PLoS MedpmedPLoS Medicine1549-12771549-1676Public Library of Science10.1371/journal.pmed.0020162Research ArticleEpidemiology/Public HealthWomen's HealthEpidemiologySocioeconomic Determinants of HealthLifetime Socioeconomic Position and Twins' HealthAn Analysis of 308 Pairs of United States Women TwinsKriegerNancy*ChenJarvis TDepartment of Society, Human Development and Health, Harvard School of Public Health, Boston,
Massachusetts, United States of AmericaCoullBrent ADepartment of Statistics, Harvard School of Public Health, Boston, Massachusetts, United States of
AmericaSelbyJoe VDivision of Research, Kaiser Permanente, Oakland, California, United States of AmericaPeltonenLeenaAcademic EditorUniversity of California, Los AngelesUnited States of America*To whom correspondence should be addressed. E-mail:
nkrieger@hsph.harvard.edu
Competing Interests: The authors have declared that no competing interests exist.
Author Contributions: NK conceived and designed the study, interpreted the data, and
prepared the manuscript. JTC helped devise the analytic plan, conducted the analyses, and assisted
with interpreting the data and preparing the manuscript. BAC advised on the analytic plan, and
assisted with analyzing and interpreting the data and with preparing the manuscript. JVS designed the
Kaiser Permanente Women Twins Study II, and assisted with interpretation of the data and preparing
the manuscript.
This is
an open-access article distributed under the terms of the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
Twins' Health and Adult Socioeconomic StatusBackground
Important controversies exist about the extent to which people's health status as adults is shaped
by their living conditions in early life compared to adulthood. These debates have important policy
implications, and one obstacle to resolving them is the relative lack of sufficient high-quality data
on childhood and adult socioeconomic position and adult health status. We accordingly compared the
health status among monozygotic and dizygotic women twin pairs who lived together through childhood
(until at least age 14) and subsequently were discordant or concordant on adult socioeconomic
position. This comparison permitted us to ascertain the additional impact of adult experiences on
adult health in a population matched on early life experiences.
Methods and Findings
Our study employed data from a cross-sectional survey and physical examinations of twins in a
population-based twin registry, the Kaiser Permanente Women Twins Study Examination II, conducted in
1989 to 1990 in Oakland, California, United States. The study population was composed of 308 women
twin pairs (58% monozygotic, 42% dizygotic); data were obtained on childhood and
adult socioeconomic position and on blood pressure, cholesterol, post-load glucose, body mass index,
waist-to-hip ratio, physical activity, and self-rated health. Health outcomes among adult women twin
pairs who lived together through childhood varied by their subsequent adult occupational class.
Cardiovascular factors overall differed more among monozygotic twin pairs that were discordant
compared to concordant on occupational class. Moreover, among the monozygotic twins discordant on
adult occupational class, the working class twin fared worse and, compared to her professional twin,
on average had significantly higher systolic blood pressure (mean matched difference = 4.54 mm Hg;
95% confidence interval [CI], 0.10–8.97), diastolic blood pressure
(mean matched difference = 3.80 mm Hg; 95% CI, 0.44–7.17), and low-density
lipoprotein cholesterol (mean matched difference = 7.82 mg/dl; 95% CI, 1.07–14.57).
By contrast, no such differences were evident for analyses based on educational attainment, which
does not capture post-education socioeconomic position.
Conclusion
These results provide novel evidence that lifetime socioeconomic position influences adult health
and highlight the utility of studying social plus biological aspects of twinship.
Differing socioeconomic positions in adult life are associated with differences in the health of
twins raised together.
Introduction
Twins have long provided a unique opportunity to study how health is shaped from conception to death
by biological and social factors [1–5]. At issue are the contributions, singly and combined, of genetic
inheritance, in utero postzygotic events before and after twinning, and familial plus societal contexts,
including the ways in which twins are treated by family members, each other, and society at large
[1–7
].
To explore these issues, one major trend in twin research has focused on comparing health status of
twins raised separately since birth or early childhood [1
–7]. Far fewer studies have investigated how twins
raised together, but who differ in their postadolescent socioeconomic position, compare on adult health
status [2,4
]. Yet such research could potentially inform current debates over the contribution of lifecourse
socioeconomic conditions to adult health [8–11], given twins' shared genetic inheritance and early life
socioeconomic plus biological exposures. Twins afford an important opportunity to examine the additional
impact of adult experiences on adult health in a population matched on early life experiences.
In particular, one important unresolved issue in the burgeoning literature on lifecourse analysis of
health concerns how well early life social circumstances are measured, since this these data are
essential for distinguishing between the influence of early life and adult conditions on adult health
status [8,9
]. At issue are the often limited data on childhood socioeconomic conditions [8,9], plus the possibility of
systematic error, by adult socioeconomic position, when adults recollect their early childhood
circumstances [12–
14]. Limited data, recall bias, and poor measurement together hinder obtaining accurate
effect estimates, due to confounding by unmeasured factors. This concern is especially salient for
studies investigating the social patterning of health, precisely because living and working conditions
influence health through myriad discrete yet entangled pathways [
8,15].
