cross-cultural variation in blood pressure: a quantitative analysis of the relationships of blood...

Sue. Sci. Med. Vol. 16. pp. 419 IO 430, 1982

Printed in Great Britain. All rights reserved 0277-9636/82/040419-l2$03.00/0

Copyright 0 1982 Pergamon Press Ltd

CROSS-CULTURAL VARIATION IN BLOOD PRESSURE: A QUANTITATIVE ANALYSIS OF THE RELATIONSHIPS

OF BLOOD PRESSURE TO CULTURAL CHARACTERISTICS, SALT CONSUMPTION AND BODY WEIGHT

INGRID WALDRON, MICHELE NOWOTARSKI, MIRIAM FREIMER, JAMES P. HENRY*,

NANCY POST and CHARLES WITTEN

Department of Biology, University of Pennsylvania, Philadelphia. PA 19104, U.S.A.

Abstract-This study has analyzed the relationships of cross-cultural variation in blood pressure lo cultural characteristics, salt consumption and body weight. The data used were blood pressures for adults in 84 groups, ratings of cultural characteristics (based on anthropological data and made by raters who had no knowledge of the blood pressure data) and, where available, salt consumption and body mass index (weight/height’).

Blood pressures were higher and the slopes of blood pressure with age were greater in groups which had greater involvement in a money economy, more economic competition, more contact with people of different culture or beliefs, and more unfulfilled aspirations for a return to traditional beliefs and values. Blood pressures were also higher in groups for which the predominant family type was a nuclear or father-absent family, as opposed to an extended family. For Negroes, groups who were descended from slaves had higher blood pressures than other groups.

The correlations between blood pressures and involvement in a money economy were substantial and significant even after controlling for level of salt consumption and, for men, also after controlling for body mass index. For men there were also significant partial correlations between blood pressure and salt consumption, controlling for type of economy. For women there were significant partial correlations between blood pressure and body mass index, controlling for type of economy.

In conclusion, cross-cultural variation in blood pressure appears to be due to multiple factors. One contributory factor appears to be psychosocial stress due to cultural disruption. including the disruption of cooperative relationships and traditional cultural patterns which frequently occurs during economic modernization. In addition, both the protective effects of very low salt consumption in some groups and differences in body weight appear to contribute to cross-cultural variation in blood pressure.

INTRODUCTION

Recent reviews of studies of blood pressure in different cultural groups are in general agreement that blood pressures for adults tend to be higher in westernized societies than in unacculturated, traditional societies [l-S]. In westernized societies, blood pressures tend to increase with age. In contrast, in many traditional societies, there is little or no relationship between blood pressure and age for adults.

Although there is general agreement concerning these findings, there is considerable controversy concerning the interpretation of these data. Henry and Cassel [l] have argued that cultural change is the critical characteristic which distinguishes the groups that have high blood pressures. They hypothesized that the primary cause of increased blood pressures is psychological stress due to “dissonance between the social milieu in later life and expectations based on early experiences”. Ostfeld and D’Atri [Z] have hypothesized that rapid sociocultural change may have no direct effect on blood pressure. Rather, increases in body weight associated with economic modernization may be responsible for the increased blood pressures in acculturated societies. Freis [3] has argued that the blood pressure levels in different societies are most closely correlated with differences

*Present address: Department of Physiology, Univer- sity of Southern California, Los Angeles, CA 90007. U.S.A.

in salt consumption. He hypothesized that increased salt consumption is the principle cause of increased blood pressures in acculturated societies.

None of these papers has presented a quantitative analysis of data from a systematic and inclusive sample of cultures in order to provide a basis for evaluating the alternative hypotheses. A systematic test of the role of cultural characteristics has been difficult because little information has been available concerning cultural characteristics. Most of the original studies of blood pressure levels present only brief, impressionistic descriptions of the cultural situation of the group studied.

The purpose of this study was to develop a more systematic and quantitative analysis of the correlates of cross-cultural variation in blood pressure. System- atic cultural data were sought for each of the groups for which suitable blood pressure measurements were available. Cultural characteristics were rated by methods designed to eliminate the possibility that the cultural ratings might be biased by knowledge of the blood pressure levels. The relationships between the ratings of cultural characteristics and blood pressure levels have been analyzed quantitatively. In addition, partial correlation analyses have been used to test whether cross-cultural variation in blood pressure can be accounted for by a single factor, such as salt consumption, or whether salt consumption, body weight and cultural characteristics each make independent

419

420 INGRID WALDRON et al.

contributions to the prediction of cross-cultural variation in blood pressure. Due to limitations of the data, the results cannot be considered definitive. However, these analyses do provide systematic, quantitative evidence in support of the hypothesis that cross-cultural variation in blood pressure is due to multiple factors, including body weight, salt consumption and cultural characteristics related to economic modernization.

