neighbourhoods and
creating infrastructure, actually ended up
encouraging urban sprawl
3
. Similar debates
continue to play out in rapidly develop-
ing cities around the world today, from
Beijing to Rio de Janeiro in Brazil, often lead-
ing to similar mistakes.
But cities supply solutions as well as
problems, as they are the world’s centres of
creativity, power and wealth. So the need is
urgent for an integrated, quantitative, pre-
dictive, science-based understanding of the
dynamics, growth and organization of cit-
ies. To combat the multiple threats facing
humanity, a ‘grand unified theory of sus-
tainability’ with cities and urbanization at its
core must be developed. Such an ambitious
programme requires major international
commitment and dedicated transdiscipli-
nary collaboration across science, economics
and technology, including business leaders
and practitioners, such as planners and
designers. Developing a predictive frame-
work applicable to cities around the world
is a daunting task, given their extraordi-
nary complexity and diversity. However, we
are strongly encouraged that this might
be possible.
Universal featUres
Cities manifest remarkably universal, quan-
tifiable features. This is shown by new analy-
ses of large urban data sets, spanning several
decades and hundreds of urban centres in
regions and countries around the world
from the United States and Europe to China
and Brazil
4,5
. Surprisingly, size is the major
determinant of most characteristics of a city;
history, geography and design have second-
ary roles4,6.
Three main characteristics vary system-
atically with population. One, the space
required per capita shrinks, thanks to
denser settlement and a more intense use
of infrastructure. Two, the pace of all socio-
economic activity accelerates, leading to
higher productivity. And three, economic
and social activities diversify and become
more interdependent, resulting in new
forms of economic specialization and cul-
tural expression.
We have recently shown that these general
trends can be expressed as simple math-
ematical ‘laws. For example, doubling the
population of any city requires only about
an 85% increase in infrastructure, whether
that be total road surface, length of electrical
cables, water pipes or number of petrol sta-
tions
4
. This systematic 15% savings happens
because, in general, creating and operating
the same infrastructure at higher densities
is more efficient, more economically viable,
and often leads to higher-quality services
and solutions that are impossible in smaller
places. Interestingly, there are similar savings
in carbon footprints
7,8
— most large, devel-
oped cities are ‘greener’ than their national
average in terms of per capita carbon emis-
sions. It is as yet unclear whether this is also
true for cities undergoing extremely rapid
development, as in China or India, where
data are poor or lacking.
Similar economies of scale are found in
organisms and communities like anthills
and beehives, where the savings are closer
to 20%9. Such regularities originate in the
mathematical properties of the multiple
A unified theory
of urban living
It is time for a science of how city growth affects society
and environment, say Luis Bettencourt and Geoffrey West.
OLIVER MUNDAY
Full content and enhanced
graphics at: nature.com/cities
SCIENCE AND THE CITY
912 | NATURE | VOL 467 | 21 OCTOBER 2010
COMMENT
© 20 Macmillan Publishers Limited. All rights reserved10
networks that sustain life, from the cardio-
vascular to the intracellular
9
. This suggests
that similar network dynamics underlie
economies of scale in cities.
Cities, however, are much more than giant
organisms or anthills: they rely on long-
range, complex exchanges of people, goods
and knowledge. They are invariably magnets
for creative and innovative individuals, and
stimulants for economic growth, wealth
production and new ideas — none of which
have analogues in biology.
The bigger the city, the more the aver-
age citizen owns, produces and consumes,
whether goods, resources or ideas4. On aver-
age, as city size increases, per capita
socio-economic quantities such as
wages, GDP, number of patents pro-
duced and number of educational
and research institutions all increase
by approximately 15% more than the
expected linear growth4. There is,
however, a dark side: negative met-
rics including crime, traffic conges-
tion and incidence of certain diseases
all increase following the same 15%
rule
4
. The good, the bad and the ugly
come as an integrated, predictable,
package.
Our work shows that, despite
appearances, cities are approximately
scaled versions of one another (see
graph): New York and Tokyo are, to
a surprising and predictable degree,
nonlinearly scaled-up versions of San Fran-
cisco in California or Nagoya in Japan. These
extraordinary regularities open a window on
underlying mechanism, dynamics and struc-
ture common to all cities.
Deviations from these scaling laws, illus-
trated by the spread of data in the figure,
measure how each city over- or under-per-
forms relative to expectations for its size6.
Relatively large deviations (as much as 30%)
are seen for quantities with small numbers,
such as patents and murders, whereas much
smaller deviations (with variances less than
10%) are seen for economic properties. We
also find that quantities such as GDP are
more variable for urban centres in developing
countries, such as China and Brazil, than for
older cities in developed areas such as North
America or Japan. It is unclear whether this
is a fundamental property of developing
nations or an artefact of data collection.
In biology, the network principles under-
lying economies of scale have two profound
consequences. They constrain both the pace
of life (big mammals live longer, evolve slower,
and have slower heart rates, all to the same
degree9), and the limits of growth (animals
generally reach a stable size at maturity10). In
contrast, cities are driven by social interac-
tions whose feedback mechanisms lead to
the opposite behaviour. The pace of urban life
systematically increases with each expansion
of population size: diseases spread faster,
businesses are born and die more often and
people even walk faster in larger cities, all by
approximately the same 15% rule4. More-
over, this social network dynamic allows the
growth of cities to be unbounded: continuous
adaptation, not equilibrium, is the rule.