Further complicating analysis of the impact of childhood and adult socioeconomic position on health
are choices regarding the socioeconomic measure(s) employed [
16–18]. As discussed in several
comprehensive review articles [16–18], considerable evidence exists demonstrating that different
socioeconomic measures—e.g., education, occupation, income, wealth, housing tenure,
etc.—are not simply “exchangeable” with each other and instead often yield
different estimates of the magnitude of the socioeconomic gradient and affect health by independent as
well as correlated pathways. For example, while education has often been valued as a socioeconomic
measure precisely because, once achieved, it is not subject to reverse causation (e.g., poorer health
leading to lower income), it also has been shown to be insensitive to subsequent changes in adult
socioeconomic position (e.g., income dynamics) that also can affect adult health status [16–22]. An
important implication is that studies concerned with the joint impact of childhood and adult
socioeconomic position on health must take into account how their choice of socioeconomic measures may
influence their results.
Of note, studies of adult twins, and especially monozygotic twins, can usefully address problems of
capturing early life circumstances and assessing the contribution of childhood and adult conditions on
adult health. This is because monozygotic twins raised together through childhood (a) are tightly matched
on both genetic endowment and the socioeconomic circumstances characterizing their gestation and early
life and childhood household resources [4–6], and (b) are the same biological sex, so they are likely to
be accorded the same gender expectations and not have differential treatment or access to household
resources because of their gender [6,15]. Thus, even without any measurement of childhood conditions, a comparison of
adult monozygotic twins who are concordant versus those who are discordant on adult socioeconomic
position allows ascertainment of the extent to which socioeconomic position after childhood affects adult
health, above and beyond the impact of childhood socioeconomic position. Twin analyses employing diverse
socioeconomic measures capturing circumstances earlier versus later in adult life could also potentially
yield insight into the impact of cumulative socioeconomic position on health, with interpretation of
results for monozygotic twins, including patterns of within-pair variability, aided by comparison to
results to same-sex dizygotic twins.
Thus, our objective, framed by ecosocial theory and its concern with the lifelong embodiment of social
conditions [15,23
], was to compare health status among a cohort of monozygotic and dizygotic women twins with
shared upbringing (at least until age 14) and concordant versus discordant adult socioeconomic position.
Outcomes included biological markers, anthropometric and health outcomes, and health behaviors. We
employed data on both adult occupational class and educational level, hypothesizing that the former might
capture relevant aspects of socioeconomic position occurring after completion of educational
attainment.
MethodsStudy Population
The study twin pairs were members of the Kaiser Permanente Women Twins Study Examination II,
conducted in 1989–1990 in Oakland, California, United States [24]. The Examination I cohort included 434 twin pairs recruited in
1978–1979 from a twin registry established in 1974 at the Northern California Kaiser Permanente
Medical Care Program. All participants resided in the San Francisco Bay Area at the time of Examination
I and were born in or prior to 1960 (mean age, 41 y; range, 18–85 y). Zygosity for each pair
was determined by analysis of 20 polymorphic loci, such that the probability of misclassification as
monozygous was less than 0.001 [24].
For Examination II (1989–1990), original cohort members were sent a self-administered
questionnaire on their health and sociodemographic characteristics plus an invitation to return for a
physical exam [24]. Cohort retention was high:
Only 72 women (8.3%) did not respond, of whom 36 were deceased. Among the 796 respondents, only
87 (10.9%) did not return for a physical exam. After additionally excluding five women whose
twin was a nonrespondent, the Examination II cohort included 352 twin pairs (58% monozygotic
and 42% dizygotic), representing 81.1% of the original cohort. Enrollment and study of
the twins in both cohorts was approved by the Kaiser Permanente Medical Care Program, Northern
California Region, Institutional Review board; analyses for this investigation were additionally
approved by the Harvard School of Public Health Human Subjects Committee.
Socioeconomic Data
Childhood and adult socioeconomic position were measured at Examination II using a self-administered
questionnaire. We employed a modified version of Erik Olin Wright's occupational class schema
[12,16,25–27], analogous
to the United Kingdom's newly established National Statistics Socioeconomic Classification system (NS-
SEC) [28]. Distinctions, in order of dominance,
were between persons classified as “nonworking class” (NWC; own a business and employ
others, self-employed, or supervisory employees), “working class” (WC; nonsupervisory
employees), or not in the paid labor force [12,16,25–27].
