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The threat of small households. Nature


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Many studies have suggested that the increasing global human population is having a negative effect on biodiversity. According to new work, another threat comes from the rising number of households.
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experiments, such as the European Space
Agency’s Eddington and NASAs Kepler
missions, will also search for extrasolar plan-
ets through their transit signatures. Avoiding
the data deterioration caused by the Earths
atmosphere, these aim to locate planets as
small as, or smaller than, the Earth. The
success of Konacki et al.
should inspire
even greater enthusiasm for the promising
projects soon to come.
Timothy M. Brown is at the High Altitude
Observatory, National Center for
Atmospheric Research, 3450 Mitchell Lane,
Boulder, Colorado 80303, USA.
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ouseholds in many countries have
become smaller in recent decades.
Between 1970 and 2000, the average
number of occupants in households in less
developed countries fell from 5.1 to 4.4. And
in more developed nations, the decrease was
from 3.2 to 2.5 people per household over the
same period (the decline began earlier; Fig.
1). From their analysis of household dynam-
ics in biodiversity ‘hotspot’ areas, Liu and
now argue (page 530 of this issue)
that the decline in household sizes has un-
intended negative effects. The global human
population has risen, not fallen, so smaller
households means more households — and
a higher demand for natural resources. This
is in addition to the increased demand result-
ing purely from population growth.
Even before the writings of Thomas
Malthus in the late eighteenth century, the
balance between population and natural
resources was a recurrent theme. Since
ancient times, statesmen and philosophers
have expressed opinions about such issues as
the optimum number of people and the dis-
advantages of excessive population growth
Although some theorists see population
expansion in a positive light
, there is
increasing concern about the negative conse-
quences for resources
. Other things being
equal, a larger population implies a greater
demand for food, water, arable land, energy,
building materials, transport and so on — a
link that was first quantified some 30 years
. A populations age structure also influ-
ences economic growth and hence resource
use: a rapid growth of the young age seg-
ments decelerates economic growth
More recently, scholars have acknowl-
edged that another demographic variable —
the number of households — also has an
important role in resource consumption
Even when the size of a population remains
constant, more households imply a larger
demand for resources. Household members
share space, home furnishings, transporta-
tion and energy, leading to significant
economies of scale. For instance, two-person
households in the United States in 1993–94
used 17% less energy per person than one-
person households
To appreciate the different effects of pop-
ulation size and number of households on
resource consumption on a larger regional
scale, consider the following example
. In
more developed regions, energy consump-
tion increased by 2.1% per year over the
period 1970–90. Population growth can
explain 0.7 percentage points of this growth
in energy usage, while changes in per capita
energy use explain the remaining 1.4 points.
However, an alternative analysis decompos-
es the growth in energy consumption into a
factor that describes the growth in number
of households and a factor describing per
household energy use. This analysis shows
that the household growth factor explains
1.6 percentage points of the energy-
consumption increase — more than twice
as much as the population growth factor.
Liu and colleagues
now draw our atten-
tion to household dynamics in biodiversity
hotspot areas — regions that are rich in
endemic species and threatened by human
activities. They find that, during the years
1985–2000, the number of households in 76
hotspot countries increased by 3.1% per year,
substantially faster than did the population
(1.8% per year). So, average household size
fell by about 1.3% per year. These changes
relate to the group of 76 countries as a whole.
For individual hotspot countries, more than
80% showed a pattern of greater growth in
household numbers than in population. In
65 non-hotspot countries, however, popula-
tion increased at roughly the same tempo as
household numbers during 1985–2000.
Many of the world’s most populated coun-
tries are hotspot countries (such as China,
India, Indonesia, Brazil and Bangladesh).
And most of the hotspot countries studied by
Liu et al. (65 out of 76) belong to the group of
less developed nations. We know that falling
birth rates were an important driving force
behind reductions in average household size
in less developed countries in the 1990s (ref.
12). Despite these falling birth rates, however,
the population in such countries did increase
(because of decreased death rates, for
instance). All of this might explain why
increases in the number of households were
relatively pronounced in hotspot countries
Liu et al. also refer to projections of popu-
lation size and the number of households
over the next 15 years. These projections
suggest that the divergence in population
growth and household numbers will become
more pronounced. So, the authors argue, it
is crucial to consider average household size
when assessing threats to biodiversity.
