Sibling composition during childhood and adult blood pressure among native Amazonians in Bolivia

Working Paper Series

SIBLING COMPOSITION DURING CHILDHOOD AND ADULT BLOOD PRESSURE IN A

NATIVE AMAZONIAN SOCIETY OF BOLIVIA

WU ZENG,1,* EDUARDO A. UNDURRAGA,1 COLLEEN NYBERG,2 DAN T.A.

EISENBERG,3 SABITA PARIDA,1 ARIELA ZYCHERMAN,4 OYUNBILEG MAGVANJAV,1

VICTORIA REYES-GARCÍA,1, 5 SUSAN TANNER,6 BOLIVIAN TAPS STUDY TEAM,7

AND RICARDO GODOY,1

1 Heller School for Social Policy and Management, Brandeis University, Waltham, MA 02454,

USA

2 Department of Anthropology, University of Massachusetts Boston, Boston, MA 02126, USA

3 Department of Anthropology, Northwestern University, Evanston, IL 60208, USA

4 Teachers College, Columbia University, New York, NY 10027

5ICREA and Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona,

08193 Bellatera, Barcelona, Spain

6 Department of Anthropology, University of Georgia, Athens, Georgia 30602

7 Tsimane’ Amazonian Panel Study, Correo Central, San Borja, Beni, Bolivia

* Correspondence to: Wu Zeng, Heller School for Social Policy and Management, Brandeis

University, Waltham, MA 02454-9110.

E-mail: [email protected]

The Cultural and Physical Anthropology Programs of the National Science Foundation (NSF),

Sibling composition and blood pressure

1

mailto:[email protected]

mailto:[email protected]

USA, provided funding for this research. The funders had no role in the study design, data

collection, analysis, decision to publish, or in the preparation of the manuscript. Dan T. A.

Eisenberg was supported by an NSF Graduate Research Fellowship. The IRB for research with

human subjects of Northwestern University (project number 0007) and the Great Tsimane’

Council approved the study. Before enrollment in the study we obtained oral rather than written

assent from participants because a large share of the studied population is illiterate.

Word count: Abstract 221; Text (excluding abstract, cover page, and acknowledgements) 6,556; 4 tables, 3 figures.


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ABSTRACT

Some early life events (e.g., low birth weight) are associated with adult blood pressure, but we

do not know whether these early life events also include sibling composition (number of older

brothers, younger brothers, older sisters, younger sisters). We draw on human capital theory to

test hypotheses about the association between sibling composition among adults (≥16 years of

age) and their systolic and diastolic blood pressure (SBP, DBP) in a native Amazonian society of

forager-farmers in Bolivia (Tsimane’). We collected data in 2007 from 426 adults in all 213

households of 13 villages. Household random-effects multiple regressions were run with SBP or

DBP as outcomes; covariates included the four sibling categories and control variables (e.g.,

sex, age, education). We found that mean SBP and DBP were 115 (SD=14) and 65 (SD=10)

mmHg, higher than other native Amazonian societies. Having an additional older sister had

statistically significant positive effects only on DBP, but the effect size was small (1-1.5%). The

effect was more prominent as people aged. Having an additional younger brother was associated

with higher SBP and DBP among young people. As people aged the effect of having a younger

brother waned. Again the effect size was small (maximum: 2.7-2.9%). Certain types of sibling

bear a statistically significant association with adult blood pressure, but the effect sizes were

biologically insignificant .

Key words: Hypertension, cardiovascular health, systolic, diastolic, Tsimane’ Amazonian Panel

Study (TAPS), sibship size


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INTRODUCTION

Research on the developmental origins of health has documented links between

early life events (e.g., low birth weight) and higher adult blood pressure [1-6]. Blood pressure is

an important indicator of adult health because it predicts risks of cardiovascular diseases [7-8],

the leading cause of death in industrializing nations [9]. Among the most important aspects of

early life is sibling composition, defined as the number of older brothers, younger brothers, older

sisters, and younger sister of a person. By being mentors, competitors, role models, and allies,

the type and number of siblings affects the current and future well-being of a person [10-11].

The literature linking (a) adult health with either (b) sibling size or with birth order is

large [11], but the literature linking sibling composition with health is smaller. Lawson and

Mace [12] followed 13,176 British children from birth to 10 years of age and found that an

additional older sibling of either sex depressed the growth rate and height of younger siblings.

Garg and Morduch [13] studied 5,203 children in Ghana and found that an additional sister was

associated with a greater Z score of height-for-age among older siblings. Wang et al. [14]

studied 7,959 junior high-school students in Japan and found that the risk of being overweight

among boys was negatively associated with the number of older sisters, whereas the risk of being

overweight among girls was negatively associated with the number of any siblings. Several

studies [15-20] suggest that having older siblings protects against the risk for allergies and

respiratory infections.