We defined “childhood household social class” as the occupational class position of
the person identified as the head-of-household when the respondent was age 14; we also ascertained the
proportion of twins who lived together until at least age 14. We measured adult household social class
using a validated, gender-appropriate approach, equal to the most dominant class position, taking into
account the usual individual class position of the respondent and her partner or other head-of-
household, if any [16,
25,26]. Using a gender-neutral approach to
measuring household class is increasingly recognized as a more valid means of assessing household
socioeconomic position than one which automatically assigns it to either (a) the respondent, whether a
woman or man, or (b) the occupation of the adult man in the household (if one is present), given the
rise of dual wage-earner households [16–18,27–32]. For example, the new UK NS-SEC measure explicitly rejects the
prior conventional practice of “males taking precedence over females” when selecting
the “household reference person” for assignment of household class, and instead chooses
based upon “the person responsible for owning or renting or who is otherwise responsible for
the accommodation,” regardless of gender [29
]. We also obtained data on the educational level attained by each twin and their father. No
data were available, however, on childhood or adult household income, wealth, or debt, or on the
educational level of the mother.
Because the twin pairs were matched, by definition, on socioeconomic position in utero through age
14, we categorized twin pairs in relation to adult socioeconomic position. For adult household social
class, three types of pairs were possible: two concordant (both WC:WC or NWC:NWC) and one discordant
(WC:NWC). For education, the pairs were defined in relation to being concordant or discordant for fewer
than 4 y versus 4 y or more of college.
Health Outcome Data
The selected health outcomes were chosen because of their well-documented associations with
socioeconomic position and because risk could plausibly be affected by both early life and adult
circumstances [8,
9,25]. Self-report data were analyzed for self-
rated health (dichotomized as excellent/good versus fair/poor) and medication use. A validated
interviewer-administered questionnaire was used to obtain data on physical activity (kcal per kg per
y); this instrument assessed the typical amount of time spent in activities of varying intensity at
home, at work, and during recreation [33].
Data on anthropometric and biological characteristics were obtained by physical examination and
laboratory analysis [24]. Height was recorded to
the nearest 0.5 cm, weight was measured to the nearest 0.1 kg, and these data were used to calculate
body mass index (BMI, in kg/m2). Participants' minimum waist girth was measured using a
steel tape at the natural indentation or at a level midway between the iliac crests and the lower edge
of the rib cage if no natural indentation was present; hip girth was measured at the level of the
greatest protrusion of the buttocks. These measurements were recorded to the nearest 0.5 cm, and the
averages of two measures (different by no more than 1 cm) were used to calculate the waist-to-hip ratio
(WHR). After participants rested for 5 min, a mercury sphygmomanometer was used to take two measures
each of systolic and diastolic blood pressure (seated, right arm); averages of these two measures were
used for data analysis. High blood pressure was defined as systolic blood pressure 140 mm Hg or higher,
or diastolic blood pressure 90 mm Hg or higher, or taking antihypertensive medication.
Blood for lipid and lipoprotein measurement was obtained after participants had fasted overnight. It
was collected into tubes containing ethylenediaminetetraacetic acid (EDTA). Total, high-density
lipoprotein (HDL), and low-density lipoprotein (LDL) cholesterol were measured by standard methods
[24]. Glucose level (mg/dl [amount
× 0.0555 = mmol/l]; measured 2 h post-load) was determined using the glucose oxidase
method [24]; analyses using data on glucose
levels excluded the five twin pairs for whom one or both twins had values 300 mg/dl or higher.
Data Analysis
First, to establish the analytic cohort, we identified twin pairs for whom we could determine both
that they had lived together until at least age 14 and their joint socioeconomic trajectory (
n = 308 pairs). This analytic cohort excluded 44 twin pairs (21 missing data on duration lived
together; one reporting separation prior to age 14; two where one twin said below age 14 and the other
age 14 or above; plus 20 pairs for whom the joint data on adult household class was either missing,
inconsistent, or not in the labor force). Second, we ascertained the retained twins' sociodemographic
and health characteristics. Third, for continuous outcomes, we calculated (a) the mean matched
difference for twin pairs discordant on adult socioeconomic position, setting the twin with the most
socioeconomic resources as the baseline, so as to determine both the magnitude and direction of
differences in the outcomes among these pairs, and (b) the mean matched absolute difference for twin
pairs in each socioeconomic stratum, to ascertain the variability of outcomes among the twin pairs both
discordant and concordant on adult socioeconomic position. Additionally, for the categorical outcomes,
we calculated the kappa statistic and associated 95% CI [34]. We do not report data on the 18 WC:WC twin pairs, because small numbers
rendered the parameter estimates uninterpretable. All analyses were done in SAS [35].
Results
As shown in Tables 1 and
2, the sociodemographic and health characteristics of the full cohort (n = 352
pairs) and the analytic cohort (n = 308 pairs) were quite similar, with about
40% having grown up in working class households and 80% in households in which the father
had less than a 4-y college education. At Examination II, 32% of the twin pairs in the analytic
cohort were discordant for adult household occupational class, and 20% were discordant for
individual college attainment.