METHODS

Blood pressure data

This study is based on blood pressure measurements for samples from 84 different groups C&61] (Supplementary Table l)*. Data were included from all studies known to the authors which met the following criteria. First, it was required that the blood pressure data were presented separately by sex and age, and that age was known precisely or could be confidently estimated with reasonable accuracy. For inclusion of data for a given sex, it was required that data be available for three or more adult age groups with at least ten subjects of that sex in each age group, and with the youngest subjects no more than 30 years old. Data from open age groups were excluded, as were data for age groups which included only individuals younger than 20 or older than 64. Samples of hospital patients, samples of insurance policy holders and samples which excluded subjects with signs of high blood pressure were excluded from this study, since for these types of samples the method of recruitment could result in a biased sample with respect to average blood pressure levels. A few groups were excluded because we were unable to obtain the publication which reported the blood pressure study or were unable to locate adequate anthropological data for that group for the approximate date at which the blood pressure measurements were made. Data for a few samples were excluded because the sample included individuals living under such diverse cultural conditions that it was not possible to make meaning- ful cultural ratings. Groups composed primarily of individuals who had migrated within the last few years were also excluded.

In some cases data were available for two groups at very similar locations and dates or for the same group at two slightly different dates, and it would have been necessary to use essentially the same anthropological information to make cultural ratings for both samples. In these cases we used the data from the study which had superioi methodology or had data for more of the relevant variables (including serum cholesterol, which will be discussed in a subsequent paper). The average absolute value of the difference in blood pressures between the ‘duplicate’ groups was 7/5 mm Hg at age 50. To test for possible bias in our selection of the groups to be included, we tested whether the blood pressures of the groups we included in the study were higher or lower than the blood pressures of the duplicate groups we excluded.

*Supplementary tables may be obtained by request to the first author. These supplementary tables are referred to in the text, and they present additional detailed data and results.

The blood pressures for the included group were equally likely to be greater or less than the blood pressures for the excluded group for the whole sample of duplicate groups, and also within specific cultural categories. This finding indicates that our selection among duplicate groups has not introduced bias in our results.

The application of the criteria for inclusion in the study resulted in the exclusion of blood pressure data for one sex or for one or more groups reported in some of the papers cited as sources for our study (Supplementary Table 1). In some cases it was necessary to make calculations to bring the data into con- formity with our criteria. Specifically, for several groups [25,29,35,38-J it was necessary to compute the weighted average of blood pressures for a pair of adjacent age groups which individually had inad- equate sample sizes. For one sample [37], age-specific average blood pressures were calculated on the basis of individual blood pressure values. For the Fijian samples [28,30] and the Amish samples [21], alge- braic calculations using the published data provided average blood pressures for two subsamples for which separate anthropological ratings were derived.

A variety of different age groups were used in the presentation of the blood pressure data in the original studies. In order to obtain comparable data for each group studied, the following computations were performed separately for each sex. Blood pressures were averaged for all groups which had an average age between 19 and 31. These average blood pressures are designated in the Results section as blood pressures for ages 2C30. Similarly, blood pressures were averaged for all age groups with an average age of 49-61 years of age, and these are designated as blood pressures for ages 5&60. Finally, the slope of blood pressure with age was computed for the age range from 20 to 65 years old. It should be noted that the slopes of blood pressure with age are derived from cross- sectional data and are consequently influenced by differences between cohorts and do not necessarily reflect actual change of blood pressure with age for any given cohort. An example which illustrates the need for caution is Donnison’s finding of lower blood pressure at older ages among Kenyan men [62], which is often interpreted as evidence of a decline of blood pressure with age. However, the observed differences could equally well represent cohort differences or situational differences between the young men, who were ‘recruits for labor’ examined at a hospital, and the older men, who were examined “in the native reserves when (they) gathered together in council” [62].

Weight and salt data

Data concerning weight and height of the sample populations were obtained where available [12, 15, 17, 19, 31-33, 42-44, 50, 51, 58, 60, 63-661. In our analyses we have used the body mass index (weight/ height*), which has proven to be a useful measure in previous analyses [12, 14,67,68]. For analyses of the relationships between body mass index and blood pressure at ages 20-30 or at ages 5(r60, the averages of body mass index for the corresponding age groups were used [68]. For analyses of the relationship between body mass index and slope of blood pressure