We draw two conclusions from this literature. First, there seems to be a link between

sibling composition and health among children, but we do not know if the links persist into

adulthood. Second, researchers have generally examined how broad categories of siblings (e.g.,


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sisters versus brothers; older versus younger siblings) affect health, but they have paid less

attention to the comparative effects of specific sibling categories (e.g., older brothers) on health.

We equate the term sibling size with the total number of siblings, and use it instead of the

synonym sibship size. We shall have more to say about the term sibling composition, but for the

most part we use it to refer to the specific number of older brothers, older sisters, younger

brothers, and younger sisters of a person.

There are several theories that have been applied to study the effect of sibling

composition on adult outcomes. Among these theories, we draw on the neoclassical theory of

human capital [21-22] to guide the empirical analysis. We do so because of its parsimony, clarity

of broad predictions, and because it has been widely used by economists [10, 13, 23-24] to

estimate the effects of sibling composition on many indicators of adult well-being, including

health (but not blood pressure). Furthermore, the neoclassical theory of human capital

complements and extends early life history theory [25-26] by explaining why events beyond

maternal, fetal, or infant health might affect adult health. There are other theoretical approaches

one could have used, including role models [11], equity heuristics used by parents [27], resource

dilution [28], sex-minority theory [25], and evolutionary models [12, 29-30]. Some of these

approaches are more useful when examining other topics, such as parental favoritism in

investments between girls and boys and between older and younger siblings[31], and the ensuing

health consequences of these investments, rather than identifying with precision the type of

sibling that might affect adult health the most. Despite their strengths, none of the theories

(including human capital theory) predicts the specific type of sibling likely to affect adult health.


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The neoclassical theory of human capital starts by assuming that parents try to behave

altruistically and that they try to maximize the well-being of all their children. However, parents

adjust their goals in response to at least three factors: sex biases in the economy, resource

constraints (e.g., time, income), and mechanisms for lifting the constraints through such things as

reciprocity or credit.

Through market signal mechanism, the theory of human capital predicts a sex-bias in

investment in children. If parents live in an economy with a pro-male bias, then boys will earn

higher wages than girls when children reach adulthood. If so, then parents will likely skew

investments to children whose sex is favored in the labor market [32]. When parents are

resource-constrained and cannot lift the constraints, they will skew investments to the offspring

with the potential of earning the highest wage as an adult. Under such conditions, holding

constant the number of siblings in the household, having more brothers will harm the health of

girls because brothers will compete for and dilute the fixed amount of parental resources

available to invest in the health of all offspring, particularly sons.

Additionally, the neoclassical theory of human capital makes clear predictions about the

strength of the effects of sibling composition on parental investments. It predicts that the effects

will be most marked in societies with a sharp gender bias in the labor market, and among

households facing more binding resource constraints and fewer possibilities of lifting those

constraints. At the other extreme, in more affluent economies without a wage premium for one

sex over the other, and with well-functioning credit markets, one should expect sibling

composition to exert weaker effects on parental investments among children. In between the

extremes -- within a society with a particular type of gender bias -- sibling composition should be


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more marked among poorer households because they have fewer possibilities of lifting resource

constraints.

If one accepts the three building blocks of the theory of human capital – market signals,

binding resource constraints, and access to mechanisms to ease these constraints – then linking

sibling composition to adult health becomes a straight-forward task. Poorer households without

access to mechanisms to ease binding resource constraints will invest more in, say, boys over

girls in response to market signals, and thus the addition of a girl will have a differential impact

on parental investments in boys over girls. Boys will accumulate more schooling and enjoy

better health than their sisters. Since schooling and adult health are positively associated [33],

children who accumulate more schooling early in life should have better adult health. Thus,

sibling composition should affect adult health through direct and indirect paths: in an economy

with a male bias, boys will have better child health, and this should contribute to better adult

health, and in these societies boys will accumulate more schooling, which should also contribute

to better adult health.

The neoclassical theory of human capital serves well as an initial platform to explain why

parents might skew investments to children of one sex over the other in response to signals from

the market, and in response to their own resource constraints, but the theory poses several

empirical challenges. First, the theory does not explain the particulars of why or how older or

younger sisters or younger and older brothers might affect adult health. In an economy that

favors boys over girls, it may not be any additional brother who competes for resources and

erodes the health of other siblings, but only an additional older (or younger) brother. Second,

human capital theory is silent about how to identify the resource constraints (e.g., credit, time)


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that matter most. Third, human capital theory has little to say about the multiple benefits and

costs that additional siblings might bring – benefits and costs that could well cancel each other

out. For example, in an economy with a pro-male bias, an additional younger brother might

increase sibling rivalry and lower per-capita parental educational investments among children,

but the additional younger brother might help in household chores and so free up time for school

work among older siblings. These countervailing effects plus the cultural specificity of resource

constraints might explain in part why empirical findings about the effects of sibling composition

on child or adult outcomes have been so mixed [10]. Note that many of these deficiencies are

also shared by other approaches.