Results pertain to both the direction and magnitude of the difference in health status among the twin
pairs, in relation to both different measures of adult socioeconomic position and zygosity. First,
regarding the magnitude of health disparities among monozygotic twins discordant on adult socioeconomic
position, for the analyses using data on adult occupational class (Tables
3 and 4), the WC twin had significantly higher systolic
blood pressure (mean matched difference = 4.54 mm Hg; 95% CI, 0.10–8.97), diastolic blood
pressure (mean matched difference = 3.80 mm Hg; 95% CI, 0.44–7.17), and LDL cholesterol
than her NWC twin (mean matched difference = 7.82 mg/dl [amount × 0.0259 =
mmol/l]; 95% CI, 1.07–14.57). Additionally, among the monozygotic twin pairs, a
greater proportion of twin pairs discordant on occupational class were discordant for self-reported
health compared to twin pairs concordant on occupational class (27.5% versus 6.9%;
p = 0.0178). Poorer health was also more likely to be reported by the working class
twin; among the 51 monozygotic pairs discordant on class, the proportion of pairs in which the WC twin
reported fair or poor health while her NWC twin reported excellent or good health (17.6%) was
almost twice that of the converse (9.3%, i.e., pairs in which the WC twin reported good or
excellent health and the NWC twin reported fair or poor health).
By contrast, corresponding analyses using data on educational level (
Tables 5 and 6) revealed little difference in patterns of
health among monozygotic twin pairs discordant on educational attainment. Dizgyotic twins discordant on
adult socioeconomic position, whether categorized by occupational class or educational level, likewise
did not notably differ on their adult health status (Tables 3
–6).
Second, regarding the variability in health outcomes among twin pairs in relation to their adult
socioeconomic position, the mean matched absolute difference was similar among both monozygotic twins who
were discordant and concordant on occupational class, and also was similar among dizygotic twins
discordant and concordant on occupational class (Tables 3 and
4). Within occupational class strata, however, for all the
continuous outcomes other than diastolic blood pressure, the magnitude of variability typically was
greater for the dizygotic than the monozygotic twin pairs (Tables
3 and 4). Similar results were obtained for analyses based
on educational level (Tables 5 and 6), with some important exceptions. Specifically, for several outcomes among the
monozygotic twins, especially average systolic and diastolic blood pressure, post-load glucose, and
physical exercise, variability was greatest among twin pairs in which both had fewer than 4 y of college,
intermediate among discordant pairs, and least among those where both had 4 y of college or more.
Discussion
Our study provides novel evidence suggesting that correlations in health outcomes among adult women
twin pairs who lived together through childhood vary by their subsequent socioeconomic position, with
results sensitive to choice of socioeconomic measure. Although small numbers limit precision of
estimates, cardiovascular factors differed more among twins who were discordant on adult occupation class
than twin pairs concordant on being professionals, and, within twin pairs discordant on occupational
class, the working-class twin typically fared worse than the professional twin. These patterns were much
weaker or not evident for analyses using data on educational attainment. Together, these results,
combined with our prior research showing that the twins who experienced cumulative deprivation had the
worst health [26], lend additional support to the
hypothesis that cumulative experiences across the lifecourse, including those after adolescence and after
completion of educational attainment, and not just early life experiences, shape adult health
[8,9].
Additionally, the greater magnitude of variability in outcomes among dizygotic compared to monozygotic
twins within the same socioeconomic strata is what would be expected, given the tighter matching on
genetic endowment among the monozygotic twins [3
–5]. However, the suggestive finding of greater
magnitude of variability, within both the monozygotic and dizygotic twins, among pairs with the least
education compared to the most education, especially for the cardiovascular-related results, has not to
our knowledge previously been reported. Given that low educational attainment is highly correlated with
low socioeconomic resources during childhood [16
–18], our results lend tentative support to the
hypothesis that increased variability of physiological traits such as blood pressure may be positively
associated with greater early-life and cumulative exposure to economic deprivation [36]. A related body of research suggests that chronic exposure to
social stressors associated with socioeconomic deprivation may result in repeated activation—and
ultimately harmful dysregulation—of physiological systems that respond to stress, thereby
increasing risk of elevated blood pressure, insulin resistance, and visceral fat deposition and thus risk
of cardiovascular disease, obesity, and diabetes [37
–39].
