Dose birthweight matter to quality of life? A comparison between Japan, the U.S., and India

Abstract

Background

Birthweight is a widely accepted indicator of infant health and has significant and lasting associations. Several studies have found that low and high birthweight have significant negative associations with adult health. A new study in the field of social sciences has established that birthweight has significant negative associations with not only adult health but also social attributes, such as income and occupation; however, no studies have evaluated the associations between birthweight and quality-of-life (QOL) attributes such as happiness.

Methods

In this study, we use data from Japan, the U.S., and India, collected in 2011, in which the respondents were asked about their own birthweights to examine the long-term associations between low and high birthweight and eight outcome variables related to the QOL: adolescent academic performance, height, education, marital status, body mass index, income, health, and happiness. We regressed each of the eight outcome variables on low and high birthweight and the interaction terms of the old age and the birthweight dummies for each country. We estimated both the reduced and the recursive-structural forms. While the former estimates the total, that is, the sum of direct and indirect associations between birthweight and each outcome, the latter reports the direct association between birthweight and each outcome.

Results

In Japan, while low birthweight is negatively associated with all outcomes, the associations of high birthweight were limited. In the U.S., low birthweight was not associated with any outcomes, but high birthweight had significantly negative associations with health and happiness. In contrast, in India, high birthweight was significantly and positively associated with income, health, and happiness, while low birthweight was associated with several outcomes negatively, similar to Japan. These associations were stronger in youth than in old age.

Conclusion

Our study demonstrated that the associations of birthweight with QOL are widely diversified across countries: low birthweight, rather than high birthweight, is a problem in Japan and India. However, the opposite is true for the U.S., indicating that policymakers in developed countries must pay closer attention to the problems caused by high birthweight, whereas those in developing countries are better to focus on low birthweight.

Introduction

This study investigates whether birthweight matters to quality of life (QOL). Studies of birthweight are classified into those in the domains of medicine and social science, each of which is classified into those investigating the causes and consequences of birthweight. In the field of social sciences, researchers are interested in whether inequality—which is measured by social attributes, such as income, education, and occupation—is passed from generation to generation, and therefore they focused on low birthweight, which might be a link in the transmission of poverty [1]. For example, with regard to the causes, it is known that parents’ education level, income level, and employment types are critical factors [2], while considering the consequences, low birthweight was found to have a negative association with grades of mathematics, as well as with health and employment in adulthood [3].

The present study falls in the social science domain, and investigates the associations of birthweight with QOL in adulthood. Compared with previous studies, this study has a merit of investigating various outcomes including not only academic performance, height, and education in adolescence, but also marital status, body mass index (BMI), income, health, and happiness in adulthood. Among these, the associations with happiness have not been reported in literature. Furthermore, this study is unique as it estimates not only reduced-form for each outcome separately, as done in most previous studies, but also the recursive and structural forms considering the simultaneity of the outcomes. While the reduced form estimates the total association, i.e., the sum of direct and indirect associations, between birthweight and each outcome, the latter reports the direct association between birthweight and each outcome.¹

The second contribution of this study is that it analyzes data from three distinct countries—Japan, the U.S., and India. Though there have been many studies investigating QOL’s link with birthweight in the U.S., in Japan, there have been no substantial studies except [4, 5]. Furthermore, in India, there have been no studies, to our knowledge, which investigated the associations between birthweight and the long-term socio-economic consequences in adulthood [refer to Additional file 1: Supplementary material A for a detailed review]. As shown in Table 1, the distributions of birthweight in our data are quite different among these countries.² India is one of the countries where the percentage of low-birthweight births is the highest in the world.³ In contrast, the share of the high-birthweight births was 15% in the U.S., which was much higher than that of Japan (1.3%) and India (0%). These facts suggest that QOL’s link with low birthweight may differ across countries, and that not only low birthweight, but also high birthweight might be related to QOL. The latter suggestion is consistent with the knowledge in obstetrics that high birthweight is associated with various dysfunctions, such as an increase in frequency of labor dystocia and consequently an increase in Caesarean section rates [8, 9]. High birthweight is caused by maternal obesity and gestational diabetes, which exposes the fetus to elevated levels of fuels such as glucose and fatty acids throughout gestation [10, 11]. Considering that overweight is a major health-related risk factor in adulthood,⁴ high birthweight has a risk of reducing QOL. In this context, the third contribution of this study is to investigate how high birthweight, in addition to low birthweight, influences QOL in adulthood, which has been, to our knowledge, seldom investigated, except for the associations with health.