Quantifying the impact of falling household
sizes, and increasing household numbers, on
biodiversity changes should have high
research priority.
Small households have adverse effects on
resource consumption both because they
are less energy-efficient in themselves and
news and views
VOL 421
30 JANUARY 2003
| 489
The threat of small households
Nico Keilman
Many studies have suggested that the increasing global human population
is having a negative effect on biodiversity. According to new work, another
threat comes from the rising number of households.
Average number of
people per household
1950 1970 1985 2000
World Less developed regions More developed regions
Figure 1Decline and fall in household sizes. Data for 1950 and 1970 are taken from ref. 8; data for
1985 and 2000 are from ref. 17.
© 2003
because they often reflect an increase in the
number of households. If this increase could
be stabilized at roughly the same level as
population growth, the adverse effects might
also stabilize. But could this be achieved?
That depends on possible explanations for
why household sizes have fallen in the first
place. Some of these explanations are as
follows. First, all other factors remaining the
same, falling birth rates reduce population
size, but do not affect the number of house-
holds; hence, household size is reduced.
Second, increased material standards of
living have an effect. Extended households
are observed in countries in an early stage of
. When these countries attain
a higher standard of living, some institutions
— such as social-security systems — provide
the assurance against risks that were formerly
supplied by the extended household.
Third, social, economic and cultural
theories of demographic behaviour point to
a variety of reasons why individuals prefer
to live in small households
. These include
less adherence to strict norms; less religiosity
and increased individual freedom on ethical
issues; female education, which has led to
women having greater economic indepen-
dence and also facilitates divorce; more
assertiveness in favour of symmetrical gender
roles; the contribution of women to the
labour market; increased economic aspira-
tions; and residential autonomy. Fourth, pop-
ulation ageing reduces household size. This is
a direct consequence of two facts: increased
longevity leads to longer periods of time when
children do not live with their parents; and
the greater mortality of men, together with
the usual age difference between spouses,
results in many widows who live alone.
Smaller households, then, are the result
of processes that cannot be reversed (such as
modern contraception and liberalization
from norms) or that we value for a number of
reasons (such as womens emancipation). So
policy interventions will have to focus on the
average household resource consumption,
in order to combat the adverse effects of
smaller households.
Nico Keilman is in the Department of Economics,
University of Oslo, Blindern, N-0317 Oslo, Norway.
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undamental advances in colloid science
often depend on physical models,
which are made by dispersing carefully
tailored particles, less than a micrometre in
size, in pure aqueous or organic liquids.
Such dispersions can be characterized by
methods such as light scattering and confocal
microscopy, and the physical and chemical
interactions between the particles, responsi-
ble for intriguing phases such as colloidal
crystals (which behave like atomic solids), can
be precisely controlled. On page 513 of this
issue, Yethiraj and van Blaaderen
describe a
new model system that can be tuned with an
electric field to display phase transitions and
unexpected crystalline structures.
Colloidal crystals first attracted interest
in the 1960s. In studies of the light scattered
from dilute dispersions, a transition was
detected from a disordered fluid to an
ordered body-centred-cubic (b.c.c.) crystal
when the screened (or reduced) Coulomb
repulsions between the colloidal particles
extended to length scales greater than the
lattice spacing
. In fact, this transition can be
controlled: adding a small amount of salt
decreases the range of the repulsive force,
because the salt dissociates into ions that
enhance the screening. As a result, the
volume fraction (or density) of particles
at the transition increases, and a denser,
face-centred-cubic (f.c.c.) crystal structure
is favoured. Adding even more salt leads to
‘hard-sphere’ transitions — as though the
particles were effectively hard spheres, with
no Coulomb repulsion. Then, entropy —
generally considered to be a measure of
disorder — favours the f.c.c. crystal, as
the number of configurations available to a
particle localized about a lattice site in the
f.c.c. crystal exceeds those accessible in a
disordered fluid or the b.c.c. crystal
Although hard-sphere behaviour of poly-
mer-based colloids could be achieved in
model systems, there was a drawback: those
colloids were opaque at even moderate densi-
ties, so little could be learned about their
structure from light scattering. More trans-
parent dispersions were sought, such as silica
spheres coated with short hydrocarbon
chains in a nonpolar solvent that eliminates
surface charge
. In the 1980s, these organo-
philic silicas and the aqueous lattices sufficed
for many studies of fluid-to-crystal transi-
tions and other colloidal phenomena. But
small silicas could not easily be made highly
uniform in size and there can be extra, van der
Waals attractions between the larger ones, so
better colloidal hard spheres were sought.