Study limitations include
the relatively small number of twin pairs (albeit similar to other twin studies
[3,11
]);
lack of data on detailed occupational class position over time and on age at obtaining
a college degree, plus prior or current data on income, poverty, wealth, and debt;
lack of data on gestational age, birth weight, birth order, and whether the twins had
shared or separate chorions and amniotic sacs [2,7,11,40];
lack of data on differences in the twins' childhood experiences and exposures (e.g.,
differential treatment accorded to first- versus second-born twins, and to monozygotic versus
dizygotic twins [6]); and
lack of data on male twins; in addition, the small number of women twins who were
concordant on adult working class position limits generalizability (but not internal validity) of
results.
Most studies assessing the impact of childhood socioeconomic position on health, however, have relied on
occupational and sometimes educational data [8,9,26,
41–46], reflecting difficulties in
obtaining income data across the lifecourse [16
–18].
By contrast, strengths of our study include:
biological confirmation of zygosity;
identical gestational age;
identical biological sex, relevant to gender expectations and gendered exposures (more
similar for same- versus opposite-sex twins [5,6]);
data on age until which the twins lived together;
use of a validated and gender-appropriate household occupational class measure, plus
data on education; and
measurement of anthropometric and physiologic characteristics, not just self-reported
health.
Moreover, by focusing on postadolescence divergence of socioeconomic position, the study avoided
concerns affecting comparisons of twins raised separately versus together, e.g., difficulties in
assessing similarities versus differences of the family of origin versus adoptive family [4–6]. A recent
analysis of United Kingdom twins' earnings in relation to educational level additionally underscores the
utility of using twin analyses to gauge the impact of childhood and adult socioeconomic conditions, at
the individual and the household level, on adult economic and health-related outcomes (e.g., smoking)
[47].
Overall, results of this study are in accord with other research suggesting that cumulative exposures
related to socioeconomic position, not only genetic inheritance and early life experiences, shape adult
health [8–11,26,41–46,48]. As with our findings, these studies typically
have documented the strongest joint impacts for outcomes pertaining to cardiovascular health
[8–11,26,41–46,48]. Unlike prior research, however, the present study
newly employed a same-gender twin design, affording comparatively tight matching on life circumstances
through early adolescence, with monozygotic twins additionally matched on genetic inheritance, thereby
circumventing important concerns raised about likely unmeasured confounders affecting results of prior
studies dependent upon adult recall of—and limited data on—childhood socioeconomic
position. Even so, generalizability of results to nontwins could be hampered if twins differ
systematically from nontwins on factors influencing associations between socioeconomic position and adult
health, as perhaps related to maternal and zygotic characteristics relevant to risk of monozygotic or
dizygotic twinning or to exposures contingent upon being a twin in utero (e.g., down-regulation of
growth) [2,4
–7,49–52].
In summary, creative use of social and biological twin data concerning both social and biological
aspects of twinship [1–3,6] has the potential to inform current
debates about the impact of lifecourse socioeconomic position on health. Suggesting such investigations
could have public health import, prior research has estimated that a reduction of 2 mm Hg in the average
diastolic blood pressure in the United States—i.e., about half the difference we observed in the
comparison of working class to nonworking class monozygotic twins—would translate to a
17% decrease in hypertension, a 6% reduction in coronary heart disease, and a 15%
reduction in risk of stroke and transient ischemic attacks [
53]. Given the longstanding fascination with twins [1–3,
6], if additional and larger twin studies of economically diverse women and men twins
confirmed the relevance of cumulative and intergenerational lifetime socioeconomic resources for health,
and were also able to include a wider array of socioeconomic measures (e.g., income, wealth, debt, and
mother's education) and data on gestational age and birth weight, the evidence would likely have high
policy salience, plus importantly enhance understanding of how embodiment of societal conditions shapes
population patterns of health, disease, and well-being [
15,23,54
].
Patient SummaryBackground
Important controversies exist about the extent to which people's health status as adults is shaped
by their living conditions in early life compared to adulthood. These debates have important policy
implications, with regard to directing resources for improving health: should they be focused on
children, on adults, or both? One obstacle to determining the relative influence of early life
compared to adulthood on health is a lack of sufficient high-quality data on childhood and adult
socioeconomic position and adult health status. Twins research can be used to answer this question,
because for twins raised together their social class early in life (here defined as before age 14)
will be the same, and study of monozygotic (identical) twins further allows researchers to look at
the impact of living conditions on people with the same genetic background.
What Did the Researchers Do?
They looked at how much education each twin had and their social class in later life, and they
analyzed these in relation to diverse health outcomes (blood pressure, cholesterol, body mass index)
in 308 pairs of female twins recruited in California.
What Did the Researchers Find?
They found that the monozygotic twins who differed later in life in their social class tended to
have differences in health, with the working-class twin having higher blood pressure and higher
cholesterol than her professional counterpart. By contrast, differences in education made no
difference to these measures of health.
What Do These Findings Mean?
It is already believed that social class in children may affect later health; these results
suggest that even individuals who had the same social class in childhood may have different health
because of adult social class, including their living conditions after completing their educations.