Table 1 Distribution of birthweights

The rest of this study is organized as follows: In the methods section, expositions of outline of survey, definitions of variables, hypotheses of this study, and estimation methods are given. In the results section, the estimation results of the reduced form, those of recursive and structural forms, and those by full information maximum likelihood (FIML) method are presented. In the discussion section, we discuss the innovation of our results and link them to those from previous studies. In the conclusion section, we argue for the uniqueness of this study, mention its limitations, and review possible future work.

Methods

Outline of the survey

This study uses the data of Japan, the U.S., and India for 2011 from the Household Panel Survey on Consumer Preferences and Satisfaction (JHPS-CPS) conducted by Osaka University.⁵ Respondents were not asked whether they were singletons or twins in the survey. In Japan, the survey was executed by Central Research Services Inc., a company with extensive experience in academic research and government contracts. Double stratified random sampling from the entire population was used to create a representative sample with respect to sex, age, and living locations. Enumerators visited the homes of the selected respondents to administer the questionnaires. The completed questionnaires were collected after several days. Meanwhile, in the U.S., the survey was conducted by a large survey company, TNS Custom Research, which mailed an English translation of the same questionnaire used in Japan to residents, randomly selected from its pool of registered respondents using the census divisions of the U.S., except for Alaska and Hawaii. In India, Nikkei Research Inc. implemented the survey in six major cities (Delhi, Mumbai, Bangalore, Chennai, Kolkata, and Hyderabad). Each city is separated into four areas; in each area, 15 points were chosen from which five respondents were selected considering their sex, age, and social class (Socio-Economic-Classification; SEC),⁶ and were interviewed because many people with no education were involved. Accordingly, respondents from the poorest to the richest strata are included in the survey. The questions asked in the three countries are nearly the same. Most of the questions in Japanese and English were checked by a bilingual researcher to ensure consistency. In India, the questionnaires in five local languages, Bengali, Hindi, Kannada, Tamil, and Telugu, were used at the interviews in addition to the questionnaire in English.

The number of surveys distributed and collected (response rate), and the observations used for analysis were as follows: Japan, 5316, 4934 (92.8%), 4850; U.S., 7046, 5313 (75.4%), 5075; India, 1280, 1037 (81.0%), 1037.⁷

Variable definitions

It is possible and sometimes effective to use the continuous variable of birthweight instead of the low- and high-birthweight dummies. However, because this study focuses on the association between low- and high-birthweight and QOL, and six options were provided for birthweight, we used birthweight dummies rather than a continuous birth weight variable.

Birthweight dummies

Respondents were asked how much they weighed when they were born, allowing them to choose from one of the following: “less than 2.5 kg,” “2.5–2.999 kg,” “3.0–3.499 kg,” “3.5–3.999 kg,” “4.0–4.499 kg,” “4.5 kg or more,” and “don’t know”.⁹ The distribution of these responses is shown in Table 1. Compared to 6.2% of the respondents in Japan and the U.S., 12.3% answered “less than 2.5 kg” in India.¹⁰

We defined low-birthweight dummy (LBW), which takes the value of 1 for those who chose “less than 2.5 kg” and 0, otherwise. High-birthweight dummy (HBW) in Japan was defined as 1 if birthweight is equal to or more than 4 kg, and 0 otherwise because only three respondents answered “4.5 kg or more” in Japan. In the U.S., 632 respondents between 4 kg and 4.5 kg were defined as HBW, and 145 respondents who weighed 4.5 kg or more are defined as very high birthweight (V_HBW). In India, since there are no respondents who weighed 4 kg or more, we defined the 17 respondents who weighed 3.5 kg or more as having a “quasi-high birthweight” (Q_HBW).¹¹ Furthermore, we defined the “do not know dummy” (DONTKNOW), which takes 1 for those who chose “don’t know” and 0 otherwise, for all the estimations in the three countries.

Outcome variables¹²

Academic performance, height, education, and health: Academic performance (ACADEMIC) is based on a self-assessment of “grade ranks of all subjects” at the age of 15 years on a five-point scale. HEIGHT (in meters) is the height at the time of the survey. EDUCATION was defined as the highest level of education completed.¹³

Marital status

MARRIAGE is a binary dummy variable, which takes 1 for “currently married” and 0 otherwise.

BMI

BMI was defined as weight (kg) / [height (m)]² at the time of the survey.

Personal income

For Japan and the U.S., the respondents were asked to select their personal income (before taxes and including bonuses) in 2010 from a list of 10 options ranging from 1 = “None” to 10= “JPY 14 million (USD 140 thousand) or more.” We estimated the value (unit = JPY 1 million; USD 10 thousand) of each classification by applying lognormal distribution to the frequency distribution of these responses. For India, respondents were asked directly for their monthly personal income (before taxes and including bonuses; unit = INR 10 thousand) in 2010, from which annual personal income was calculated by multiplying with 12. We defined this as “personal income” (INCOME). Though personal income of housewives/househusbands, students, and the retired is often extremely low, it is not appropriate to evaluate that their QOL is low. Therefore, this study excluded them from the sample for the estimation of personal income.¹⁴

HEALTH was defined as the responses to the question “How would you describe your current health status?” based on a five-point Likert scale.