Eventually a standard emerged: poly(methyl-
methacrylate) (PMMA) spheres coated with
a low-molecular-weight polymer
In a solvent that also contains soluble
polymer, neighbouring spheres are pushed
together by osmotic pressure due to expul-
sion of polymer chains from small gaps
between the particles. This attractive force
increases roughly linearly with polymer
concentration and can easily cause a dilute
gas-like dispersion to condense into a
colloidal fluid, and then into a solid f.c.c.
crystal. In reality, the hard spheres pass
through an intermediate, random hexa-
gonal close-packed (r.h.c.p.)
phase and only
slowly convert to the f.c.c. structure. For
larger colloids or smaller polymer chains,
the transition directly from ‘gas’ to f.c.c.
crystal is more favourable
Thus long-range attractions or repul-
sions yield condensed phases with low den-
sity and coordination number, such as dense
fluid or b.c.c. crystal phases. Short-range
repulsions and attractions produce denser
f.c.c. crystals with higher coordination
number. But crystals with lower coordina-
tion numbers than the b.c.c. phase or more
complex structures have not been achieved
with spheres of a single size. Yethiraj and van
confront this issue by devising
a model system in which the forces between
particles can be tuned, combining a soft
repulsion with a long-range, anisotropic
The authors laced PMMA spheres (with
radii between 1 and 2 mm) with fluorescent
dye and dispersed them in an organic
mixture whose refractive index and density
were chosen to aid confocal imaging of the
spheres. The solvent also preserves sufficient
dielectric contrast for an applied electric
field to induce strong dipole–dipole inter-
news and views
VOL 421
30 JANUARY 2003
Condensed-matter physics
Tunable colloidal crystals
William B. Russel
Microscopic particles dispersed in a solvent — a colloidal dispersion —
can be a useful model for phase transitions and crystal nucleation. A
colloid that can be ‘tuned’ using an electric field is a valuable new tool.
© 2003
... First, smaller households use more energy per capita than large households do (MacKellar et al. 1995, O'Neill & Chen 2002, De Sherbinin et al. 2007). Second, global households grow faster in numbers than world population (Keilman 2003). Gu et al. (2015) report ratios of annual growth rates of total numbers of households over annual growth rates of total population size between 1950 and 2010. ...
... Why do we see falling average household size? We can indicate some of the mechanisms (Keilman 2003, Cohen 2010. First, all other factors remaining the same, falling birth rates reduce population size but do not affect the number of households; hence, household size is reduced. ...
... The important point I want to make is that future resource use is not only a function of future population but also of the number of households. Average household size has declined worldwide since 1970, and demographers expect that this will continue (Keilman 2003). Therefore, resource use forecasts based on households will be different compared to those based on population. ...
The paper reviews a number of issues related to uncertain population forecasts, with a focus on world population. Why are these forecasts uncertain? Population forecasters traditionally follow two approaches when dealing with this uncertainty, namely scenarios (forecast variants) and probabilistic forecasts. Early probabilistic population forecast models were based upon a frequentist approach, whereas current ones are of the Bayesian type. I evaluate the scenario approach versus the probabilistic approach and conclude that the latter is preferred. Finally, forecasts of resources need not only population input, but also input on future numbers of households. While methods for computing probabilistic country-specific household forecasts have been known for some time, how to compute such forecasts for the whole world is yet an unexplored issue. You can access the article via the following e-print URL:
... This might also contribute to higher family dependency for financial and welfare support, and hence higher household sizes. The reliance on extended family for assurance against risks of ill health, unemployment or poverty could be reduced with higher standards of living and the provision of stronger social-security systems in these countries [61]. ...
... Yet, the trend of smaller households results from a myriad of processes, some of which cannot be reversed (e.g., falling birth rates or liberation from norms), or which we consider valuable for other reasons (e.g., female emancipation, financial independence or residential autonomy) [48,61]. For example, higher divorce rates worldwide may result in an increase of energy use and GHG emissions per capita [13]; however, the freedom to divorce is also a matter of human rights and social justice. ...