The implication is that interventions to eliminate social inequalities in health must take into
account adult as well as childhood living conditions.
Where Can I Get More Information?
There are many twin sites on the Web. One site with many links, including to registries, is that
of the International Society for Twin Studies. http://www.ists.qimr.edu.au/links.html
The source of funding was the National Institutes of Health, National Heart, Lung, and Blood Institute
(1 R29 HL51151–01). The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript.
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sexes of the twin pairInt J Cancer1996674724788759603HeslopPSmithGDMacleodJHartCThe socioeconomic position of employed women, risk factors and mortality
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cardiovascular outcomes: A systematic reviewBMC Public Health200551715661071MartinJAParkMMTrends in twin and triplet births: 1980–97Natl Vital Stat Rep199947116AlexanderGRKoganMMartinPPapiernikEWhat are the fetal growth patterns of singletons, twins, and triplets in the United
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characteristics in adult womenTwin Res2001446447711780939HallJGTwinningLancet200336273574312957099CookNRCohenJHebertPRTaylorJOHennekensCHImplications of small reductions in diastolic blood pressure for primary
preventionArch Intern Med19951557017097695458KriegerNEmbodying inequality: Epidemiological perspectives2004Amityville (New York)Baywood PublishersAbbreviationsBMI
body mass index
CI
confidence interval
NWC
nonworking class
WC
working class
WHR
waist-to-hip ratio
Citation: Krieger N, Chen JT, Coull BA, Selby JV (2005) Lifetime socioeconomic position
and twins' health: An analysis of 308 pairs of United States women twins. PLoS Med 2(7): e162.
Sociodemographic Characteristics: Kaiser Permanente Women Twins Study, Oakland, California,
United States, 1989-1990, Full Cohort (n = 352 Pairs) and Analytic Cohort (n = 308 Pairs)
Sociodemographic Characteristics
Full Cohort (n = 352 Pairs)
Analytic Cohort (n = 308 Pairs)
Pairs
Missing or Discordant Data
Pairs
Missing or Discordant Data
n
%a
n
(%)
n
%a
n
(%)
Age (y)
30–44
128
35.8
0
(0.0%)
109
35.4
0
(0.0%)
45–64
154
43.8
137
44.5
≥ 65
72
20.5
62
20.1
Race/ethnicity
White
316
89.8
0
(0.0%)
278
90.3
0
(0.0%)
Black
26
7.4
20
6.5
Asian
9
2.6
9
2.9
American Indian
1
0.3
1
0.3
Childhood household class (at age 14)
WC (nonsupervisory employee)
105
40.5
93
(26.4%)
95
40.8
75
(24.4%)
NWC (professional/supervisory)
151
58.3
135
57.9
Not in the paid labor force
3
1.2
3
1.3
Adult household class
Both twins working class (WC:WC)
20
5.7
20
(5.7%)
18
5.8
0
(0.0%)
Discordant (WC:NWC)
102
29.8
97
31.5
Both twins professional/ supervisory (NWC:NWC)
230
64.5
193
62.7
Father's educational level
< 4 y college
221
77.3
66
(18.8%)
199
78.3
54
(17.5%)
≥ 4 y college
65
22.7
55
21.7
Adult educational level
Both twins < 4 y college
181
51.4
0
(0.0%)
161
52.3
0
(0.0%)
One twin < 4 y college, onetwin ≥ 4 y college
74
21.0
63
20.5
Both twins ≥ 4 y college
97
27.6
84
27.3
Lived with twin at least until age 14
Yes
309
99.7
42
(11.9%)
308
100.0
0
(0.0%)
No
1
0.3
Zygosity
Monozygotic
206
58.5
0
(0.0%)
178
57.8
0
(0.0%)