As an exploratory study, we use a sample of 206 men and 220 non-pregnant women ≥ 16

years of age from a foraging and farming society of native Amazonians in Bolivia (Tsimane’)

surveyed during 2007 to estimate the association between (a) systolic blood pressure (SBP) and

diastolic blood pressure (DBP) (outcome variables) and (b) the number of younger sisters,

younger brothers, older sisters, and older brothers people currently had (main explanatory

variables).

AIMS

Aim 1: To test two hypotheses from the theory of human capital about the effects of sibling

composition on adult blood pressure:

H1: Sibling composition bears an association with adult blood pressure.

H2: The effects of sibling composition will be more marked among Tsimane’ with fewer

resources.


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Aim 2: To complement Aim 1, we try to answer the question: “what type of sibling will bear the

strongest association with adult blood pressure?”

Besides these two aims, we also contribute to the small literature

on blood pressure among contemporary native Amazonians. Descriptive studies of adult blood

pressure among native Amazonians matter because they provide the benchmark against which to

assess changes in blood pressure as these populations become increasingly integrated into the

market economy. Market exposure changes diet, physical activity, and stress, which contribute to

changes in blood pressure.

THE TSIMANE’

Setting

The Tsimane’ are a native Amazonian society of farmers and foragers who live mainly in

the department of Beni, Bolivia. They number ~8,000 people and live in ~120 villages, each of

which has an average of 19.3 households (standard deviation [SD] = 11.8 households).

Tsimane’ have been in sporadic contact with Westerners since the early 1950s [34,35]. Tsimane’

subsistence centers on hunting, fishing, plant foraging, and slash-and-burn farming. 75% of

Tsimane’ practice preferential, bilateral, cross-cousin marriage, meaning that a man marries the

daughter of his mother’s brother or his father’s sister [36]. Elsewhere [37,38,] we have shown

that Tsimane’ society is relatively egalitarian between the sexes, particularly in early life; we

have found no significant differences in anthropometric indicators of nutritional status,

schooling, academic skills, spoken Spanish language skills, or infant or childhood mortality.

Two recent publications contain ethnographies, histories, and descriptions of the

geographic setting of the Tsimane’ [34,35]. Recent publications also contain descriptions of the


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general health of the Tsimane’ [39-41]. Since background material has been covered elsewhere,

we limit the discussion of this section to sibling composition.

Sibling size and sibling composition

Adult Tsimane’ had an average of 5.8 siblings and a median of six siblings (SD = 3.0

siblings; range: 0-14). Table 1 suggests that between younger sisters and younger brothers, or

between older sisters and older brothers, the number of brothers exceeded the number of sisters.

Specifically, Tsimane’ had an average of 1.4 younger sisters (SD = 1.4), but they had an average

of 1.7 younger brothers (SD = 1.6). The difference was statistically significant (t = 4.33,

p=0.001). On average, Tsimane’ had 1.5 older brothers (SD = 1.5), but they had only 1.3 older

sisters (SD = 1.3). The difference between the mean number of brothers and sisters was

statistically significant (t = 2.68; p = 0.007).

INSERT TABLE 1 ABOUT HERE

The skewed ratio in the number of brothers compared with the number of

sisters could reflect (a) greater omission when recalling the number of sisters than when recalling

the number of brothers and (b) selective mortality of females in the past.

Tsimane’ children have roles and responsibilities for other siblings, and the roles and

responsibilities are clearer for girls than for boys. The oldest unmarried sister still living at home

is responsible for her youngest sibling. This means that by the time the youngest child is old

enough to walk, the older sister, even if she is young, is given the responsibility of watching the

newly mobile younger sibling. Responsibilities include watching the child, cleaning up after the

child stools or urinates, making sure the child is fed, making sure the child receives gifts or food

equally to the other children in the family, comforting the crying child, changing the child’s


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clothing, and bathing the child. The oldest sister retains this responsibility for the youngest

sibling as more children enter the family. Middle sisters then take care of second and third

youngest siblings. The mother is the primary caregiver for the youngest child, though if a

babysitter is needed she will turn to the oldest daughter to take her place. This responsibility of

the oldest sister is only relinquished once she has her own children. If she moves into her own

house before having her own children and remains near to her mother’s house, the oldest sister

maintains her responsibility for the youngest walking sibling. In the past (and to a lesser extent

today) Tsimane’ practiced sororal polygyny, so sisters would accompany each other into

adulthood and share all household responsibilities

Boys do not have as many responsibilities to their siblings as girls do. If a boy is the

oldest sibling, the mother continues to care for the youngest children until there is a girl in a

care-giving position. Rather, when they get older, the boy’s responsibilities become clearer in a

financial capacity, particularly towards their brothers. Older brothers help younger brothers

construct homes and find paid work and a wife. Younger brothers will seek the appropriate

sibling to ask for money or to help answer experiential questions.