Cross-cultural variation in blood pressure

Table 1. Cultural rating scales

421

(a) Hunter-gatherers (b) Pastoral; i.e. herding, primarily for family consumption (c) Traditional agriculture, primarily for family consumption (d) Unmechanized agriculture not primarily for sale, but a significant fraction (about lo:,;,) of total production goes to

nonfamily members for taxes or sale (e) Unmechanized family agriculture, but more than one-quarter of total production for sale (f) Plantations (large-scale unmechanized production of agricultural products for sale) (g) Mechanized agriculture in an industrial society (h) Industry (any steam or electrical powered industry), trade, etc. in a small town (up to 50,000) (i) Industrial urban society (a) Economic system more competitive than cooperative (b) Some economic cooperation but mainly each family seeks livelihood independently or a little cooperation with a

little competition (c) Economic livelihood depends to a significant extent on cooperation within the group For this question consider contacts by members of the group with other people who have different beliefs. Include any contacts which are sufficient to communicate differences in beliefs or culture.. Indicate the average amount of contact of this type for adults in this society in hours per week What fraction of these people experience unfulfilled aspirations for a return of society to traditional beliefs and values’! (a) More than half the people live in extended families; i.e. adults often live with their parents or siblings (b) Less than half the people live in extended families, but more than half live in polygamous families (c) More than half the people live as nuclear families inside dwellings, but have at least one additional relative living

within half a mile or see each other several times a week (d) Most people live in nuclear families (e) More than one-quarter of the children not living with both parents (father-absent families)

with age, the slope of body mass index with age was tural information can be obtained from the first used [68,69]. author upon request.

Data concerning salt consumption or sodium excretion of the sample populations were used where available [9,39,41,43,44,47,52,56,70]. In addition; data for closely related samples were used to provide estimates of salt consumption or excretion for a few groups [71-761. Most of these data were for 24-hr urinary excretion of sodium. In a few cases estimates were based on dietary information and/or casual urinary sodium concentration [9,41,47,75]. Infor- mation from groups for which more than one type of data were available suggest that these different types of data are reasonably comparable [44,70]. Data concerning salt consumption or excretion were available for men and women separately in only a few cases and there was generally relatively little sex difference [43,44,52,70], so a single average figure for men and women was used.

Cultural data

Examples of the cultural rating scales which were used are shown in Table 1. The first scale shown is referred to as type of economy. The type of economy rating for each group in the study is given in Supple- mentary Table 1. In the early weeks of the research project, raters worked together to rate cultural characteristics for the same groups in order to develop explicit and mutually agreed criteria for making the cultural ratings. If adequate, reliable information was not available to provide a basis for rating a particular cultural characteristic for a particular group, then no rating was made for that scale for that group. Some indication of the reliability of ratings is provided by the independent, duplicate ratings of type of economy which were made for seven cultural groups. In six cases these duplicate ratings were in complete agreement and in the other case the duplicate ratings were adjacent categories on the rating scale.

Ratings of cultural characteristics were made by students who did not know which health variables were under study and, or course, had no information about specific blood pressure levels for the groups being rated. Cultural data from the articles reporting the blood pressure measurements were made available to the raters only if they were explicit factual data, such as the distribution of occupations for the sample. The sources of the data for the cultural ratings were primarily anthropological articles and books. One important source of information was the Human Relations Area Files, a compilation of anthropological data for a large number of societies. A copy of these files is located in the Anthropology Department of the University of Pennsylvania. In a few cases information was obtained from informants who lived or had lived as part of or near to the group in question. A bibliography of major sources of cul-

Methods of analysis

For most of the cultural characteristics, the relationships between the cultural ratings and the blood pressure measures were tested using Pearson product moment correlation coefficients. Significance tests for Pearson correlation coefficients are valid if the distribution of either of the correlated variables is normal [77], and the blood pressure measures were normally distributed. For a few of the cultural variables, the distribution was sufficiently non-normal that the significance tests for the Pearson correlation coefficients, although statistically valid, could be sensitive to a few deviant data points. In these cases we have used Ken- dall r rank order correlation coefficients or, for a dichotomous cultural variable, t-tests. Partial correlation coefficients were computed to test whether blood pressures were associated with one independent variable, after controlling statistically for another

422 INGRID WALDRON er al.

independent variable. Computations were performed using the Statistical Package for the Social Sciences [78]. The number of observations is given for each row in the tables, but the number of observations for a specific result may be slightly lower due to oc- casional missing data for specific blood pressure measures.

There are substantial intercorrelations between many of the cultural characteristics and also among the various blood pressure measures, so the significance tests for the correlation coefficients presented in a table in the Results section cannot be considered independent tests. Because we have made multiple tests of significance and these are not all independent tests, the significance levels must be interpreted con- servatively. In interpreting the results, we will not put weight on individual significant findings, but rather will be concerned with the patterns of observed relationships.

In addition to the analyses described thus far, two additional subsidiary types of analysis were performed in order to test the sensitivity of the results to variation in methodology. The results of these subsidiary analyses are not presented in the tables or figures of this paper, but are described in the text and are presented in the supplementary tables, available from the first author upon request. The first set of subsidiary analyses included data only from the studies with the best methodology and the best quality data. Specifically, these analyses of ‘best quality samples’ included only data from studies of representative, probability population samples with participation rates of at least 74:/A, for whom age was known precisely, with at least 40 subjects for each included sex.