Happiness

Respondents were asked the question “Overall, how happy would you say you are currently?” and were requested to choose on a scale of 0 to 10, with 0 being “very unhappy” and 10 being “very happy”. We defined the variable HAPPINESS with the responses.

Control variables

The control variables include parents’ education (F_EDUCATION: father’s education, M_EDUCATION: mother’s education), parents’ age at birth (F_AGE_BIRTH: father’s age at birth, M_AGE_BIRTH: mother’s age at birth), standard of living at 15 years old (S_LIVING), no siblings at age 15 (ONLYCHILD), and mother’s employment status at age 15 (M_FULLTIME: full-time work dummy, M_PARTTIME: part-time work dummy, and M_HOUSEWIFE: housewife dummy).¹⁵ Other control variables include male dummy (MALE), age (AGE), age squared (AGESQ), depth of religious faith (RELIGION), and prefectures and state at the age of 15 years in Japan and the U.S., respectively, and current residence in the six cities in India (region dummies).¹⁶ The definitions of these variables are summarized in Table 2. Also, Table 3 presents the descriptive statistics of the variables used in the estimation. In Fig. 1, we present the mean and 95% confidence interval (95%CI) of each outcome variables by birthweight categories by country. Looking at the means, Fig. 1A (Japan) reveals that LBW is lower for all the categories except MARRIAGE and BMI, while HBW is higher for HEIGHT, EDUCATION, BMI, HEALTH, and HAPPINESS. Figure 1B (U.S.) reveals that LBW is lower for ACADEMIC, HEIGHT, EDUCATION, INCOME, HEALTH, and HAPPINESS, while HBW is higher for HEIGHT, BMI, INCOME, and lower for MARRIAGE and HAPPINESS. Figure 1C (India) reveals that LBW is lower for HEIGHT, EDUCATION, INCOME, HEALTH and HAPPINESS, while HBW is higher for ACADEMIC, HEIGHT, EDUCATION, BMI, INCOME, HEALT, and HAPPINESS. In sum, the tendency of lower outcomes for LBW is recognized in three countries, while HBW is associated with lower outcome in the U.S. but higher outcomes in India and Japan. Since 95% CI is large from several cases, however, regression analysis is required to get more reliable results.

Table 2 Definition of the variables

Table 3 Descriptive statistics of the variables

Fig. 1

A Japan: Each mean outcome variable by birthweight categories with 95% CIs. Notes: This figure implies associations between birthweight and each outcome variable in the case of Japan. LBW is lower for all the categories except MARRIAGE and BMI, while HBW is higher for HEIGHT, EDUCATION, BMI, HEALTH, and HAPPINESS in Japan. B USA: Each mean outcome variable by birthweight categories with 95% CIs. Notes: This figure implies associations between birthweight and each outcome variable in the case of the U.S. LBW is lower for ACADEMIC, HEIGHT, EDUCATION, INCOME, HEALTH and HAPPINESS, while HBW is higher for HEIGHT, BMI, INCOME and lower for MARRIAGE, and HAPPINESS in the U.S. C India: Each mean outcome variable by birthweight categories with 95% CIs. Notes: This figure implies associations between birthweight and each outcome variable in the case of India. LBW is lower for HEIGHT, EDUCATION, INCOME, HEALTH and HAPPINESS, while HBW is higher for ACADEMIC, HEIGHT, EDUCATION, BMI, INCOME, HEALTH, and HAPPINESS

Hypotheses

As mentioned in the Introduction, studies in developed countries have found that low birthweight has a negative association with health and education [also refer to Additional file 1: Supplementary material A]. Therefore, we speculate that LBW has an unfavorable association with QOL and examine:

• Hypothesis 1: LBW has a negative association with the outcomes concerning QOL, specifically, academic performance, height, highest educational attainment, personal income, health, and happiness.

While several studies that have reported associations between LBW and health in young adulthood, relatively few have reported associations of LBW with outcomes in adulthood and beyond. Therefore, we posit the following hypothesis.

• Hypothesis 2: The association between LBW and outcomes concerning QOL is stronger in youth, and weaker in old age.

Although there have been only a few studies, some indicated that HBW causes overweight later in life, resulting in various health problems [12, 14, 15]. Moreover, as pointed out in the Introduction, a high BMI prevails in the U.S. (a developed country), whereas underweight prevails in India, which leads us to the following:

• Hypothesis 3: HBW has an association with outcomes concerning QOL. Specifically, it has a negative association in the U.S. and a positive association in India.