MATERIALS AND VARIABLES

Materials

Information comes from the 2007 survey of an annual panel study among all Tsimane’ in

13 Tsimane’ villages along the Maniqui River, department of Beni [43]. The panel study started

in 2002 and continued uninterruptedly until 2010. At the start of the panel, we selected the 13

villages to capture geographic variation in proximity to the market town of San Borja (mean


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village-to-town distance = 25.96 km; SD = 16.70 km), the only town along the Maniqui River.

For this article we use the 2007 survey because it is the only time we asked about sibling

composition and measured blood pressure at the same time.

The 2007 survey included 426 adults or people ≥ 16 years of age (non-pregnant women =

220; men = 206) who came from 213 households. We chose 16 years of age as a cut-off to

define adulthood because at that age Tsimane’ set up their households and stop attending school.

The median age of first union for women was 16 years (SD = 3.14) and the median age of first

union for men was 19 years (SD = 3.14).

Surveyors visited villages twice during June-September, 2007. During the first visit,

surveyors collected socioeconomic and demographic data. During the second visit, surveyors

took anthropometric measures, asked about perceived health, and measured blood pressure.

Surveys took place in the home of study participants, but blood pressure and anthropometric

measures were taken in the village school. Four Bolivian university graduates did the surveys

and four Tsimane’ who worked in the panel study from its beginning translated survey questions.

Variables: Blood pressure and sibling composition

Using an automatic blood pressure monitor (Omron HEM-711DLX; rating 6V 4W),

surveyors took three measures of blood pressure for each adult on the upper left arm according to

standard protocol, with the participant’s feet on the floor and arm rested at heart level. Each

participant was seated and rested for at least five minutes before the initial measure and for a

minimum of two minutes between successive measures. Participants remained still until we

completed the three measures. For the analysis we used the average of the three measures. A


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recent university graduate with a bachelor’s degree in biology and a Tsimane’ translator who had

worked in the project from the beginning took blood pressure; Nyberg and Tanner trained the

staff to measure blood pressure.

Surveyors asked each adult in the household for the current number of living younger

sisters, older sisters, younger brothers, and older brothers. We took two steps to reduce errors in

the measure of sibling composition. First, we only asked about siblings from the same mother;

we did not ask about half siblings on the father’s side to reduce the response burden on

participants. Second, we only asked about currently living siblings. Asking retrospective

questions about a person’s sibling composition during childhood would have provided a better

metric of the possible sibling rivalry faced by the target subject when the subject was a child, but

the measure would have been vulnerable to recall bias from omission. Based on the four

categories of siblings, we calculated sibling size and an index of birth order, expressed as (birth

order -1)/(sibling size -1)[10]. The index of birth-order allows us to address the concern that

children with low birth order tend to have a higher probability of being in a small household.

We transformed sibling size to its natural logarithm in the analysis.

Control variables

In the regressions of Table 3 we control for a person’s: (a) current age, (b) sex (men = 1;

women =0), (c) maximum schooling level attained (hereafter, education), and (d) standing

height, a proxy for long-term health. We use education as a proxy for resource constrain since

education is documented to be positively associated with income [44,45]. In the regressions of


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Table 4 we drop (d) and include monetary income from wage labor and from the sale of local

goods for the 14 days before the interview

ANALYSIS

We analyzed the data in two stages.

[a]. First stage: Effect of different sibling categories. We first estimated the effects of

different sibling categories on blood pressure. To enhance statistical power, we did the analysis

with women and men combined, but controlled for the person’s sex and for interaction effects

between sibling categories and sex. Table 3 contains those results. One problem with using

different sibling categories as explanatory variables is that it leaves out the possibility of more

general effects of total sibling size. For this reason we move to the second stage.

[b]. Second stage: Effect of total sibling size and birth order. In the second stage we

merged different sibling categories and estimated the effect of total sibling size and birth order

on blood pressure, separately for women and for men. Those results are shown in Table 4.

The regressions used in both stages include the natural logarithm of either systolic or

diastolic blood pressure as an outcome. We used a household random-effects model to address

individual household impacts. We did not use ordinary least squares (OLS) regressions since

OLS regressions are unable to deal with household heterogeneity and would produce inefficient

estimates. For the statistical analysis we used Stata for Windows, version 10 (Stata Corporation,

College Station, Texas).