The second set of subsidiary analyses is related to the problem that different methodologies were used to collect the blood pressure data for different samples included in this study, and the differences in methodology can influence the blood pressures obtained and reduce comparability between studies [34,79]. To assess the impact of these methodological differences, a set of subsidiary analyses were performed using blood pressures which had been adjusted for several of the more important methodological differences, and results were compared to the results of the primary analyses which used unadjusted blood pressures. For diastolic blood pressures, the adjustments were as follows. If fourth phase rather than fifth phase diastolic blood pressures were recorded, then 5 was subtracted from the blood pressure values [79, p. 233; 80; unpublished data from 19 and I. Waldron]. If readings were rounded to the nearest 5 or 0 below or if the lowest of two or three readings was recorded, then 2.5 was added. If the authors mentioned that the subjects were anxious during blood pressure measurements, then 8 was subtracted [79, pp. 327 and 376; 81: 821. For systolic blood pressures, the second and third of these adjustments were used and, in addition, if the setting in which the blood pressure measurements were made was unfamiliar to the subject, then 3 was subtracted from the blood pressure values [34]. These adjustments are clearly somewhat arbitrary in magnitude, but they represent reasonable, somewhat conservative estimates of the probable effects based on available data, and these adjusted blood pressures

Systolic blood pressure Diastolic blood pressuie

140 t

Fig. I. Average blood pressures for men in societies with different types of economy. More detailed definitions of each type of economy are given in Table 1. Slope of blood pressure with age is expressed in units of mm Hg per year of age. Parentheses indicate that the data point is based on fewer than five groups. None of the relationships shown deviate significantly from a linear relationship. Correlation

coefficients are given in Table 2.

have been used only in subsidiary analyses to check the robustness of the results of the main analyses. It is emphasized that these adjusted blood pressures were not used in the analyses presented in the tables and figures of this paper; the adjusted blood pressures were used only in the subsidiary analyses presented in Supplementary Tables 2, 8 and 9.

RESULTS

Cultural characteristics

Figures 1 and 2 illustrate the relationships between type of economy and blood pressures. Average blood pressures in the hunter-gatherer, pastoral or traditional agricultural groups which did not produce

Fig. 2. Average blood pressures for women in societies with different types of economy. See notes for Fig. 1.

Cross-cultural variation in blood pressure 423

Table 2. Correlations between cultural characteristics and blood pressure

Blood pressure Blood pressure Slope of blood at ages 2&30 at ages 5G60 pressure with age

Systolic Diastolic Systolic Diastolic Systolic Diastolic IV

Hunter-gatherer-Urban industrial Men

Women

Economic cooperation-Economic competition

Men

Women

Average contact with people of different culture or beliefs

Men

Women

Desire for return to traditional beliefs and values

Men

Women

Extended family-Nuclear family or father-absent

Men

Women

+ 0.42 (0.001) +0.26 (0.02)

+ 0.36 (0.001) +0.21 (0.05)

+0.41 (0.001)

+0.39 +0.55 f0.38 (0.02) (0.001) (0.03) +0.56 + 0.63 +0.51 (0.003) (0.001) (0.01)

+0.31 (0.004)

+0.33 (0.002)

+ 0.40 (0.001)

f0.41 (0.001)

+0.28 (0.01)

+0.56 (0.001) + 0.50 (0.001)

+0.45 (0.001) +0.35 (0.003)

+0.36 (0.001) + 0.24

(0.04)

+ 0.42 (0.001) + 0.48 (0.001)

+0.61 (0.001) + 0.54 (0.001)

+ 0.50 (0.001) +0.38 (0.002)

+0.41 (0.001) +0.26 (0.03)

+ 0.39 (0.02) + 0.44 (0.02)

+ 0.44 (0.001) + 0.40 (0.001)

f0.55 (0.001) +0.57 (0.001)

+0.33 (0.002) +0.38 (0.001)

+ 0.24 (0.02) +0.27 (0.02)

+0.32 (0.03) f0.50 (0.001)

+0.51 (0.001) +0.58 (0.001)

+ 0.26 (0.01) f0.38 (0.001)

f0.27 (0.01) +0.22 (0.05)

+0.28 0.01

+ 0.45 (0.001)

77

66

74

64

70

60

28

22

76

65

The values given are Pearson product moment correlation coefficients (with one-tailed probabilities in parentheses), A dash indicates P > 0.05.

For each cultural characteristic, a positive correlation indicates that blood pressures were higher in societies which received ratings toward the end of the scale indicated by the second alternative listed. More detailed information concerning the rating scales for the cultural characteristics is given in Table I.

goods for sale were substantially lower than the aver-

age blood pressures in societies where the principle economic activity was the production of either agricultural or industrial goods for sale. Average systolic blood pressures at ages 5&60 were about 20 mm Hg lower in the groups which were not involved in a money economy, and average diastolic blood pressures were about 10 mm Hg lower. There was also a substantial difference in the slope of blood pressure with age, with little or no association between blood pressure and age among the hunter-gatherer, pastoral or traditional agricultural groups, and a much steeper gradient of blood pressure with age in the groups which were engaged primarily in the production of agricultural or industrial goods for sale (Figs 1 and 2). These relationships are summarized in correlation coefficients (Table 2) which indicate the substantial correlations between type of economy and blood pressures.