Method of estimation

To test Hypotheses 1 through 3, we regressed each of the eight outcome variables on LBW and HBW as well as the interaction terms of the old age dummy and the birthweight dummies for each country.¹⁷ We estimated both reduced form and recursive-structural form.

Reduced form

We estimated the reduced form based on the literature in this field, regressing each outcome variable over exogenous variables including birthweight dummies. All exogenous variables were used in all the equations without venturing into the issue of identification. The estimates of the reduced form represent the total association of each regressor with each outcome variable as the outcomes of QOL. The formula of the reduced form is given by Eq. (1).

$${OUTCOME}_{ik}={\alpha}_k+{\beta}_{k,m}\sum \limits_m{BW}_{im}+{\upgamma}_{k,m}\sum \limits_m{BW}_{im}{OLD}_i+{\delta}_k{OLD}_i+{\varepsilon}_k{X}_i+{e}_{ik},\kern0.36em k=1,\dots, 8$$

(1)

Here, OUTCOME_ik is the kth outcome of the i th individual, BW_im is the mth birthweight dummy variable (LBW and HBW, etc.), X_i is the set of control variables, and e_ik is the disturbance term. If the mth birthweight dummy lowers the kth outcome, then β_{k, m} will be negative. We also expect that the association of birthweight dummy is stronger for younger people and will gradually disappear as they get older (Hypothesis 2). To measure this change, we created an old age dummy (OLD_i), which takes 1 if the respondent’s age is higher than or equal to 50 years old, and 0 otherwise. We made its interaction terms with birthweight dummies. Therefore, the coefficient of birthweight dummies, β_{k, m}, represents its association for young respondents, while the coefficient of the interaction terms, γ_{k, m}, represents the difference in the associations between the old and the young. For BMI, INCOME, HEALTH, and HAPPINESS, γ_{k, m} is expected to take the opposite sign of β_{k, m}, because the influence on these outcomes will be smaller for the older respondent. On the other hand, because ACADEMIC, HEIGHT, EDUCATION, and MARRIAGE are outcomes corresponding to a younger age for most respondents, we expect that they are independent of their age when the survey was conducted, so that γ_{k, m} is expected to be zero.

As control variables, we used sex and age as basic attributes, parents’ education and parents’ age at birth (as confounding factors), standard of living at the age of 15 years, presence or absence of siblings at the age of 15 years, mother’s employment status at the age of 15 years, region of residence at the age of 15 years, and the depth of religious faith, which we regarded as exogenous to the outcome variables.

We estimate Eq. (1) with ordinary least squares (OLS) for HEIGHT and BMI, with logit for MARRIAGE and with ordered logit for ACADEMIC, EDUCATION, INCOME, HEALTH, and HAPPINESS.¹⁸

Recursive and structural form

Since the eight outcome variables are endogenous, the problem of identification arises. Therefore, in this study, in addition to the reduced form, we estimate the recursive and structural form.¹⁹ Figure 2 depicts the timeline, or possible paths, from birth until the survey, through which birthweight has a connection with each outcome concerning QOL. As Fig. 2 shows, birthweight is connected with the adult outcomes directly and indirectly.

Fig. 2

Path diagram: Associations of birthweight on long-term outcomes. Note: This figure shows the timeline path from birth until the survey, meaning that birthweight is associated with each outcome related to quality-of-life (QOL)

We assume the recursive system of the outcomes to be occurring in the order of ACADEMIC, HEIGHT, EDUCATION, and MARRIAGE.²⁰ Therefore, the specification for ACADEMIC is the same as in Eq. (1). As for HEIGHT, the specification becomes the one which added ACADEMIC to Eq. (1). For EDUCATION, HEIGHT is added to this equation, and for MARRIAGE, EDUCATION is further added to this equation. Each equation is estimated separately by OLS.

Note that the time-ordering simply implies that predetermined variables are not affected by later events. However, the ordering is not precise, so there should exist some who do not follow it: For example, some people graduate from university after they marry. Therefore, this timeline includes advantages and drawbacks. We should consider the tradeoff between assuming slightly unsound time-ordering and solving the difficult endogeneity problem among eight instead of four outcome variables. This paper chose the former approach in consideration of the fact that the burden of the endogeneity problem is enormous.

Among the assumed orderings, many people may question if higher academic performance will lead to higher height. However, we do not argue this causation, or any causation, in Fig. 2. We just assume that height, which is determined around 18 years of age for many people, does not affect academic performance in 15-year-olds. Many people may also question if height affects performance in later life. Though we again do not assume this causation, there have been studies that report that height is positively associated with wage level and so there is a height premium in the labor market (e.g., [16,17,18]).