RESULTS

Description of blood pressure among Tsimane’


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Figures 1-2 show the distribution of systolic and diastolic blood pressure separately for

adult women and men. Adults in the sample had a mean systolic blood pressure of 115 mmHg

(mmHg = millimeters of mercury) and a median systolic blood pressure of 114 mmHg (SD =

14.84). Mean and median diastolic blood pressure were both 65 mmHg (SD = 10.82). Diastolic

blood pressure had more variation than systolic blood pressure; the coefficients of variation (SD/

mean) for systolic and diastolic blood pressure were 0.13 and 0.17. After controlling for age,

men had 9.10 mmHg or 7.97% higher systolic blood pressure than women (103 mmHg) and 3.84

mmHg or 5.79% higher diastolic blood pressure than women (61 mmHg). The differences in

mean systolic or diastolic blood pressure between women and men were statistically significant

at the 99% confidence interval.

INSERT FIGURES 1-2 ABOUT HERE

Only 5.87% of the sample (25 out of 426) had hypertension, defined as diastolic blood

pressure >90 mmHg or systolic blood pressure >140 mmHg. Our estimate of 5.87% is slightly

higher than an estimate from an earlier study (3.5%) done by other researchers [50] with a

different sample of Tsimane’. A χ2 test showed no statistically significant difference between the

share of women (12 out of 220 [5.40%]) and the share of men (13 out of 206 [6.30%]) with

hypertension (χ2=0.15; p = 0.70). A larger share of the sample, 30.75% (131 out of 426), showed

evidence of pre-hypertension, defined as follow: 80 mmHg ≤ diastolic ≤ 89 mmHg or 120

mmHg ≤ systolic ≤ 139 mmHg. 41.75% of the men but only 20.45% of the women had pre-

hypertension, and of the 131 people with pre-hypertension, 65.65% were men and 34.35% were

women.


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Comparison with other native Amazonian societies

In Table 2 we summarize published studies on mean blood pressure among other native

Amazonian societies. To enhance the validity of the comparison, we specify the age and the sex

composition of the comparison group, and then compute the statistic for the same demographic

group among the Tsimane’. For example, if a study estimated systolic blood pressure among

women 21-40 years of age, we compute the same statistic for Tsimane’ women 21-40 years of

age.


We found five published studies of blood pressure among other native

Amazonian societies besides the Tsimane’. Studies contained an average of 144 people, roughly

evenly split between women (mean = 70) and men (mean = 75). The sample size of our study

(men = 206; women = 220) was larger than the sample size of the other studies.

The results of Table 2 suggest that Tsimane' had higher systolic blood pressure and, for

the most part, also higher diastolic blood pressure than other native Amazonian groups. Only the

Cofán of Ecuador, and the Parakateje and Karib Xingu of Brazil had higher diastolic blood

pressure than the Tsimane’. The sample used in this article may be capturing the higher end of

the distribution of blood pressure among native Amazonians. Since this study took place in 2007

and most of the studies summarized in Table 2 took place on or about the same time

(1994-2009), we doubt that the differences could reflect secular changes in blood pressure in the

region.

Regression results


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[A]. The effect of different sibling categories (Table 3, section I). Among the four

categories of siblings, the number of older sisters bore a consistent association with adult blood

pressure, but only when using diastolic blood pressure as an outcome, and even then results

varied across models (Table 3, section I, row b, Columns A2, B2, C2). Overall, the addition of

an older sister was associated with an increase in diastolic blood pressure among adults of the

following amounts: (i) 1.05% (p=0.04; column A2) if we only include main effects and (ii)

1.27-1.58% if in addition to main effects we include interaction effects with the person’s sex

(1.58%, p=0.022, column B2) or education (1.27%, p=0.044, column C2).

Column D2 provides more details regarding the impact of older sisters. The main direct

effect on diastolic blood pressure of having an older sister ceased to be positive or statistically

significant (section I, row b, column D2) after interacting sibling categories with age. The

interaction effects between older sisters and age show that the positive effect of having an older

sister on adult diastolic blood pressure grew more prominent as people aged (section IV, row a,

column D2). Having an additional older sister was associated with 0.06% increases of diastolic

blood pressure for every one year increase in age.

Besides the number of older sisters, the number of younger brothers (row c) also affected

adult systolic and diastolic blood pressure. Although having an additional younger brother

showed no statistically significant overall impact on blood pressure, having an additional

younger brother was associated with 2.7-2.9% higher systolic and diastolic blood pressure

(section I, row c, column D) when people were 16 years old. As people aged, the impact of

having more young brothers waned by 0.06-0.08% per year (section IV, column D).


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The addition of one more younger sister lowered diastolic (but not systolic) blood

pressure by 1.5% (section I, column A2; p=0.01) or 3.5% (section I, column D2; p=0.001), but it

had not effect on systolic blood pressure, and even with diastolic blood pressure the effect was

found in only two of the four regressions. Last, we found no evidence that the number of older

brothers (section I, row d) bore an association with adult blood pressure.


