1 Version of 27/08/2014 The Intergenerational Transmission of Adiposity across Countries Abstract There is a worldwide epidemic of obesity. We are just beginning to understand its consequences for child obesity. This paper addresses one important component of the crisis – namely the extent to which obesity – or more generally – adiposity - is passed down from one generation to the next. Using the Body Mass Index (BMI) as a measure of adiposity, we find that the elasticity of intergenerational transmission is relatively constant – at 0.2 per parent. Our second finding is that this elasticity is very comparable across time and countries - even if these countries are at very different stages of economic development. Our third finding is that this intergenerational transmission mechanism is substantively different across the distribution of children’s BMI. Most specifically, it is more than double for the fattest children what it is for the thinnest children. These findings have enormous consequences for the health of the world’s children 1 . JEL reference numbers: I15 Key words: intergenerational; adiposity 1 We wish to thank Oscar Marcenaro-Gutierrez, Alma Sobrevilla and Qisha Quarina for preliminary research assistance on the Spanish, Mexican and Indonesian data respectively.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Version of 27/08/2014
The Intergenerational Transmission of Adiposity across Countries
Abstract
There is a worldwide epidemic of obesity. We are just beginning to understand its
consequences for child obesity. This paper addresses one important component of the crisis –
namely the extent to which obesity – or more generally – adiposity - is passed down from one
generation to the next. Using the Body Mass Index (BMI) as a measure of adiposity, we find
that the elasticity of intergenerational transmission is relatively constant – at 0.2 per parent.
Our second finding is that this elasticity is very comparable across time and countries - even if
these countries are at very different stages of economic development. Our third finding is that
this intergenerational transmission mechanism is substantively different across the distribution
of children’s BMI. Most specifically, it is more than double for the fattest children what it is
for the thinnest children. These findings have enormous consequences for the health of the
world’s children1.
JEL reference numbers: I15
Key words: intergenerational; adiposity
1We wish to thank Oscar Marcenaro-Gutierrez, Alma Sobrevilla and Qisha Quarina for preliminary research
assistance on the Spanish, Mexican and Indonesian data respectively.
2
1. Introduction
There is a worldwide epidemic of obesity. We are just beginning to understand its
consequences for child obesity - which has become one of the foremost major public health
problems in most countries. This paper addresses one important component of the crisis –
namely the extent to which obesity – or more generally – adiposity - is intergenerational – i.e.
it is passed down from one generation to the next. We examine the extent to which the BMI of
the children is inherited from the BMI of their parents. Explicitly we use data on the heights
and weights of approximately 100,000 children and their parents, measured by health care
professionals from across 6 countries: the UK, USA, China, Indonesia, Spain and Mexico. Our
analysis applies to all ages of children up to 18 years and in all countries, from the most to the
least developed, and with the most (USA) to least (Indonesia) obese population. Using the BMI
as a measure of adiposity, we find that the elasticity of intergenerational transmission of BMI
is remarkably constant – at around 0.2 per parent.
In 2013, the US spent 190 billion dollars on obesity-related health expenses. The US is by no
means alone in experiencing this epidemic. Countries like Mexico, the UK and other European
countries are all alarmed by the upward obesity trend from the epidemiological evidence. It is
also the case that many developing countries are seeing huge rises in the fraction of children
who are becoming obese in literally one generation. Countries like China and Indonesia are
our relevant comparators. We are only slowly beginning to understand the causes and
consequences of childhood obesity. This paper addresses the intergenerational dimension of
this crisis by examining how adiposity is passed down from one generation to the next and
compares it to other intergenerational processes.
Hence, our central underlying question is: what is the driving force behind rising childhood
obesity? Adiposity - or fatness - is a result of both the biological process of genetic inheritance
and a consequence of decisions made in families – loosely termed the ‘family environment’.
Most clearly, the family decisions relating to what to eat, how much to eat, how much exercise
to take, how to spend family time, and other key lifestyle choices will all have a bearing on the
outcomes on individuals in the family. But, to what extent is the problem of an individual’s
adiposity as reflected in their BMI ‘not directly their responsibility’ in the sense that their body
shape, weight and height – and hence their BMI - is passed down to them through their parents
and their genetic legacy? This is our central concern.
3
Our second focus is to pose the question of whether the process of intergenerational
transmission of adiposity is the same across countries – irrespective of their stage of
development, degree of industrialisation, or type of economy. The motivation here is to
understand the extent to which the process driving intergenerational transmission is related to
the type of economy and society under consideration. To this end we sought to examine data
from literally all the countries from which we could retrieve a reasonable sample with the
appropriate information. This is a considerable undertaking as there are not many datasets in
the world where we have - both children’s and parents heights and weights, preferably on more
than one occasion, which are medically measured rather than self-reported. We were able to
obtain data from diverse countries – from those with the most obese population – USA – to
some of the least obese countries in the world – China and Indonesia.