Dizygotic
146
41.5
130
42.2
Health Characteristics: Kaiser Permanente Women Twins Study, Oakland, California, United
States, 1989-1990, Full Cohort (n = 352 Pairs) and Analytic Cohort (n = 308 Pairs)
Health Characteristics
Full Cohort (n = 352 Pairs)
Analytic Cohort (n = 308 Pairs)
Women
Missing Data
Women
Missing Data
na
Meanb
(SD)
%b
n
(%)
n
Meanb
(SD)
%b
n
(%)
Continuous outcomes
Systolic blood pressure (mm Hg)
704
118.8
(20.2)
–
0
(0.0)
616
119.0
(20.1)
–
0
(0.0)
Diastolic blood pressure (mm Hg)
702
68.5
(11.3)
–
2
(0.3)
614
68.7
(11.2)
–
2
(0.3)
BMI (kg/m2)
702
25.7
(5.8)
–
2
(0.3)
614
25.7
(5.9)
–
2
(0.3)
WHR
701
0.8
(0.1)
–
3
(0.4)
613
0.8
(0.1)
–
3
(0.5)
LDL cholesterol (mg/dl)
703
120.6
(38.3)
–
1
(0.1)
615
120.2
(38.2)
–
1
(0.2)
Post-load glucose (mg/dl)
658
106.9
(49.8)
–
46
(6.5)
574
107.1
(51.5)
–
42
(6.8)
Physical activity (kcal/kg/y)
699
6,699.0
(3,302.9)
–
5
(0.7)
613
6812.7
(3,320.9)
–
3
(0.5)
Categorical outcomes
High blood pressure (% yes)
704
–
–
8.9
0
(0.0)
616
–
–
8.8
0
(0.0)
Self-rated health: Fair or poor
704
–
–
10.5
0
(0.0)
616
–
–
10.2
0
(0.0)
Comparison of Health Outcomes for 290 Twin Pairs Concordant and Discordant on Adult Household
Occupational Class (WC and NWC): Continuous Outcomes, by Zygosity: Kaiser Permanente Women Twins
Study, Oakland, California, United States, 1989-1990
Continuous Health Outcomes
Adult Class
Monozygotic Twins (n = 167 pairs)
Dizygotic Twins (n = 123 pairs)
n (Pairs)
Matched Mean Difference (95% CI)
Mean Matched Absolute Difference (95% CI)
n (Pairs)
Matched Mean Difference (95% CI)
Mean Matched Absolute Difference (95% CI)
BMI (kg/m2)
WC:NWC
51
0.61
(−0.39–1.60)
2.46
(1.72–3.21)
46
−0.64
(−2.72–1.45)
4.38
(2.73–6.03)
NWC:NWC
115
–
–
2.81
(2.32–3.30)
76
–
–
4.89
(3.76–6.02)
WHR
WC:NWC
51
0.00
(−0.01–0.02)
0.04
(0.03–0.04)
46
−0.02
(−0.05–0.00)
0.08
(0.06–0.09)
NWC:NWC
115
–
–
0.04
(0.03–0.06)
75
–
–
0.06
(0.05–0.07)
Average systolic blood pressure (mm Hg)
WC:NWC
51
4.54
(0.10–8.97)
12.30
(9.20–15.40)
46
−0.98
(−7.54–5.58)
17.54
(13.44–21.65)
NWC:NWC
116
–
–
11.42
(9.71–13.13)
77
–
–
13.89
(11.07–16.71)
Average diastolic blood pressure (mm Hg)
WC:NWC
51
3.80
(0.44–7.17)
9.82
(7.57–12.06)
46
0.33
(−2.90–3.56)
9.16
(7.35–10.97)
NWC:NWC
116
–
–
7.41
(6.34–8.48)
75
–
–
8.85
(7.14–10.57)
LDL cholesterol (mg/dl)
WC:NWC
50
7.82
(1.07–14.57)
19.58
(15.08–24.08)
46
−9.37
(−22.03–3.29)
32.80
(24.10–41.51)
NWC:NWC
116
–
–
22.91
(18.71–27.10)
77
–
–
27.95
(23.86–32.04)
Post-load glucose (mg/dl)
WC:NWC
44
−3.80
(−15.76–8.17)
26.39
(17.32–35.46)
39
−2.54
(−14.74–9.66)
31.77
(24.88–38.66)
NWC:NWC
103
–
–
25.49
(20.51–30.46)
69
–
–
30.82
(23.82–37.63)
Physical activity (kcal/kg/yr)
WC:NWC
51
750.0
(−206.5–1706.5)
2,566.9
(1,894.7–3,239.1)
45
1,329.0
(−237.2–2,420.8)
2,987.3
(2,234.3–3,740.4)
NWC:NWC
115
–
–
2,378.8
(2,009.6–2,748.0)
77
–
–
3,296.4
(2,655.2–3,927.7)
Comparison of Health Outcomes for 290 Twin Pairs Concordant and Discordant on Adult Household
Occupational Class (WC and NWC): Categorical Outcomes, by Zygosity: Kaiser Permanente Women Twins
Study, Oakland, California, United States, 1989-1990
Categorical Health Outcomes
Adult Class
Monozygotic Twins (n = 178 pairs)
Dizygotic Twins (n = 130 pairs)
na