The results of section V suggest no consistent, systematic joint effect of the four

categories of siblings on either systolic or diastolic adult blood pressure. In rows a-b, section V,

we show the results of F tests to assess whether the total number of sisters (older + younger) had

the same effect on blood pressure as the total number of brothers (older + younger) (row a) and

whether the total number of younger sibling (younger sisters + younger brothers) had the same

effect as the total number of older siblings (older sisters + older brothers) (row b). Out of the 16

tests, only four were statistically significant at the 95% confidence interval or above.

[B]. Additional analysis: Effects of total sibling size and birth order (Table 4). The broad

conclusion of the analysis presented so far is that for the most part, specific categories of sibling

do not affect adult blood pressure in a consistent way, and when they do, the size effect is

biologically small. It is possible that the weak effects might stem from our having done the

analysis with disaggregated sibling categories; merging different sibling categories into more

conventional groups or variables, such as sibling size or birth order, might produce stronger

results. In the next section we present the results of additional analysis without specific sibling

categories, but with the total number of siblings (sibling size) or birth order as the main


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explanatory variables, and additional estimates for a wide range of interaction effects. Table 4

contains the regression results of the additional analysis.


First, of the 16 coefficients for the main direct effects of the total number of siblings

(sibling size) and birth order (section I, rows a-b), only one coefficient was statistically

significant. A 20% increase in the number of siblings was associated with a 1.9 %-increase in

systolic blood pressure (p=0.08) among men without education at age 16. Lumping or splitting

different sibling groups produced essentially the same weak results.

Second, we found no evidence of interaction effects between sibling size and age [row e]

or between birth order [row f] and age. None of the eight coefficients measuring interaction

effects between age and sibling size or between age and birth order was statistically significant.

Third, we found no evidence of interaction effects between education and sibling size

(row c) and little evidence of interaction effects between education and birth order (row d). Of

the eight coefficients indicating interact effects between education and either sibling size (row c)

or birth order (row d), only one was statistically significant. Although the main effect of

education is not shown in Table 4, the significant interaction effect with education suggests a

stronger negative association between a women’s birth order and her systolic blood pressure as

the level of education increases. For a woman with one year of education, an increase in the

birth order index of 0.1 was associated with a 1.2% drop in systolic blood pressure (p=0.00) (row

d). Again, note the biologically modest (albeit statistically significant) effect of birth order on

adult blood pressure.


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Fourth, we found significant three-way interaction effects between sibling size,

education, and age on systolic blood pressure (row g). Two out of the four coefficients for

interaction effects were statistically significant, and the effect was consistent across gender, but

only for systolic blood pressure. The three-way interaction suggests that the impact of sibling

size on blood pressure depends on the level of education and age. Among women, as education

and age increased, a 20% increase in the number of siblings was associated with an additional

reduction in systolic blood pressure of 0.032% (p=0.00) per year of education per year of age.

Among men, as education and age increased, the effect of an additional 20% increase in the total

number of sibling on systolic blood pressure was 0.010% per year of education per year of age

(p=0.06). All these three-way interaction effects are net of main effects and secondary

interaction effects. As an illustration, in Figure 3 we plot the effect of the three-way interaction,

examining how the impact of sibling size on systolic blood pressure varies by age and the level

of education of men.

INSERT FIGUE 3 ABOUT HERE

Limitations

Parameter estimates for the four sibling categories of Table 3 or for the

variable sibling size or birth order of Table 4 might be biased if (a) people reported the number

of siblings with random measurement error from poor recall and (or) (b) we omitted variables

linked with both blood pressure and with sibling categories or with sibling size and birth order.

We had no instrumental variables to correct for random measurement error of sibling categories.

Correcting for random measurement error would have produced larger coefficients and stronger

statistical results. Thus, if random measurement error was the only methodological shortcoming


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affecting our regression results, then one could read the coefficients of Tables 3-4 as lower

bounds of true magnitudes.

We tried to control for some confounders by including a person’s education

(a proxy for permanent or lifetime income) and their height (a proxy for long-run health), but we

had no data to control for other variables, such as parental attributes or preferences about ideal

sibling composition [7] or other aspects of early life. Sibling composition is endogenous because

parents have some choice in the number of daughters and sons or total sibling size in a

household. In addition, sibling composition might result from endogenous biological

characteristics of parents [51, 52]. Unfortunately, we do not have data on these omitted

variables.

DISCUSSION AND CONCLUSIONS

Aim 1: Test hypotheses.

H1. Sibling composition bears an association with adult blood pressure. We found that

having an additional older sister or an additional younger brother was associated with higher

adult blood pressure. Having an additional older sister had statistically significant positive effects

on diastolic blood pressure overall, but the size effect was small (1-1.5%). The effect increased

as people aged. Having an additional younger brother was associated with higher adult blood

pressure. As people aged, the effect diminished. Having an additional older brothers had no

effect on adult blood pressure.