Two subsidiary analyses were performed to test the sensitivity of these findings to variation in methodology. As described in the Methods section, the first of these analyses included only data from the 32 best quality samples with blood pressure data for representative population samples with high participation rates. The second of these analyses used blood

pressure levels which were adjusted to correct for differences in methodology in the original studies (such as use of fourth vs fifth phase diastolic blood pressures). The analysis for the best quality samples and the analysis for adjusted blood pressures both yielded results (Supplementary Table 2) which were very similar to the results of the primary analysis (Table 2) which used data from all groups included in this study and blood pressures which had not been adjusted. Thus, the relationship between involvement in a money economy and higher blood pressures is robust and appears not to be sensitive to variation in methodology.

Other cultural characteristics which were correlated with high blood pressures included economic competition, greater contact with people of different culture or beliefs, more widespread desire for a return to traditional beliefs and values, and predominant family type nuclear or father-absent, as opposed to extended families (Table 2). (Correlations for additional cultural characteristics are given in Supplementary Table 3.) Most of these other cultural characteristics were correlated with the type of economy (Supplementary Table 4). Partial correlation analyses, controlling for type of economy, were performed to test whether these cultural characteristics were independent predic-

424 INGRID WALDRON et (11.

Table 3. Relationship of enslavement of forbearers to blood pressure in Negroes

Blood pressure Blood pressure Slope of blood at ages 2G30 at ages X&60 pressure with age

Systolic Diastolic Systolic Diastolic Systolic Diastolic N

Descendants of slaves vs not descendants of slaves

Men 127/121 80/75 1461129 SO/SO 0.66/0.25 0.35/0.16 22 (0.02) (0.01) (0.002) (0.002) (0.002) (0.002)

Women 77174 166/142 95185 1.4JO.67 0.60/0.35 I6 (0.05) (0.001) (0.001) (0.001) (0.01)

Values given are average blood pressures (or slopes of blood pressure with age) for samples of Negroes who were descendants of slaves/those who were not descendants of slaves. Values in parentheses are one-tailed probabilities from t-tests.

tars of blood pressure, above and beyond any prediction which could be attributed to differences in type of economy (Supplementary Table 5). Significant partial correlations with blood pressure were found for family type and, for women, for desire for a return to traditional beliefs and values.

Race

For two racial groups, the number of samples was large enough to permit analyses of the relationships between type of economy and blood pressure within the racial group. There were substantial correlations between economic modernization and higher blood pressures within both the Negro and the Oceanian racial groups (Supplementary Table 6). (The Oceanian group included Polynesians, Micronesians and Mela- nesians, which can be considered as one racial group

C831.) For the Negro samples, blood pressures were sub-

stantially higher for groups who were descendants of slaves than for other groups (Table 3). Analyses of covariance, with type of economy as the covariate, were used to test whether the differences between descendants of slaves and other Negro groups remained significant after adjusting for type of economy. These analyses of covariance showed substantial and significant differences for blood pressures for older women and for slopes of blood pressure with age for women (Supplementary Table 7). For men, each of the corresponding four analyses just missed usual levels of statistical significance with 0.05 < P < 0.10. In conclusion, for Negro women, and, to a lesser extent, for Negro men, being descendants of slaves was an independent predictor of high blood pressure, above and beyond any effects associated with economic modernization.

Salt

Figure 3 illustrates the relationships among type of economy, salt consumption, and slope of blood pressure with age for men. Salt consumption was low in the groups which were not involved in a money economy and was generally higher in economically more modernized groups. Both economic modernization and higher salt consumption were associated with greater slopes of blood pressure with age. These observations lead to the question, ‘Is there a relationship between economic modernization and blood pressure, after controlling for salt consumption?. Inspection of Fig. 3 reveals that, if attention is restricted to societies

with salt consumption of less than 70mEq sodium per day or societies with salt consumption between 125 and 200mEq sodium per day, then, even within these relatively narrow ranges of salt consumption, the slope of blood pressure with age tends to be greater in societies with greater economic modernization.

Further information is provided by the partial correlation coefficients between blood pressure and type of economy, controlling statistically for salt consumption (Table 4). At older ages and for slopes of blood pressure with age, these coefficients were substantial and significant, and this indicates that economic modernization was associated with higher blood pressures, above and beyond any association which could be attributed to differences in salt consumption. Partial correlation coefficients between blood pressures and salt consumption, controlling for type of

>300 t

+.96 +.K?