... Proximate causes of the reduction in household sizes worldwide include lower fertility rates, higher divorce rates and a decline in the frequency of multi-generational families with increasing non-family provision of care among others [61,64]. There is some evidence that the trend of decreasing fertility rates and increasing divorce rates is reversing since the early 2000s [65,66], which may also stabilize or even reverse the trend of smaller households. ...
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As households get smaller worldwide, the extent of sharing within households reduces, resulting in rising per capita energy use and greenhouse gas (GHG) emissions. This article examines for the first time the differences in household economies of scale across EU countries as a way to support reductions in energy use and GHG emissions, while considering differences in effects across consumption domains and urban-rural typology. A country-comparative analysis is important to facilitate the formulation of context-specific initiatives and policies for resource sharing. We find that one-person households are most carbon- and energy-intensive per capita with an EU average of 9.2 tCO2eq/cap and 0.14 TJ/cap, and a total contribution of about 17% to the EU’s carbon and energy use. Two-person households contribute about 31% to the EU carbon and energy footprint, while those of five or more members add about 9%. The average carbon and energy footprints of an EU household of five or more is about half that of a one-person average household, amounting to 4.6 tCO2eq/cap and 0.07 TJ/cap. Household economies of scale vary substantially across consumption categories, urban-rural typology and EU countries. Substantial household economies of scale are noted for home energy, real estate services and miscellaneous services such as waste treatment and water supply; yet, some of the weakest household economies of scale occur in high carbon domains such as transport. Furthermore, Northern and Central European states are more likely to report strong household economies of scale—particularly in sparsely populated areas—compared to Southern and Eastern European countries. We discuss ways in which differences in household economies of scale may be linked to social, political and climatic conditions. We also provide policy recommendations for encouraging sharing within and between households as a contribution to climate change mitigation.
... Looking at building stock level, a higher residential density leads to many benefits in terms of resource efficiency, including a lower energy use, as it contributes to an economy of scale where the energy needed for each individual to heat, ventilate air, and use hot water for cooking and washing will be reduced [92]. The fact that household size is decreasing, causing the average living space per capita to increase, in many parts of the world has been argued as a great challenge for reduced resource use and carbon footprint per capita [93]. ...
... Finally, Table 1 shows a synthesis of the estimated energy-saving potential, barriers, and political/general recognition of the three main approaches for reduced energy use in the housing stock that were described in this chapter. world has been argued as a great challenge for reduced resource use and carbon footprint per capita [93]. Finally, Table 1 shows a synthesis of the estimated energy-saving potential, barriers, and political/general recognition of the three main approaches for reduced energy use in the housing stock that were described in this chapter. ...
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While the energy transition of the EU housing stock is now being intensified with the launch of the Renovation Wave, economic inequalities are increasing in many OECD countries, which has effects on housing-related inequalities and the demand of affordable housing. The Renovation Wave is thus an opportunity to improve housing quality for low-income households, but also entails risks for increased rents. In Sweden, the standard of housing is relatively high and energy poverty in multifamily housing is rare, meaning that there are limited social benefits to be achieved from extensive energy retrofitting; moreover, Sweden lacks a social housing sector, which limits protection of the worst-off residents. This paper thus explores whether the limited social benefits of the Renovation Wave weigh up against the risks that it entails for the worst-off in the Swedish context. This is done within a normative framework for just energy transitioning that is developed within the context of the Renovation Wave and increasing economic inequalities, consisting of four ordered principles: (1) The equal treatment principle; (2) The priority principle; (3) The efficiency principle; and (4) The principle of procedural fairness. Analysis showed that to be considered just according to our framework, the Swedish energy transition of housing should, in contradistinction to what is suggested in the Renovation Wave, limit the imposition of extensive energy retrofitting in low-income areas. Finally, having identified a mismatch between the most effective approaches in terms of energy savings and the most acceptable approaches in terms of social justice, we offer policy recommendations on how to bridge this mismatch in a Swedish context.
... Household-based consumption has been routinely used as a major indicator of demand in macroeconomics. The household is also highlighted in studies of human impact on environment and sustainable development, as it is the unit of home-based energy consumptions (Keilman 2003;Liu et al. 2003;Abrahamse et al. 2005;Bradbury et al. 2014;Gu et al. 2015). ...