Our third line of investigation is to explore the extent to which the relationship between a
parent’s BMI and that of their child is potentially different at different points in the distribution
of a child’s BMI. In other words, to what extent is the intergenerational mechanism the same
for fat children and thin children? One could easily hypothesise that the relationship could be
different at different points in the distribution. Specifically, if we see societies getting fatter,
then we need to know whether the fatter children are more likely to come from fat parents or
not, and to what degree these sudden changes in rates of obesity may be driven by ‘within
generation’ experiences – i.e. decisions taken by this young generation – as they are growing
up – and, as a consequence, have nothing to do with their parents at all. Our research findings
show the effect of parents’ BMI on their children’s BMI varies by what the BMI of the child
is. Consistently, across all populations studied, we find it to be lowest for the thinnest children
and highest for the fattest. The elasticity of BMI transmission for the former is 0.1 per parent
and in the latter, 0.3 per parent. As a consequence, we can say that the children of obese parents
are much more likely to be obese themselves when they grow up. These findings have
enormous consequences for the health of the world’s children.
To understand the process of obesity it is crucial to understand the intergenerational
transmission mechanism behind it. Evidence suggests that adiposity is affected by both
environmental and genetic factors. Clearly, the intergenerational transmission mechanism we
4
are studying operates through both these two channels. So it is transmitted through family
environmental factors, which directly relates to the intra-household mechanism (how the
resources are allocated within the family), and it is also affected by genetic factors through a
direct channel. Therefore, through exploring the elasticity of adiposity across generations in
different countries, we attempt to reveal the underlying intergenerational relationship in
anthropometric characteristics.
In order to provide some basic perspective of the underlying relationship between parents and
child’s BMI – we first of all present some basic non-parametric graphs of the aggregate data,
using a kernel plot based on the raw data. Figure 1a below is the local weighted scatter
smoothing of the log of father’s BMI variable against the log of their child’s BMI variable.
Figure 1b is the corresponding figure for the relationship between the child and their mother.
The slopes of the estimated lines capture the magnitude of the intergenerational elasticity. They
suggest that the slopes have a fairly constant gradient and are nearly all parallel across countries.
This remarkable finding shows that the intergenerational elasticity is relatively high and
approximately constant across countries, i.e. that the underlying gradient of the relationship
between adiposity across generations is fundamentally constant and that the stage of
development of the country only shifts up the intercept with the least developed country having
the lowest intercept and the most developed country the highest intercept.
5
Figure 1a Lowess Plot of Log (Father’s BMI) and Log (Child’s BMI)2
Figure 1b Lowess Plot of Log (Mother’s BMI ) and Log (Child’s BMI)
2 We drop the observations with the log of BMI less than 2.5.
2.7
2.8
2.9
33.1
log(B
MI o
f child
)
2.5 3 3.5 4log(BMI of father)
China 1989-2009 Indonesia 1993-2007
British 1970-1996 England 1995-2010
US 1988-1994
Lowess Father-Child
2.7
2.8
2.9
33.1
log(B
MI o
f child
)
2.5 3 3.5 4log(BMI of mother)
China 1989-2009 Indonesia 1993-2007
British 1970-1996 England 1995-2010
US 1988-1994
Lowess Mother-Child
6
The features of these two figures are quite remarkable. Naturally, the western countries, whose
populations typically have larger, fatter body types are above the less developed countries
whose populations have thinner, smaller frames. This is unsurprising and what we would, of
course, expect. The other thing we would expect is that some of the country profiles start much
further along the x-axis than others – for example, Indonesia and China – simply because there
are relatively few fat children with low BMIs in these countries. But the most important thing
to notice is our central finding in this research – namely that the lines for each country are, for
the most part, parallel. This suggests that the elasticity – here the slope of the line in log-log
space - is essentially a very similar number in each country. This is a quite remarkable result
– which is the main motivation of our research. In simple terms, this research presents, the
substantive – hitherto unreported finding - that the proportionate increase in a child’s BMI
which is associated with their parent’s BMI, is approximately constant – at around .2 across
countries and populations which are substantively different in epidemiological terms. This
suggests that literally a unit increase in an adult’s BMI will have an overall, 20% effect, on
their child at the mean. And this impact is, in practice, nearly doubled when we consider the
effect of both parents.