Fig. 3. Slope of men’s systolic blood pressure with age by salt consumption and type of economy. Each number on the graph gives the slope of men’s systolic blood pressure with age for a group with the indicated salt consumption and type of economy. Slope is given in units of mm Hg per year of age. 100 mEq (milliequivalents) of sodium is equiv-

alent to 5.85 g of salt.


Table 4. Partial correlation coefficients of blood pressure with type of economy and salt consumption

Blood pressure Blood pressure Slope of blood at ages 2(r30 at ages 50-60 pressure with age


Partial correlation with type of economy, controlling for salt consumption

Men

Women

Partial correlation with salt consumption, controlling for type of economy

Men

Women

- +0.55 + 0.62 + 0.57 +0.52 20 (0.01) (0.004) (0.01) (0.02) + 0.46 +0.54 + 0.57 +0.61 20 (0.03) (0.01) (0.01) (0.003)

+0.53 - +0.58 f0.53 20 (0.02) (0.01) (0.01)

+0.40 - 20 (0.05)

One-tailed probabilities are given in parentheses. A dash indicates P > 0.05.

economy, were also calculated (Table 4). For men, several of these coefficients were significant, indicating that there was an association between salt consumption and slope of blood pressure with age, above and beyond any association which could be attributed to differences in type of economy. Both types of partial correlation coefficients were also calculated for the best quality samples and also for the adjusted blood pressures: these analyses yielded results similar to those described above (compare Supplementary Table 8 and Table 4).

Body weight

Economic modernization was also associated with increased body weight. The partial correlations between blood pressure and type of economy, controlling for body mass index are presented in Table 5. For men, these were substantial and significant, indicating that economic modernization was associated with higher blood pressures above and beyond any association which might be due to greater body

weights. For women, only one of these partial correlation coefficients was significant. Table 5 also shows the partial correlation coefficients between blood pressure and body mass index, controlling for type of economy. These were not significant for men, but were significant for older women and for slope of blood pressure with age for women. An analogous analysis for adjusted blood pressures yielded similar results, except that the partial correlations between blood pressure and type of economy, controlling for body mass index, were significant for young women as well as for men (Supplementary Table 9).

We also calculated the partial correlation coefficients between blood pressure and type of economy, controlling for both body mass index and salt consumption. There were only eight samples with the requisite data, so the results must be considered suggestive only. Nevertheless, it is of interest that, for males, three of the six partial correlation coefficients between blood pressure and type of economy were significant, with values of +0.73 to +0.83 (one-tailed P < 0.05).

Table 5. Partial correlation coefficients of blood pressure with type of economy and body mass index

Blood pressure Blood pressure Slope of blood at ages 20-30 at ages 50-60 pressure with age


Partial correlation with type of economy, controlling for body mass index

Men

Women

Partial correlation with body mass index, controlling for type of economy

Men Women

+ 0.49 +0.55 +0.52 + 0.67 + 0.47 +0.41 23 (0.01) (0.004) (0.01) (0.001) (0.01) (0.03)

- - - + 0.42 18 (0.05)

- - - 23 + 0.49 +0.56 +0.41 +0.63 18 (0.02) (0.01) (0.05) (0.004)

One-tailed probabilities are given in parentheses. A dash indicates P > 0.05.

426 INGRID WALDRON er al.

DISCUSSION

Methodoloyicul issues

In evaluating the findings from this study, or other analyses of the same type, several methodological issues must be addressed. One issue is the extent to which the results may have been influenced by the fact that blood pressures for different groups were measured by different. observers under different conditions and using different methodologies [34,79]. Our analyses suggest that this is probably not a major source of error in our findings; subsidiary analyses which used blood pressures adjusted for several important differences in methodology yielded results (Supplementary Tables 2, 8 and 9) similar to the results of the main analyses which used blood pressures as reported in the original studies (Tables 2. 4 and 5).

Another important methodological issue concerns the comparability of cultural data for different groups and the possibility that cultural ratings can be biased if raters know the blood pressure data and the hypotheses to be tested. In this study, problems of bias were avoided by using cultural ratings which were based on information from independent anthropological sources and were made by raters who had no knowledge of the blood pressure data or of the hypotheses under investigation. Problems of possible non-comparability of cultural ratings could not be en- tirely avoided, because the cultural ratings were based on anthropological data from many different sources. However. comparability of cultural ratings was increased by the use of uniform methods for all groups for evaluating the anthropological data and rating the cultural characteristics.

Another methodological issue is whether the samples used in the original blood pressure studies were representative of the groups studied. As described in Methods, blood pressure studies were excluded from our analyses if samples were recruited by methods likely to result in bias in average blood pressures. In addition, subsidiary analyses were performed which used blood pressure data only from representative population samples with high participation rates. The analyses for these 32 groups yielded results (Supplementary Tables 2 and 8) which were similar to the results of the analyses for the more inclusive sample of 84 groups (Tables 2 and 4). A related issue is whether there was bias in the selection of groups for inclusion in our study. The possibility of bias was minimized by the use of systematic, explicit criteria for the selection of the groups to be included, as described in the Methods section.