... Projections of the future household changes are gaining more attentions nowadays, as the declining size and changing types of households lead to major socioeconomic and environmental challenges for human beings in this new millennium (Keilman 2003;Liu et al. 2003;Bradbury et al. 2014). Forecasts of households at the subnational levels including counties/cities are becoming a frontline of research in this field (Ip and McRae 1999;Crowley 2004;Rao 2003;Egan-Robertson et al. 2008;Zeng et al. 2013), since local governments need information of household and living arrangement projections for planning public services, infrastructure, transportation, education, health service, and social welfare. ...
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... This is because population growth is very slow now and will become negative around 2035, but the number of households which truly determines home-based energy demands will continue to substantially increase in the next four decades in Hebei. Clearly, conducting household projections by types and sizes is crucially important in forecasting residential energy demands and strategic planning of environmental protections, and considering population changes only would seriously under-estimate future energy demands and mislead policy makers (Bradbury et al., 2014a, b;Keilman, 2003). ...
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... In the past decades, the land use development in many parts of the world has mostly been characterised by a steady increase in settlement and transportation areas and is contrary to the principles of sustainability [1][2][3][4][5][6]. The causes are quite complex and land consumption can often only be explained by a bundle of influencing factors [7,8]. ...
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China has been exposed to two major population security problems: (1) the rapid population aging without good social security system especially in rural areas, which leads to farmers’ high demands of old age support from their sons, and consequently at least to a large extent caused another major problem: (2) abnormally high sex ratio at birth. These two major problems are highly correlated each other, but they were administratively managed by two independent governmental agencies of National Health & Family Planning Commission and National Committee on Aging. Such administrative system makes it hard to efficiently mobilize the public and uneasy to fully utilize the available resources. Thus, we proposed in 2017 a reform to integrate the national and local aging committee network into health and family planning system, and establish the National Health and Family Welfare Commission. The national commission’s main administrative work responsibilities are nationwide citizens’ health, family planning, old ages services, and family well-being development including population aging works. We also suggest to adopt a policy of encouraging adult children living with or nearby their old parents, which may not only be helpful to face the serious challenge of rapid population aging, but also can contribute to environment protection and sustainable development by reducing energy consumption caused by increasing numbers of small nuclear households and empty-nested elderly.
The COVID-19 pandemic brings a surge in household electricity consumption, thereby enabling extensive research interest on residential carbon emissions as one of the hot topics in carbon reduction. However, research on spatial-temporal driving forces for the increase of residential CO2 emissions between regions still remains unknown in terms of emissions mitigation in post-pandemic era. Therefore, we studied the residential CO2 emissions from the electricity consumption of China during the period 1997–2019. Afterward, the regional specified production emission factors, combining with electricity use pattern, living standard and household size, were modelled to reveal the spatial-temporal driving forces at national and provincial scales. We observed that the national residential electricity-related CO2 increased from 1997 to 2013, before fluctuating to a peak in 2019. Guangdong, Shandong and Jiangsu, from East China were the top emitters with 27% of the national scale. The decomposition results showed that the income improvement was the primary driving force behind the emission increase in most provinces, while the household size and production emission effects were the main negative effects. For the spatial decomposition, differences in the total households between regions further widen the gaps of total emissions. At the provincial scale of temporal decomposition, eastern developed regions exhibited the most significant decrease in production emissions. In contrast, electricity intensity effect showed negative emission influences in the east and central regions, and positive in north-eastern and western China. The research identified the different incremental patterns of residential electricity-related CO2 emissions in various Chinese provinces, thereby providing scientific ways to save energy and reduce emissions.