However, simultaneity cannot be denied for the variables, BMI, INCOME, HEALTH, and HAPPINESS because the respondents were asked about their current state at the time of the survey. To overcome the simultaneity bias, we estimated the structural form of these four endogenous variables with two-stage least squares (2SLS) using instrument variables. For the robustness check, we also conducted full-information maximum likelihood (FIML) estimation of the whole system of eight outcomes, using the same instrument variables and the same structure as the recursive and structural forms.

We should be aware that the coefficients of the birthweight dummies of the recursive-structural form represent the “direct associations.” Therefore, the estimates should differ from the estimates of the reduced form, which represent the total association including the “indirect association.”

Results

Estimation results of the reduced form

In Tables 4, 5 and 6, we report the odds ratio for categorical variables and the OLS estimates for all the outcome variables. We report the OLS estimates even for categorical variables because the marginal effects in ordered regressions are difficult to interpret, and the estimates are better than odds ratios for comparison with those in other Tables.²¹

Table 4 The estimates of the reduced form: Japan

Table 5 The estimates of the reduced form: U.S.

Table 6 The estimates of the reduced form: India

Japan

We present the estimates of the reduced form (Eq. (1)) for Japan in Table 4. LBW was associated with all the outcomes significantly at the 1% or 5% level. It was negative for ACADEMIC, HEIGHT, INCOME, HEALTH, and HAPPINESS, while positive for MARRIAGE and BMI. Though previous studies in Japan reported that low birthweight is negatively associated with adolescent academic performance and health in adulthood, our results indicate that low birthweight was associated with more outcomes. The results that LBW was associated with BMI positively is consistent with the results of [21]. Finally, the result that LBW was negatively associated with happiness is a novel finding of this study. These results support Hypothesis 1.

The interaction terms between LBW and old age dummy showed opposite signs to those on LBW dummies for all the outcomes except HEIGHT, indicating that the associations of LBW were weaker for older respondents. However, for ACADEMIC, HEIGHT, EDUCATION, and HEALTH, the coefficients of the interaction terms were much smaller than those of LBW, indicating that the associations for older and younger respondents were not largely different. On the other hand, for MARRIAGE, BMI, INCOME, and HAPPINESS, the absolute values of the coefficients of the interaction terms were comparable with those of LBW, suggesting that the association for older adults is small. Indeed, except for academic performance and height, the associations for the elderly were insignificant. These results were consistent with Hypothesis 2. However, health was the exception, on which low birthweight continued to have an association with older adults significantly, which is consistent with previous studies in Japan.

The coefficients of HBW were not significant except for HEIGHT and BMI, while the interaction terms were significantly negative only for BMI. These results imply that the associations with HEIGHT continued to have an association with older adults, whereas the association with BMI was recognized only for younger respondents.

The results are summarized as follows: (1) In Japan, LBW was associated with all the outcomes examined in this study for respondents under 50 years old, while the associations were mitigated for those over 50 years old; (2) In Japan, HBW had a positive association only with HEIGHT and BMI.²² These conclusions are confirmed with the odds ratio (OR) estimated with an ordered logit model, though there were three differences in the significance level.

In addition to the association based on significance of the coefficients, we also evaluated economic importance using standardized regression (Estimates are presented in Additional file 3: Supplemental material C; Table C-1). Age and gender exhibited the largest associations with many outcomes: the association with LBW was merely one tenth to one fourth. Though low birthweight has a smaller association with education than that of father’s education and living standards in childhood, it has a comparable association with personal income and health.

The U.S.

In Table 5, we present the estimates of Eq. (1) for the U.S. LBW was associated with no outcomes. The magnitudes of the coefficients of LBW for various outcomes were less than half of the corresponding estimates in Japan. In addition, the interaction term of LBW and old age dummy was not significant. In contrast, HBW had a significantly positive association with HEIGHT and BMI, and a significantly negative association with MARRIAGE. V_HBW had a significant and positive association with HEIGHT and BMI, while it had a significantly negative association with HEALTH and HAPPINESS. Further, as for the outcomes significantly associated with HBW and/or V_HBW, the interaction terms with old age dummy took the opposite signs, indicating that these associations are mitigated for older adults.

The above results are summarized as follows: (3) in the U.S., though LBW was not associated with any outcomes significantly, HBW had a negative association with HEALTH and HAPPINESS, and (4) the significant associations of HBW for those under 50 years old were reduced for older adults. Though these results were not consistent with Hypothesis 1, they support Hypotheses 2 and 3.²³

These conclusions are confirmed with odds ratio estimated with an ordered logit model though there were two differences in the significance level.