There are also important methodological issues related to the interpretation of correlational analyses. It is well known that significant correlations do not necessarily represent direct causal relations between the correlated variables. Problems of interpretation are particularly difficult for cross-cultural data, since different cultural groups differ in many interrelated characteristics. so the possibility of ‘spurious’ correlations is high. As an initial approach to this problem we have used partial correlation analyses to assess the strength of relationships with blood pressure for one variable while controlling for other important potential confounding variables. The results of these analyses must be interpreted with caution because

partial correlation analysis (like multiple regression analysis, analysis of covariance and matching analysis) does not fully correct for the controlled variables and thus may overestimate the independent predictive power of the independent variable of interest [84,85]. Issues concerning the interpretation of the partial correlation coefficients are discussed further in the next section.

Multiple factors related to cross-cultural variation in blood pressure

The partial correlation analyses revealed substantial and significant correlations between type of economy and blood pressures, even after controlling for salt consumption (Table 4), and, for men, also after controlling for body mass index (Table 5). The partial correlation analyses also showed that for men. there were significant correlations between salt consumption and blood pressures, controlling for type of economy, and for women there were significant correlations between body mass index and blood pressures, controlling for type of economy. These findings suggest that no single factor is solely or primarily responsible for cross-cultural differences in blood pressure, but rather cross-cultural differences are determined by the simultaneous effects of several important factors. The results of our analyses must be interpreted with caution because of the methodological limitations of multivariate analyses discussed in the previous para- graph. It should also be pointed out that, due to limitations of available data, we were not able to test certain important hypotheses, such as the hypothesis of Ostfeld and D’Atri [Z] that rate of weight gain after childhood may be the critical factor responsible for cross-cultural variation in blood pressure. Despite these limitations, our analyses provide suggestive evidence for the hypothesis that cross-cultural variation in blood pressure is due to multiple factors.

The hypothesis that no single predictor can account for all or most of the cross-cultural variation in blood pressure receives additional support from the repeated observation that any given characteristic, such as salt consumption or weight, is unable to account for observed differences in blood pressure between particular pairs of groups [l, 31. For example, reported salt consumption in several Java- nese villages [9] was as high or higher than in a mining valley in Wales [73], and yet blood pressures for the Javanese villagers were substantially lower than for the residents of the Welsh mining valley, with differences of over 2OjlS mm Hg by ages 50-60 C9.343. Similarly, the level of reported salt consumption was as great for the Lau in the Solomon Islands [41] as for town dwellers in New Zealand [70]. and yet blood pressures for older adults were substantially lower for the Lau. Concerning body weights, Miall and co- workers found for samples of Jamaican women that the urban women weighed more but the rural women had higher blood pressures [35]. Several other authors have reported that, for the groups they studied, although the group with higher blood pressures did have higher body weights, the differences between groups in blood pressures were larger than would be predicted on. the basis of the differences between groups in weight and the relationships between weight and blood pressure within groups [45,50.51].


Observations .such as these provide further evidence for the hypothesis, supported by our analyses, that cross-cultural variation in blood pressure is related to multiple factors and cannot be accounted for by any single factor such as salt consumption or body weight.

Cultural factors

In the discussion thus far, we have been dealing with correlations or associations, and have not di- rectly addressed the question of causal interpretations of the observed correlations. Data available from physiological, experimental and longitudinal studies of individuals support the interpretation that the significant partial correlation coefficients observed in this study represent causal relationships, at least in part. For example, there is a good deal of evidence that when people gain weight their blood pressures generally increase, and when people lose weight their blood pressures generally decrease [68,69,79,86-891. These findings and other available data [S&89] support the interpretation that the significant partial correlations between blood pressures and body mass index for women probably reflect, at least in part, a causal contribution of differences in body weight to cross-cultural variation in women’s blood pressures.

We turn now to the question, ‘What are the critical cultural and social changes which occur during economic modernization and are associated with increased blood pressures?. Several observations suggest that urbanization is not a critical factor. First the average blood pressures were similar for urban samples, samples from smQ1 towns, and samples of rural groups engaged in mechanized or unmechanized production of agricultural goods primarily for sale (Figs 1 and 2). Also, previous studies have shown that differences in blood pressure between urban and rural samples are not consistent, with urban samples hav- ing higher, similar or lower blood pressures than rural samples, depending on the region studied [2, 11, 35,49,53,57,79].