As the main body to promote economic development, the role of demographic structure on carbon emissions cannot be ignored. Based on data from the three national censuses in 2005, 2010 and 2015, this paper constructs urban-level demographic structure indicators and uses the Geographically Weighted Regression Model to study the spatial heterogeneity of demographic structure changes on carbon emissions and the corresponding mechanism at the microlevel. The results show that: (i) there was a negative correlation between household size and carbon emissions; (ii) the effects of labour ratio and dependency ratio on carbon emissions between coastal and northeastern cities are significant differences, for example, there is a significant negative relationship between labour ratio and carbon emissions in most cities in the Northeast. Overall, in most cities, the labour force ratio is positively correlated with carbon emissions, while the juvenile dependency ratio is negatively correlated with carbon emissions; (iii) demographic structure affects carbon emissions through cost mechanisms and consumption upgrade mechanism; and (iv) the medical and housing needs of the elderly are positively correlated with carbon emissions. Therefore, cities should seize the positive aspects of demographic changes. For example, policies dedicated to increasing fertility willingness in the short term are effective measures to deal with the aging population, declining birthrate and environmental challenges. It is necessary to respect the inherent laws of population development, meanwhile, based on the city's functional orientation, to cultivate differentiated leading industries, and build a green and low-carbon city through the coordinated development of population structure and employment structure.
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Rapid population growth is often blamed for economic stagnation in less developed countries. Theoretically, rapid population growth forces scarce capital to be spent on nonproductive segments of the population (e.g., children) and encourages undercapitalization of the economy, underemployment, low wages, and anemic market demand. Alternative views regard rapid labor force growth as economically beneficial. In this cross-national investigation of the economic growth rates of 75 developing countries, we regress the annual average percentage change in real gross domestic product per capita from 1965 to 1990 on demographic models that incorporate either total population growth rates and labor force growth rates or age-specific population growth rates. We find that an increase in the child population hinders economic progress, while an increase in the adult population fosters economic development. We posit a demographic windfall effect whereby the demographic transition allows a massive, one-time boost in economic development as rapid labor force growth occurs in the absence of burgeoning youth dependency. We also speculate on a demographic ratchet effect whereby economies lie fallow during "baby booms," but grow rapidly as "boomers" age and take up their economic roles in society.
The model linking environmental impact to population, affluence, and technology, or I = PAT, is reformulated in terms of households (i.e., I = HAT) as opposed to persons. Such an approach may be preferable in the case of environmental impacts that arise from activities, such as residential heating and automobile transport, for which there exist significant household-level economies of scale. Because of changes in average household size, the I = HAT model gives rise to a very different decomposition of the sources of historical growth of environmental impacts than does I = PAT. Taking growth of global energy consumption as an example, the authors find that I = PAT attributes 18 percent of the annual increase (in absolute terms) over the period 1970-90 to demographic increase in more developed regions, whereas I = HAT attributes 41 percent because the number of households grew faster than the number of persons. The I = PAT and I = HAT models also give rise to substantially different projections of CO_2 emissions in the year 2100. The authors conclude that decomposition and projection exercises are sensitive to the unit of demographic account chosen. Until more is known about the nature of the many activities that give rise to environmental impacts, it would be unwise to draw far-reaching conclusions from one choice of model without a substantive justification of that choice.
Estimates of the maximum number of people the earth can support have been developed and put forth routinely in environmental debates in UN reports and in papers by scholars or academic politicians trained in ecology economics sociology geography soil science or agronomy and other disciplines. However professional demographers tend to focus upon the composition and growth of populations restricting their predictions to the near term and framing them in conditional terms. How many people the earth can support depends upon nature as well as humankinds social economic cultural and political choices. There is not and will not be one single number of people the earth an support. Earths ultimate human carrying capacity hinges upon future constraints and possibilities which cannot yet be known since they remain in the future. The world still has room to accommodate additional population. However any major population growth is pointless. The population density needed for mankind to obtain all of the advantages of cooperation and social intercourse has in most populous countries already been attained.
This study uses data from recent household surveys in 43 developing countries to describe the main dimensions of household size and composition in the developing world. Average household size varies only modestly among regions, ranging from 5.6 in the Near East/North Africa to 4.8 in Latin America. These averages are similar to levels observed in the second half of the nineteenth century in Europe and North America. About four out of five members of the household are part of the nuclear family of the head of the household. Household size is found to be positively associated with the level of fertility and the mean age at marriage, and inversely associated with the level of marital disruption. An analysis of trends and differentials in household size suggests that convergence to smaller and predominantly nuclear households is proceeding slowly in contemporary developing countries.
"… seeks to describe and interpret the main changes in family patterns that have occurred over the past half-century in Japan, China, India, the West, Sub-Saharan Africa, and the Arab countries and to relate them to various alterations in other institutional areas." (PsycINFO Database Record (c) 2012 APA, all rights reserved)