In public health terms this finding is of substantive importance as it suggests that a substantial
fraction of the obesity problem is directly related to the process of intergenerational
transmission of health outcomes within families from mother and father to son and daughter.
The finding also directly relates to the context of the other studies of the intergenerational
transmission mechanism.
2. Evidence on the Intergenerational Transmission Mechanism
Intergenerational studies originate with Francis Galton (1869). By running a regression of the
offspring’s height on their parents’ height, he argued that an individual’s characteristics are
correlated with those of their parents and at the same time “regress to mediocrity”. More
specifically, the individual characteristics (such as height and weight) are closer to the
population mean than those of their parents (Galton, 1877). This finding was the basis of
Becker-Tomes model (1986) of intergenerational human capital transmission (Goldberger,
1989; Han & Mulligan, 2001; Mulligan, 1999).
7
Most intergenerational transmission studies are about the transmission of income or
educational achievement outcomes. The focus in this transmission mechanism research relates
to the equality (or otherwise) of individual opportunity over time, which exerts profound
influences on the social mobility. The strength of this transmission is usually measured with
respect to income and education outcomes. Specifically, by the elasticity of children’s income
with respect to their parents’ income (or the intergenerational elasticity of income, hereafter
called IIE). The larger is the IIE the more it means that the children’s position on the “income
ladder” is determined by their parents’ income position. We would naturally be concerned if
this elasticity of the transmission mechanism was (too) large as it would imply that both equity
and efficiency of the society would be undermined. Specifically, it means that the smaller is
this elasticity, the smaller is the role played by one’s parents in the determination of the child’s
outcome in school or the labour market.
In terms of the discrepancy in IIE across countries, partly due to the restriction of data which
covers multiple generations, most of these studies are conducted in the US or European
countries. In the US, the consensus estimated IIE is “ 0.4 or a bit higher” (for instance, 0.473
using PSID by Grawe (2011), 0.542 using NLSY sample born 1957-64 by Bratsberg et al.
(2007), this is higher than Canada (0.2 using register data (Corak and Heisz 1999), 0.152 using
IID Canadian Intergenerational Income Data and 0.381 using PSID Panel Study of Income
Dynamics data (Grawe, 2011)) and most of the European countries except for Britain (0.45
using NCDS 1958 cohort (Bratsberg,et al., 2007)) and Italy (0.48 using Italian data from the
Survey on Household Income and Wealth (SHIW) (Piraino, 2007)). The IIE estimates in
Nordic countries and Scandinavian societies are often the lowest, ranging from 0.2 to 0.3 (see
Pekkarinen, et al., 2009; Björklund and Jäntti, 2012). In contrast, the IIE in China is perhaps at
the top of the list with 0.63: i.e. a Chinese father’s income 10 percent above the paternal cohort
mean will be associated with his son having an income 6.3 percent above the filial cohort mean
(Gong et al, 2012). Using the Urban Household Education and Employment Survey (UHEES)
and the Urban Household Income and Expenditure Survey in 1987-2004 (UHIFS), Gong et al.
(2012) show the IIE in China is 0.63 for father-son, 0.97 for father-daughter, 0.36 for mother-
son, and 0.64 for mother-daughter, and education is one of the most crucial channels through
which earnings ability is transmitted across generations. However, other factors such as genes
and health are also potentially important pathways of intergenerational transmission in China.
8
The intergenerational transmission of education achievement can be thought of in the following,
where the child’s education achievement is measured by their human capital 𝐻𝑐 = 𝐹(Yc,Hp,Ac),
The mechanism operates through three main channels. The first channel is through the effects
of parental income, higher educated parents tend to have more income, so they have more
resources to invest in child’s education (Yc,) ; the second is through the effects of parental
education (𝐻𝑝 ), since higher-educated parents may invest in child’s education in a more
efficient way. In addition to these two indirect channels, parental education may also affect
child’s education through a direct channel, which is usually proxied by the genetic inheritance
of ability (Ac). Empirically, the first channel can be decomposed into the effects of current
parental income and the effects of permanent parental income, of which the latter normally
plays the dominant role and might be measured by family fixed effects (Carneiro and Heckman,
2003). The second channel also includes the motivation effects since more highly educated
parents may encourage their children to achieve a higher level of education (Boudon, 1974).