According to the standardized regression, the magnitude of the coefficient of V_HBW in the equation for HEALTH and HAPPINESS was around − 0.06, which is tantamount to one sixth of that on age. It was comparable with that of S_LIVING for HEALTH and HAPPINESS, and F_EDUCATION and M_AGE_BIRTH for HEALTH, revealing that the association was fairly large. (Estimates are presented in Additional file 3: Supplemental material C; Table C-2).

India

The estimates for India are presented in Table 6, which are summarized as follows: (5) LBW is negatively associated with HEIGHT, EDUCATION, INCOME, and HEALTH, and positively associated with MARRIAGE. (6) As for these outcomes, the coefficients of interaction dummies with the old age dummy took the opposite signs to those on LBW except for HEALTH, indicating that the associations with these outcomes were mitigated for older adults. However, as for HEALTH, the negative association of LBW was augmented for those over 50 years old. (7) Q_HBW was significantly positive for BMI, HEALTH, and HAPPINESS.²⁴

These conclusions are confirmed with the odds ratio estimated with an ordered logit model except for the negative association between LBW and INCOME, though there were three differences in the significance level.

The standardized regression analysis revealed that the magnitude of the coefficient of LBW for EDUCATION, INCOME, and HEALTH were about half to one-third of that of F_EDUCATION for these outcomes. The coefficient of Q_HBW for HEALTH was about two-thirds of that on F_EDUCATION and half of that on M_HOUSEWIFE. (Estimates are presented in Additional file 3: Supplemental material C; Table C-3).

Comparing the results of the reduced form estimations across the three countries, while LBW was associated with various outcomes negatively in Japan and India, it was not associated with any outcome in the U.S. However, in the U.S., HBW was negatively and significantly associated with MARRIAGE, and V_HBW was negatively and significantly associated with HEALTH and HAPPINESS. In contrast, in India, Q_HBW was positively and significantly associated with HEALTH and HAPPINESS. These different associations of HBW in the U.S. and India support Hypothesis 3.

Estimation results of recursive and structural form

We estimated the recursive system consisting of consisting of ACADEMIC, HEIGHT, EDUCATION, and MARRIAGE using OLS, while we estimated the structural form using BMI, INCOME, HEALTH, and HAPPINESS, considering that these four variables are endogenous. Specifically, we adopted the averages of the endogenous outcomes across the samples of the same gender, age group, and residence as the instruments.²⁵ Though we included all the control variables in all the estimations, they are not shown in the tables to save space.

Japan

The estimation results for Japan are presented in Table 7.²⁶LBW had a significantly negative association with HEIGHT and EDUCATION, and a significantly positive association with MARRIAGE and BMI. Though all coefficients had the same signs as those of the reduced form, their absolute values were smaller, except for HEIGHT and BMI. These results suggest that the indirect associations have the same signs as direct associations, except HEIGHT and BMI. Meanwhile, INCOME, HEALTH, and HAPPINESS, whose coefficients were significantly negative in the reduced form estimations, became insignificant in the structural estimations, indicating that LBW was associated with these outcomes through indirect paths, but not directly.

Table 7 The estimates of the recursive and structural forms: Japan

HBW was significantly positive for HEIGHT and BMI, but insignificant for other outcomes. In addition, the magnitudes of the significant coefficients were similar to the estimates of the reduced form, suggesting that the indirect associations of HBW were marginal. In sum, these results are consistent with those of the reduced form. The results concerning Hypothesis 2 (i.e., interaction terms of old age and birthweight dummies) by the recursive-structural form are consistent with those of the reduced form.²⁷

The U.S.

We present the estimation results for the U.S. in Table 8. Estimates of LBW were not significant for any outcomes, as were in the reduced form. Coefficients of HBW were significantly positive for HEIGHT and significantly negative for MARRIAGE, as in the reduced form, whereas it was insignificant for BMI. In addition, the absolute values of the estimates became smaller than those of the reduced form representing the total associations, suggesting that indirect associations have the same signs as the direct associations. V_HBW had a significantly positive association with HEIGHT and BMI, and a significantly negative association with HEALTH.²⁸

Table 8 The estimates of the recursive- and structural-forms: U.S.

India

The results for India are presented in Table 9. The magnitude of the absolute values of the coefficients of LBW were similar to those in the reduced form, albeit slightly smaller. Consequently, the association of LBW with HEALTH became insignificant. The associations of Q_HBW became insignificant for BMI and HAPPINESS, and significant only for HEALTH.^29,30

Table 9 The estimates of the recursive- and structural-forms: India

Results by full information maximum likelihood (FIML) method

In addition to the recursive-structural form, the whole system of the eight outcomes associated with QOL was estimated with FIML. The estimates did not differ largely from those by recursive-structural estimations, both of which represent the direct association. They confirmed the conclusions deduced in the reduced form estimation. (Estimates of FIML are presented in Additional file 4: Supplemental material D; Tables D-1 to D-3.)