% Discordant
Kappa Statistics
na
% Discordant
Kappa Statistics
κ (95% CI)
Equality of κ
κ (95% CI)
Equality of κ
High blood pressure (yes)
WC:NWC
51
7.8
0.29 (0.21–0.80)
χ2 = 0.32
46
21.7
−0.10 (−0.19–[−0.02])
χ2 = 3.36
df = 1
df = 1
NWC:NWC
116
6.9
0.46 (0.15–0.78)
p = 0.5732
77
13.0
0.22 (−0.11–0.54)
p = 0.0669
Self-rated health: Fair or poor
WC:NWC
51
27.5
−0.14 (−0.23–[−0.05])
χ2 = 5.62
46
13.0
0.43 (0.05–0.80)
χ2 = 1.40
df = 1
df = 1
NWC:NWC
116
6.9
0.30 (−0.06–0.65)
p = 0.0178
77
16.9
0.14 (−0.15–0.43)
p = 0.2373
Comparison of Health Outcomes for 290 Twin Pairs Concordant and Discordant on Educational
Level (Less Than Versus at Least 4 y of College): Continuous Outcomes by Zygosity: Kaiser Permanente
Women Twins Study, Oakland, California, United States, 1989-1990
Continuous Outcomes
Adult Educational Levela
Monozygotic Twins (n = 178 pairs)
Dizygotic Twins (n = 130 pairs)
n (Pairs)
Mean Matched Difference (95% CI)
Mean Matched Absolute Difference (95% CI)
n (Pairs)
Mean Matched Difference (95% CI)
Mean Matched Absolute Difference (95% CI)
BMI (kg/m2)
<4 coll : <4 coll
86
–
–
2.84
(2.23–3.46)
73
–
–
5.04
(3.76–6.32)
<4 coll : ≥4 coll
33
0.91
(−0.44–2.26)
2.91
(1.95–3.87)
30
−2.38
(−4.78–0.02)
4.76
(2.88–6.63)
≥4 coll : ≥4 coll
58
–
–
2.27
(1.73–2.80)
26
–
–
3.90
(2.34–5.47)
WHR
<4 coll : <4 coll
86
–
–
0.04
(0.03–0.06)
72
–
–
0.08
(0.06–0.09)
<4 coll : ≥4 coll
33
0.00
(−0.01–0.02)
0.03
(0.02–0.05)
30
−0.02
(−0.05–0.01)
0.07
(0.05–0.08)
≥4 coll : ≥4 coll
58
–
–
0.04
(0.03–0.05)
26
–
–
0.05
(0.04–0.07)
Average systolic blood pressure (mm Hg)
<4 coll : <4 coll
87
–
–
15.19
(12.47–17.92)
74
–
–
18.04
(14.58–21.50)
<4 coll : ≥4 coll
33
2.40
(−2.72–7.53)
11.11
(7.63–14.59)
30
4.44
(−2.16–11.05)
13.51
(8.82–18.20)
≥4 coll : ≥4 coll
58
–
–
8.48
(6.58–10.38)
26
–
–
11.69
(8.07–15.31)
Average diastolic blood pressure (mm Hg)
<4 coll : <4 coll
87
–
–
10.01
(8.35–11.69)
72
–
–
10.29
(8.52–12.05)
<4 coll : ≥4 coll
33
0.31
(−2.98–3.61)
7.22
(5.07–9.37)
30
0.23
(−2.97–3.44)
6.86
(4.84–8.87)
≥4 coll : ≥4 coll
58
–
–
6.82
(5.33–8.31)
26
–
–
7.59
(5.33–9.85)
LDL cholesterol (mg/dl)
<4 coll : <4 coll
87
–
–
22.76
(18.96–26.55)
74
–
–
29.24
(23.73–34.76)
<4 coll : ≥4 coll
33
−10.30
(−23.60–2.99)
25.64
(15.12–36.15)
30
15.03
(1.76–28.30)
32.90
(29.49–40.82)
≥4 coll : ≥4 coll
57
–
–
19.60
(14.44–24.75)
26
–
–
24.50
(16.20–32.80)
Post-load glucose (mg/dl)
<4 coll : <4 coll
70
–
–
32.90
(24.99–40.81)
63
–
–
33.25
(27.15–39.35)
<4 coll : ≥4 coll
29
−1.03
(−12.34–10.28)
22.00
(14.15–29.85)
26
−7.54
(−22.30–7.23)
27.62
(17.15–38.08)
≥4 coll : ≥4 coll
56
–
–
19.43
(14.99–23.87)
24
–
–
32.08
(18.76–45.41)
Physical activity (kcal/kg/yr)
<4 coll : <4 coll
86
–
–
2,743.8
(2,232.4–3,255.3)
74
–
–
3,040.6
(2,380.1–3,701.0)
<4 coll : ≥4 coll
33
−757.0
(−1,770.0–256.9)
2,384.6
(1,741.0–3,028.1)
30
706.7
(−945.6–2,359.0)
3,625.5
(2,598.4–4,652.6)
≥4 coll : ≥4 coll
58
–
–
2,197.1
(1,658.2–2,735.9)
25
–
–
3,010.1
(2,066.1–3,954.2)
Comparison of Health Outcomes for 290 Twin Pairs Concordant and Discordant on Educational
Level (Less Than Versus at Least 4 y of College): Categorical Outcomes by Zygosity: Kaiser Permanente
Women Twins Study, Oakland, California, United States, 1989-1990