The third channel is normally identified by comparing children of twin pairs (Behrman and
Rosenzweig, 2002) or between biological and adopted children with variation in education
(Björklund et al., 2006), the general conclusion is that the intergenerational correlation in
education cannot be fully attributed to the genetic factors. The intergenerational correlation of
education is estimated to be between 0.3 and 0.4 (Alburg, 1998), but a more popular measure
of this intergenerational educational relationship is the intergenerational education elasticity
(hereafter called IEE), which varies from 0.14~ 0.45 in the USA (Mulligan, 1999) to 0.25-0.4
in the UK (Dearden et al.,1997). It is worth mentioning that some of the literature looks at the
intergenerational elasticity of IQ (hereafter called IQE), which is used to measure the
intergenerational relationship in the third channel, the magnitude of IQE ranges from 0.3 to 0.5
(Solon, 2004; Anger and Heineck, 2010;Van Leeuwen et al., 2008 ).
There is also a small but growing literature on the intergenerational correlation in various health
outcomes, such as weight, height, BMI, self-rated measures (Coneus and Spiess, 2012),
depression (Akbulut and Kugler, 2007), and smoking behaviour (Loureiro et al.,2006). Using
data from the German Socio-Economic Panel (SOEP), Coneus and Spiess (2012) estimate the
intergenerational relationship of both father and mother and children. Their fixed effects
estimates, using actual BMI and obesity as the anthropometric measure, suggest that father’s
BMI has a significantly positive effect on child’s BMI (with a coefficient of 0.57, but the effects
9
of mother’s is not significant), whereas mother’s obesity is strongly associated with child’s
obesity with a coefficient of 0.26. They claim this is a “transmission” rather than merely “link”.
However, in their data, child’s health outcomes are provided by the mother rather than medical
professionals, additionally, father and mother’s health are self-reported, this might lead to
biased estimates from measurement error. Classen (2010) estimates the intergenerational
elasticity of BMI when both generations are between the ages of 16 and 24. Applying a
regression which only controls for mother and child’s BMI, he finds an elasticity of 0.35
between mother and child’s BMI, but typically this sort of long run panel data is not available.
As Black and Devereux (2005) review, few papers have claimed a causal link, since the family
environmental factors may affect the health outcome of both parents and children. Some studies
try to address this by differencing out fixed family characteristics through comparing “sibling
mothers” or looking within “twin pairs of mothers”, assuming twins share the same
environment and genetics (Currie and Moretti, 2003; Black, Devereux, and Salvanes, 2005;
Royer,2009)
In the case of income and education, the intergenerational elasticity varies consistently across
countries, and the environmental channels has been more fully explored than the genetic
channel. Whether this transmission mechanism is applicable to the case of an anthropometric
outcome such as adiposity is the main motivation of our study. The difference of IBE from IIE
and IEE hinges on the relative role and the interaction of environmental and genetic forces in
the intergenerational transmission. Our hypothesis assumption is that in the transmission of
BMI variable, a smaller fraction of the process is open to manipulation (such as the diet changes
within the household), and a larger fraction of the mechanism is driven by the “natural process”.
In other words, in the case of health outcome such as the BMI variable, they are more likely to
be inherited genetically regardless of the change in environment. If this hypothesis is true, our
estimation for IBE may provide a lower bound of the intergenerational correlation in any
characteristics including the income and education. In other words, assuming the
intergenerational transmission of an anthropometric outcome is entirely determined by the
genetic traits, if our IBE is closer to the IIE in Scandinavian societies where the IIE is the lowest
(at 0.2), it may imply that the relationship between parents and child cannot be lower than this
threshold, in spite of the change in either family environment (such as the shift of nutrition
pattern) or socioeconomic environment (such as the innovation or marketing campaign in food
industry).
10
In addition to “regression to the mean” in the inheritability of BMI, the degree of this
inheritability (IBE) may vary across child’s BMI distribution and this variation usually relates
to the family’s socioeconomic status in the society. The general conclusion in the literature is:
in either developed countries or developing countries, the intergenerational correlation in
health measure tends to be stronger at lower SES levels (see, for example, Currie and Moretti,
2007; Bhalotra and Rawlings, 2013). In developing countries, this strong correlation emerges
at the lower levels of BMI, whereas in developed countries such as the US, this also occurs at
higher levels of BMI (Classen, 2010; Laitinen et al., 2001; Scholder et al., 2012), one
explanation is that in these countries, fast food industry is more developed, these “unhealthy”
food are generally cheaper than “healthy” food, thus lower income families tend to consume
these “unhealthy” food which is viewed as one important contributory cause of obesity.
Thus far, we can see these intergenerational studies are essentially derived from the model of
“regression to the mean” (due to Becker but strictly speaking it dates back to Galton). Based
on this framework, the intergenerational transmission of BMI variable in this paper can be