The SEM command of STATA reports the estimates of direct, indirect, and total associations based on the FIML estimation. (The estimates of direct, indirect, and total associations based on FIML are presented in Additional file 5: Supplemental material E; Tables E-1 to E-3.) Therefore, we can compare the estimates of the total associations by FIML with the coefficients estimates in the reduced form. These results were similar, indicating consistency of the estimates by the reduced form and structural form (Additional file 5: Supplemental material E; Tables E-4 to E-6).

Discussions

The estimation results across the three countries were quite different: while LBW was negatively associated with QOL in Japan and India, it did not matter in the U.S. In contrast, HBW was a negatively associated with QOL in the U.S. but it was a positively associated with QOL in India.

We now explore whether these results are consistent with previous studies [refer to Additional file 1: Supplemental material A: Literature survey]. As confirmed in our study, in Japan, [4] reported a significantly negative association of low birthweight on academic performance, education, and personal income while using OLS [5]. found that low birthweight was adversely associated with academic performance, which is confirmed in our results. In addition, our finding that the adverse association among younger respondents was mitigated in older respondents is consistent with their finding. However, their conclusion that low birthweight was not associated with education and primary job status contradicts our results.³¹ In sum, using a representative sample of Japan, our study found significant associations with more life outcomes. Furthermore, the analysis of high birthweight has not been done before in Japan.

Since various contradicting results have been reported on the associations of birthweight in the U.S., it is not easy to compare them with ours. For example, though [22] found a positive relationship between birthweight and the variables educational attainment, height, and wage rates, [23] reported only a minor association with education, but no association with health [24]. reported that birthweight showed a significant positive impact on math and reading ability in childhood. Meanwhile, we found that LBW was not significant for any outcome, whereas HBW had a significant association with several outcomes, including having a negative association with HEALTH and HAPPINESS.

In addition, the comparison became more difficult because most of the previous studies in the U.S. investigated the linear relationship between birthweight and outcomes concerning QOL, while we examined the associations of birthweight dummies. The linear specification of birthweight precludes the possibility that both low and high birthweights are simultaneously worse than the standard birthweight, which possibly produces contradicting results from ours.

To ease the comparison, we estimated Eq. (1) substituting birthweight (BWEIGHT) for birthweight dummies such as LBW, HBW, and V_HBW. (Estimates are presented in Additional file 6: Supplemental material F; Table F-1). The estimates revealed that BWEIGHT had a significantly positive relationship with HEIGHT and BMI, whereas it was significantly and negatively correlated with HEALTH and HAPPINESS. These results are consistent with previous studies of the U.S. At the same time, these results correspond to the results of the estimates on V_HBW for these four outcomes, indicating that the insignificant estimates of LBW in the U.S. (Table 5) does not necessarily contradict the results of previous studies, which showed that birthweight is significantly associated with some outcomes concerning QOL.

In addition, [22] reported that the upper quartile (highest 25%) of birthweight negatively affected wage rate, whereas the bottom quartile (lowest 25%) of birthweight had a positive association, which is consistent with our result of negative associations of V_HBW with various outcomes.³²

There have been no papers in India, which to our knowledge, investigated the associations between birthweight and QOL in the field of social science. This study found that LBW has a significant and negative association with ACADEMIC, HEIGHT, EDUCATION, and HEALTH, while it has significantly positive association with MARRIAGE. Meanwhile, Q_HBW has a significant and positive association with BMI, INCOME, and HEALTH: that is, higher birthweight has an opposite association compared to that in the U.S.

In sum, the associations of birthweight are widely diversified across the countries: in Japan, while low birthweight was negatively associated with many adult outcomes, the associations of high birthweight were limited. In India, while low birthweight was associated with many outcomes negatively as in Japan, quasi-high birthweight was associated with income, health, and happiness positively. In the U.S., while low birthweight did not have any significant associations, very high birthweight was negatively associated with health and happiness. These are visualized using the estimates by standardized regression (Fig. 3A and B).