Open-ended growth is the primary
assumption upon which modern cities and
economies are based. Sustaining that growth
with limited resources requires that major
innovations — such as those historically asso-
ciated with iron, coal and digital technology
— be made at a continuously accelerating rate.
The time between the ‘Computer Age’ and
the ‘Information and Digital Age’ was some
20 years, compared to thousands of years
between the Stone, Bronze and Iron Ages.
Making major technological paradigm shifts
systematically faster is clearly not sustainable,
potentially leading to collapse of the entire
urbanized socio-economic fabric. Avoiding
this requires understanding whether we can
continue to innovate and create wealth with-
out continuous growth and its compounded
negative social and environmental impacts.
acting on evidence
The job of policy-makers is to enhance the
performance of their city relative to base-
lines for their size defined by scaling laws.
Although a scientific understanding of how
cities work may not be prescriptive for pol-
icy-makers, recent work should help them to
encourage positive urban development.
Our research shows that cities are remark-
ably robust: success, once achieved, is sus-
tained for several decades or longer6, thereby
setting a city on a long run of creativity and
prosperity. A great example of success is
metropolitan San Jose, home to the Silicon
Valley, which has been consistently over-
performing relative to expectations for its size
for at least 50 years, well before the advent of
modern hi-tech industry. Unfortunately, the
reverse is also true: it is very hard to turn
around urban decay swiftly. Ineffective
policy and unrealistic short-term expec-
tations can condemn a city to decades
of under-performance: witness former
industrial cities such as Buffalo, New York.
Today’s rapid development and urbani-
zation provides an opportunity to collect
detailed data that will illuminate the links
between economic development and its
undesirable consequences. Policy initiatives
in developed and developing cities should
be viewed as experiments that, if carefully
designed and measured, can help support
the creation of an integrated, predictive the-
ory and a new science of performance-based
planning. Examples of this approach are
increasingly common, both among
poster children such as Barcelona in
Spain or Curitiba in Brazil, and as part
of new initiatives in New York or Lon-
don. Ideally, by coupling general goals
(such as lower carbon emissions) to
actionable policies and measurable
indicators of social satisfaction, suc-
cesses and failures can be assessed and
corrected for, guiding development of
theory and creating better solutions.
Cities are the crucible of human
civilization, the drivers towards
potential disaster, and the source
of the solution to humanity’s prob-
lems. It is therefore crucial that we
understand their dynamics, growth
and evolution in a scientifically pre-
dictable, quantitative way. The dif-
ference between ‘policy as usual’ and policy
led by a new quantitative understanding of
cities may well be the choice between creat-
ing a “planet of slums” or finally achieving
a sustainable, creative, prosperous, urban-
ized world expressing the best of the human
spirit.
Luis Bettencourt is a scientist at Los
Alamos National Laboratory and external
professor at the Santa Fe Institute. Geoffrey
West i s dis tinguished profe ssor at the Santa
Fe Institute and senior fellow at Los Alamos
National Laboratory.
e-mail: gbw@santafe-edu
1. Schellnhuber, H. J., Molina, M., Stern, N., Huber,
V. & Kadner, S. (eds) Global Sustainability: A Nobel
Cause (Cambridge Univ. Press, 2010).
2. UN-Habitat. State of the World’s Cities 2010/2011
— Cities for All: Bridging the Urban Divide (2010);
available at http://www.unhabitat.org
3. Jacobs, J. The Death and Life of Great American
Cities (Random House, 1961).
4. Bettencourt, L. M. A., Lobo, J., Helbing, D.,
Kühnert, C. & West, G. B. Proc. Natl Acad. Sci. USA
104, 7301–7306 (2007).
5. Batty, M. Science 319, 769–771 (2008).
6. Bettencourt, L. M. A., Lobo, J., Strumsky, D. &
West, G. B. PLoS ONE (in the press).
7. Brown, M. A., Southworth, F. & Sarzynski, A. Policy
Soc. 27, 285–304 (2009).
8. Dodman, D. Environ. Urban. 21, 185–201 (2009).
9. West, G. B., Enquist, B. J. & Brown, J. H. Science
276, 122–126 (1997).
10. West, G. B., Brown, J. H. & Enquist, B. J. Nature
413, 628–631 (2001).
Data from 360 US metropolitan areas show that metrics such as
wages and crime scale in the same way with population size.
PREDICTABLE CITIES
−2.0 −1.5 −1.0 −0.5 00.5 1.0 1.5 2.0
log (city population/city population average)
log (metric/metric average)
−2.0
−1.5
−1.0
−0.5
0
0.5
1.0
1.5
2.0
2.5
Income
Crime
METRIC:
Patents
GDP
L. BETTENCOURT; N. RODRIGUEZ; G. WEST
21 OCTOBER 2010 | VOL 467 | NATURE | 913
COMMENT
© 20 Macmillan Publishers Limited. All rights reserved10
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Título de cubierta: The death and life of great American cities : the failure of town planning Reprinted: 1974
Global Sustainability: A Nobel Cause
  • H. J. Schellnhuber
  • M. Molina
  • N. Stern
  • V. Huber
  • S. Kadner
  • G B West
  • J H Brown
  • B Enquist
West, G. B., Brown, J. H. & enquist, B. J. Nature 413, 628–631 (2001).