For salt consumption, there is good evidence that restriction of dietary sodium to extremely low levels of less than 10 mEq per day results in reduced blood pressures in a substantial fraction of patients with essential hypertension [9@93]. However, above 10mEq per day, findings have not been consistent; some analyses have indicated a relationship between changes in sodium intake and blood pressure [94-981, but many analyses have not found evidence of such a relationship [91-93,97-1011. It may be that individual differences in blood pressure responses to changes in sodium intake have contributed to the variability in results [93]. Methodological deficiencies such as small sample sizes may have been a factor in some of the studies which found no relationship. It is also possible that placebo effects played a role in the studies which found that reduced sodium intake resulted in lower blood pressures. Thus, it is unclear whether variation in sodium intake in intermediate and higher ranges makes a causal contribution to variation in blood pressure levels. However, the available evidence does indicate that very low levels of sodium intake can reduce blood pressures, and this effect presumably contributes to the significant partial correlations observed between salt consumption and blood pressure for men.

The data presented in Figs 1 and 2 suggest that one critical factor may be increasing involvement in a money economy. High blood pressures were observed for the groups which were engaged primarily in the production of goods for sale, including groups engaged in mechanized agriculture or large-scale, unmechanized agriculture, as well as groups living in small towns or cities. Low blood pressures were observed in groups which had very little or no involvement in a cash economy, including hunter-gatherers, pastoral, and traditional agricultural groups. Inter- mediate blood pressures were observed for the agricultural groups which sold a portion of their produce, and thus were partially involved in a money economy. Thus, increasing involvement in a money economy may be one of the important features of economic modernization which is associated with increased blood pressures.

Further evidence concerning aspects of economic modernization which may contribute to elevated blood pressures is provided by two of the cultural characteristics which were correlated both with economic modernization and with higher blood pressures (Table 2 and Supplementary Table 4). These were (1) less economic cooperation and more economic competition and (2) increased contact with people who had different culture or beliefs. These findings suggest that economic modernization is associated with the disruption of cooperative relationships and exposure to conflicting values and beliefs, and this type of social and cultural disruption may contribute to the higher blood pressures in economically more modernized groups.

The partial correlations between type of economy Several cultural characteristics were significantly and blood pressure were significant for men, even related to higher blood pressures, even after controll- after controlling for salt consumption and body mass ing for type of economy (Supplementary Table 5). index. This suggests that, at least for men, there is an One of these was a scale of predominant family type additional causal link between economic moderniza- which ranged from extended families, through nuclear tion and high blood pressures. A variety of hypoth- families with nearby relatives, to isolated nuclear eses have been proposed by other authors, and the families and families with fathers absent (Table 1). reader is referred to [l, 4,79, 1021 for reviews of the Extended families generally involve a larger group of evidence concerning these hypotheses. We will focus cooperating adults, with other family types represen- attention on the causal link which appears to us to ting successively decreasing extent of cooperation. It play the most important role, that is, the effects on is hypothesized that the disruption of cooperative re- blood pressure of psychosocial stresses due to cultural lationships may contribute to the correlation between and social changes which accompany economic family type and blood pressures. For women, there modernization. Again, the interpretation of stress as a were significant correlations of blood pressure with potential causal factor influencing blood pressure is more widespread desire for a return to traditional supported by the results of physiological, experimen- beliefs and values, after controlling for type of econ- tal and longitudinal studies [79,81,82, 103-106-j. omy. These findings provide additional suggestive evi-


dence that cultural and social disruption, including the disruption of cooperative relationships, may contribute to higher blood pressures.

For Negroes, groups who were descendants of slaves had substantially higher blood pressures, and this effect remained significant (or, for men, approached significance) after controlling for type of economy (Table 3 and’ Supplementary Table 7). These findings could be interpreted as evidence of the contribution of cultural and social disruption to elevated blood pressures, since enslavement of Negroes involved extreme forms of cultural and social disruption. On the other hand, it is possible that groups who were descendants of slaves had higher body weights and/or higher salt consumption than other Negro groups, and this could account for the observed blood pressure differences. Unfortunately, data were not available to evaluate these alternative interpretations.

In conclusion, the evidence from this analysis and from previous studies indicates that cross-cultural differences in blood pressure are due to multiple factors. Factors which appear to play a role include differences in body weight. the protective effects of very low

levels of salt consumption in some groups. and psychosocial stress due to cultural disruption, including the disruption of cooperative social relations and the disruption of traditional beliefs and cultural patterns. Economic modernization is associated with increases in body weight and salt consumption and with the disruption of cooperative relations and traditional cultural patterns. and each of these changes may contribute to the elevated blood pressures observed in economically modernized societies.

AcL,lo~~/etlgeme,trs-Funding for computer use was provided by the University of Pennsylvania. We are grateful to Myles Alpert. Craig Coleman. Raymond Falls. Rachel Green. Lorri Kanter. Cathy McPherson and the Inter- librarv Loan Staff of the Medical School Library of the University of Pennsylvania for assistance in gathering data used in this studv. We wish to thank Drs V. M. Haw- thorne. S. W. Hodbler. and R. D. Remington for providing unpublished data concerning blood pressure and weight for several samples. We also thank Drs J. Eyer and P. Sterling for helpful comments on an earlier draft of this paper.

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