Fig. 3

A Mean and 95% CI of the coefficient on LBW estimated by standardized regression. Notes: The graph is based on the estimates reported in tables C-1 to C-3 in Additional file 3: Supplemental material. In this figure, we present the estimates by standardized regression because it allows the comparison of the magnitudes of estimated coefficients among different regressions. B Mean and 95% CI of the coefficients on HBW, V_HBW, Q_HBW for Japan, the U.S., and India, respectively, estimated by standardized regression. Notes: The graph is based on the estimates reported in tables C-1 to C-3 in Additional file 3: Supplemental material. In this figure, we present the estimates by standardized regression because it allows the comparison of the magnitudes of estimated coefficients among different regressions

Probably, the diverse results across the three countries come from, in part, the differences in BMI in adulthood. It is known that obesity due to affluent society is a problem in the U.S., while malnutrition due to poverty is a serious problem in India.³³ In our data for the U.S., BMI is negatively correlated with HEIGHT, EDUCATION, MARRIAGE, INCOME, HEALTH, and HAPPINESS at the 1% level, whereas in India, it is positively correlated with MARRIAGE, HEALTH, and HAPPINESS at the 1% level (results not shown). Meanwhile, we found that HBW had a significantly positive association with BMI in the three countries (Tables 4, 5 and 6). These results suggest the possibility that high birthweight led to high BMI in adulthood, which in turn connected negatively to the outcomes in the U.S. and positively to those in India. Furthermore, the result that HBW was associated with the outcomes in adulthood, such as MARRIAGE, BMI, HEALTH, INCOME, and HAPPINESS, but was not associated with ACADEMIC and EDUCATION in adolescence, is opposite to the results of LBW, suggesting that the mechanism of the associations of low and high birthweight might be different.

Conclusions

This study analyzed the long-term association between birthweight and QOL using the large-scale survey data conducted in Japan, the U.S., and India in 2011. This study is unique in the following ways: First, it investigated the eight outcomes concerning QOL, adolescent academic performance, height, education, marriage, BMI, income, health, and happiness; second, it analyzed the data for three countries, Japan, the U.S., and India, which are widely diversified with respect to birthweight. Third, it investigated whether the associations tend to be mitigated when older; Fourth, it investigated not only the association of low birthweight, but also the association of high birthweight; Fifth, it investigated using not only the estimation of reduced form, but also the recursive-structural forms, accounting for the transmission of the associations between outcomes.

The estimates of the reduced form, which represent the total association of birthweight dummies with outcomes, revealed that low birthweight had significant and negative associations with most of the outcomes in Japan, while high birthweight did not have an association with any outcomes. In contrast, in the U.S., whereas low birthweight did not have an association with any outcomes significantly, very high birthweight was significantly and negatively associated with some outcomes including happiness. In India, low birthweight was associated with most of the outcomes significantly and negatively, whereas quasi-high birthweight was associated with some outcomes including happiness significantly and positively. In addition, these associations were stronger for younger respondents than older ones. Further, the results of the recursive-structural form were consistent with those of the reduced form.

These findings have important implications. First, the finding that the association between low and high birthweight and later QOL are different between countries suggests that the association may partly depend on how society helps citizens facing difficulties from their birthweight. In particular, this study found that, whereas high birthweight has a negative association with HEALTH and HAPPINESS for younger people in the U.S., no such association was found for older people (Table 5), and that in India, whereas there was a positive association between high birthweight and INCOME for older people, no such association was found for younger people. These complex findings might have a root in diversified social conditions among countries.

In this study, we not only investigated the associations assuming them to be independent of each other, but also assuming that they constitute a system in which they heavily depend on each other. Such an approach can elucidate how the association between low- and high-birthweight and one outcome transfers to another outcome. Because the transmission can be prevented by appropriate treatment, such a finding might elucidate weaknesses in society to mitigate the association between low- and high-birthweight and later QOL. We hope that such a study will be made in the future.

This study has limitations. First, the birthweight data were self-reported. Therefore, the data may suffer from memory bias. How might this affect the findings in this study? Since older people probably recalled their birthweights less accurately, the use of recalled birthweights could partly explain the weaker associations between birthweight and outcome variables in the older age group. To be immune from this problem, however, we should collect longitudinal cohort data, which has not been done in Japan.³⁴

Second, the lack of information regarding gestational period such as gestational age at birth is another serious limitation of this study, though this study controlled for confounders such as parents’ education and their ages at birth, living standards in childhood, mother’s working status in childhood, and the presence of brothers and sisters. To confirm the causality from birthweight to life outcomes, the information on gestational period is important. Analysis using twin fixed-effects model is an alternative method. Nonetheless, as twin fixed-effects model estimates the effect of the difference in birthweight that occurred in gestation period, the mechanism of how the difference emerges may not be identical between twins and singletons [refer to Additional file 1: Supplementary material A]. Therefore, whether results using twins have external validity to singletons is questionable, which calls for research using singletons, as in this study.

Third, to focus on the associations between low and high birthweight and QOL, we set the entire standard birthweight as the baseline. However, people’s fate may depend on their birthweight even when they are born within the standard birthweight, which constitutes an interesting future study.