H2. The effects of sibling composition will be more marked among Tsimane’ with fewer

resources. Human capital theory predicts stronger effects of sibling composition on blood


21

pressure among people with less education, since people with more education have more

employment opportunities and higher income. If so, then the interaction effects of education

with sibling categories (Table 3, section III) or education with number of sibling (sibling size) or

with birth order (Table 4, section II) should have been significant. Most of these interaction

effects were insignificant, suggesting that resource constraints within Tsimane’ society do not

affect blood pressure.

Aim 2: What type of sibling will bear the strongest association with adult blood pressure? We

noted in the introduction that human capital theory and other theoretical approaches do not make

predictions about the specific type of sibling likely to affect adult health. For this reason, we

tried to identify the sibling type most closely associated with adult blood pressure, and found that

among all siblings, younger brothers and older sisters had the strongest effects on blood pressure.

For example, the regression coefficients of Table 3 (section I, row c, column D) suggest that

having a younger brother was associated with 2.7% higher systolic blood pressure and with 2.9%

higher diastolic blood pressure, and that younger brothers also had interaction effects with age

(section IV, row c, column D). The effect of having an additional younger brother waned by

0.06% per year of age. The effect of having an additional older sister on blood pressure was

small, amounted to only about 1%, and applied only to diastolic blood pressure (section I, row

b); this effect grew by +0.06% per year of age.

Younger brothers and older sisters might affect blood pressure through different

mechanisms. Studies suggest that the presence of younger siblings is associated with poorer

mental health and behavior problems. Children see the arrival of a younger sibling as a stressor


22

because the newborn competes for parental investments [53]. In a pro-male economy, having

additional younger brothers would exert a larger impact. The persistence of such stressors would

results in higher blood pressure [54]. As time goes by, people are more independent from their

parents, and thus the effect wanes. The persistence in the effect of having an additional older

sisters may result primarily from the role that older sisters play in a family. As noted, among the

Tsimane’ older sisters have more caretaking responsibilities for younger siblings, and this

support may last through the adulthood of younger siblings.

In closing, we turn to two topics: the value of examining how disaggregated

sibling categories might affect adult health and a possible reason for the weak effects. One

striking result of section I, Table 4, is the weakness of the standard coefficients of sibling size or

birth order on adult blood pressure. Contrast those results with the results of section I, Table 3.

What the disaggregated analysis buys is the ability to see that some types of sibling increase

blood pressure, but others decrease it. For example, in column A we see that having a younger

and an older sister produce effects on blood pressure of roughly similar magnitudes, but of

opposite signs. Thus, the more conventional approach of examining total sibling size or birth

order on health outcomes might, by construction, blur the significant role played by specific

types of sibling.

The weak association between sibling types and adult blood pressure raises the question

of the type of early life event likely to have a lasting impact. Perhaps only large adverse shocks

experienced in early life (e.g., maternal smoking) leave an imprint on adult health; next to those

large adverse shocks, sibling composition might be too modest to produce lasting effects. As


23

noted in the introduction, childhood sibling composition affects contemporaneous health

outcomes during childhood, but those links do not seem to persist into adulthood.


24

TABLE 1. Sibling composition of Tsimane’ adults (≥16 years of age; n=426), 2007

Number of siblings:

Sibling category:Sisters: Brothers:

a. Younger a. Older a. Younger a. OlderObs % Obs % Obs % Obs %

0 146 34.27 147 34.51 133 31.22 133 31.221 105 24.65 125 29.34 87 20.42 122 28.642 94 22.07 82 19.25 81 19.01 77 18.083 47 11.03 46 10.80 61 14.32 50 11.744 24 5.53 20 4.69 39 9.15 23 5.405 6 1.41 4 0.94 15 3.52 16 3.766 4 0.94 2 0.47 7 1.64 3 0.707 3 0.70 2 0.47Total 100.00 100.00 100.00 100.00

Mean 1.3 1.2 1.7 1.4SD 1.3 1.2 1.6 1.4


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TABLE 2. Comparison of mean diastolic (DBP) and systolic blood pressure (SBP) among other

native Amazonian societies and Tsimane'

Other native Amazonian societies: Tsimane’:Study Group Sex Age N DPB SBP DBP SBP n

1 Cofán M 21-76 43 70 NA 68 NR 168F 27 68 64 179

2 Yanomano M20-59

99 64 101 68 120 160F 96 56 90 64 110 169

Xingu M 99 65 103 68 120 160F 99 59 96 64 110 169

3 Karaja M >17 26 66 106 68 120 197F 37 64 103 64 111 207

4 Parakateje M 20-79 56 73 111 68 120 178F 34 70 107 64 111 190

5 Karib Xingu M 20-79 125 70 107 68 120 178F 126 65 100 64 111 190

Notes: NA= not available; NR=not relevant for the comparison. Study: 1=Fitton (2005)[46];

2=Mancilha-Carvahol et al. (2003)[47]; 3=Silva and Eckhardt (1994)[48]; 4=Tavares et al.

(2003) [49]; 5=Gimeno et al. (2009)[55].


26

TABLE 3. Household random-effect multiple linear regressions with natural logarithm of blood

pressure as an outcome and sibling composition as main explanatory variables, Tsimane’ adults

(≥ 16 years of age; n=426), 2007

Covariates:

Dependent variable - natural logarithm of mean of three readings of systolic or diastolic blood pressure:

A B C D

1. Systolic

2.Diastolic 1.Systolic 2.Diastolic 1.Systolic 2.Diastolic 1.Systolic 2.Diastolic

I. Direct effects of sibling (Q1):

a.#- younger sisters -0.001 -0.015** -0.002 -0.009 -0.003 -0.007 -0.007 -0.035**

b.#-older sisters 0.002 0.010** 0.008 0.015** 0.002 0.012** -0.011 -0.015

c.#-younger brothers 0.0004 0.006 -0.009** 0.008 -0.006 0.004 0.027*** 0.029***

d.#-older brothers -0.001 -0.006 -0.003 -0.007 -0.004 -0.010 0.001 0.013

II. Interaction: sibling and male:

a.#-younger sisters*male ^ ^ 0.003 -0.014 ^ ^ ^ ^

b.#-older sisters*male ^ ^ -0.015 -0.012 ^ ^ ^ ^

c.#-younger brothers*male ^ ^ 0.016** -0.002 ^ ^ ^ ^

d.#-older brothers*male ^ ^ 0.007 0.001 ^ ^ ^ ^

III. Interaction: sibling and education:

a.#-younger sisters*education ^ ^ ^ ^ -0.001 -0.003 ^ ^

b.#-older sisters*education ^ ^ ^ ^ 0.00001 -0.001 ^ ^

c.#-younger brothers*education

^ ^ ^ ^ 0.002** 0.0009 ^ ^

d.#-older brothers*education ^ ^ ^ ^ 0.001 0.001 ^ ^ IV. Interaction: sibling and agea.#-younger sisters*age ^ ^ ^ ^ ^ ^ 0.0001 0.0005

b.#-older sisters*age ^ ^ ^ ^ ^ ^ 0.0003 0.0006**

c.#-younger brothers*age ^ ^ ^ ^ ^ ^ -0.0008*** -0.0006**

d.#-older brothers*age ^ ^ ^ ^ ^ ^ -0.00005 -0.0005

V. Results of F test for joint statistical significance of various combinations of sibling categories:

a. Sisters = brothers0.01 0.13 2.68 0.13 0.39 0.41 5.03** 11.89***

a. Younger = older0.17 2.27 3.92** 0.77 1.38 0.24 3.85** 0.04

Note: Regressions include robust standard errors, clustering by household, constant, and the

following controls (none shown): sex, age, standing height, and education. **, and ***

significant at the ≤5% and ≤1% level.


27

TABLE 4. Household random-effect multiple linear regressions with natural logarithm of blood

pressure as an outcome and sibling size and birth order as main explanatory variables, Tsimane’

adults (≥ 16 years of age; n=426), 2007

Covariates

Female MaleBasic model Full model Basic model Full model

Systolic Diastolic Systolic Diastolic Systolic Diastolic Systolic DiastolicI. Direct effects of total sibling size and birth order

[a] Sibling size -0.009 0.008 -0.027 0.009 0.027 0.010 0.0934* 0.116[b] Birth order 0.025 0.058 0.071 0.051 -0.025 0.006 0.017 -0.022

II. Interaction effects[c] Sibling size * education ^ ^ 0.029 0.008 ^ ^ -0.002 -0.018[d] Birth order*education ^ ^ -0.1215*** -0.050 ^ ^ -0.012 0.017[e] Sibling size * age ^ ^ 0.001 0.000 ^ ^ -0.002 -0.002[f] Birth order*age ^ ^ -0.001 0.002 ^ ^ 0.000 0.001[g] Sibling size * education*age ^ ^ -0.0016*** -0.001 ^ ^ -0.0005* 0.000[h] Birth order*education*age ^ ^ 0.0082*** 0.002 ^ ^ 0.0004 -0.001

Note: Same notes as in Table 3, except a person’s birth order is determined as (birth order-1)/

(sibling size-1), monetary income for the two weeks before the interview is added as additional

control variables, and age is relative to 16 for ease of interpretation. .


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Figure 1. Comparison of systolic blood pressure between adult (≥ 16 years of age) Tsimane’

women (n = 220) and men (n = 206), 2007


29

Figure 2. Comparison of diastolic blood pressure between adult (≥ 16 years of age) Tsimane’

women (n = 220) and men (n = 206), 2007


30

Figure 3. Percentage effects of a 20%-increase in number of siblings on systolic blood pressure

among men: Interaction effects with age and education


31

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Documents

Sibling composition during childhood and adult blood pressure among native Amazonians in Bolivia