Twin Birth and Maternal Condition

TWINS have intrigued humankind for more than a century (Thorndike, 1905). In behavioral genetics, demography, and psychology, monozygotic twins are studied to assess the importance of nurture relative to nature (Polderman et al., 2015). In the social sciences, twin births are also used to denote an unexpected increase in family size, which assists causal identification of the impact of fertility on investments in children and on women's labor supply (Rosenzweig & Wolpin, 2000, 1980a; Bronars & Grogger, 1994; Black, Devereux, & Salvanes, 2005). A premise of studies that use twin differences or the twin instrument is that twin births are quasi-random and have no direct impact (except through fertility) on the outcome under study. We present new population-level evidence that challenges this premise. Using 16,962,165 births in 72 countries, of which 462,246 (2.73%) are twins, we show that the likelihood of a twin birth varies systematically with maternal condition. In particular, our estimates establish that mothers of twins are selectively healthy.¹

We document that the association of twin births and maternal condition is meaningfully large and widespread. We show that is evident in richer and poorer countries and that it holds for sixteen different markers of maternal condition, including health stocks and health conditions prior to pregnancy (height, obesity, diabetes, hypertension, asthma, kidney disease, smoking), exposure to unexpected stress in pregnancy, and measures of the availability of medical professionals and prenatal care.² The effects are sizable, with a 1 standard deviation improvement in the indicator tending to increase the likelihood of twinning by 6% to 12%.

Previous research has documented that twins have different endowments from singletons; for example, twins are more likely to have low birthweight and congenital anomalies (Hall, 2003; Rosenzweig & Zhang, 2009). We focus not on differences between twins and singletons but rather on differences between mothers of twins and singletons, which indicate whether occurrence of twin births is quasi-random. It is known that twin births are not strictly random, occurring more frequently among older mothers, at higher parity, and in certain races and ethnicities (Hall, 2003; Bulmer, 1970), but as these variables are typically observable, they can be adjusted for (as in Rosenzweig & Wolpin, 1980a).³ Similarly, it is well documented that women using artificial reproductive technologies (ART) are more likely to give birth to twins (Vitthala et al., 2009), but ART use is recorded in many birth registries, so it can be controlled for and a conditional randomness assumption upheld (Cáceres-Delpiano, 2006; Angrist, Lavy, & Schlosser, 2010). The reason that our finding is potentially a major challenge is that maternal condition is multidimensional and almost impossible to fully measure and adjust for. To take a few examples, fetal health is potentially a function of whether pregnant women skip breakfast (Mazumder & Seeskin, 2015), they suffer bereavement in pregnancy (Black et al., 2016), or they are exposed to air pollution (Chay & Greenstone, 2003).

Our underlying hypothesis is that twins are more demanding of maternal resources than singletons, and as a result, conditions that challenge maternal health are more likely to result in miscarriage of twins than of singletons. We discuss the role of alternative mechanisms including nonrandom conception and maternal survival selection. We provide evidence in favor of the selective miscarriage mechanism using U.S. Vital Statistics data for 14 to 16 million births. Selective miscarriage is similarly the mechanism behind the stylized fact that weaker maternal condition is associated with a lower probability of male birth (Trivers & Willard, 1973; Almond & Edlund, 2007). We confirm this in our data, showing that twin births are more likely to be girls. Our findings add a novel twist to recent literature documenting that a mother's health and her environmental exposure to nutritional or other stresses during pregnancy influence birth outcomes, with many studies documenting lower birthweight (Currie & Moretti, 2007; Bernstein et al., 2005; Quintana-Domeque & Ródenas-Serrano, 2017). If birthweight is the intensive margin, we may think of miscarriage as an extensive margin response or the limiting case of low birthweight.

Our findings have implications for research that has exploited the assumed randomness of twin births. Studies using twins to isolate exogenous variation in fertility will tend to underestimate the impact of fertility on parental investments in children and on women's labor supply if selectively healthy mothers invest more in children after-birth, and are more likely to participate in the labor market (as discussed in Bloom, Kuhn, & Prettner, 2015). In table 1 we summarize studies using twin births to instrument fertility, documenting the mother-level controls in each study. In some cases, the validity of the conditional randomness assumption is directly probed—for instance, with respect to mother's education (Black et al., 2005; Li, Zhang, & Zhu, 2008; Rosenzweig & Zhang, 2009). However, as is acknowledged in each case, any such tests are at best partial evidence in support of instrumental validity. Importantly, no previous study has attempted to control for maternal health conditions or behaviors. This is pertinent, as it could resolve the ambiguity of the available evidence on the impacts of fertility. In particular, recent studies using the twin instrument challenge a long-standing theoretical prior of Becker and Lewis (1973) in rejecting the presence of a quantity–quality (QQ) fertility trade-off in developed countries (Black et al., 2005; Angrist et al., 2010), but our estimates suggest that this rejection could in principle arise from ignoring the positive selection of women into twin birth. Similarly, research using the twin instrument tends to find that additional children have relatively little influence on female labor force participation (FLFP; see Lundborg, Plug, & Rasmussen, 2017). But, again, these estimates are likely to be downward biased. The results of studies in economics, psychology, education, and biology that instead exploit the genetic similarity of twins will not be biased but will tend to have more restricted external validity than previously assumed.⁴

In this section, we discuss two distinct approaches to testing our hypothesis that twins are selectively born to healthier mothers. We identify variation in the mother's health before she gives birth to twins and before she knows she will give birth to twins. In the first approach, we use information on her health condition (morbidities, height, weight), health-related behaviors, access to health care, and environmental health stressors. In our second approach, we use as a marker of maternal health the fetal or infant survival rate of her births prior to the birth at which she has twins (with parity-matched counterfactuals). We discuss below the methods used to investigate potential mechanisms.

We conduct three robustness checks. First, we restrict the sample to non-ART births. It is important to demonstrate that our hypothesis holds independent of ART use because there is a positive association of ART with the likelihood of twin births (Vitthala et al., 2009), and ART users are typically more educated and wealthy (Lundborg et al., 2017). Another potential concern is that we are capturing genetic traits that, for instance, are associated with the woman's height or weight and also correlated with her predisposition toward twin birth. This would appear to be a second-order concern since we do not only rely on woman-specific measures of health but also show a positive association of twinning with environmental stressors, health facilities, and health-related behaviors. We nevertheless investigate this concern in two different ways. First, we test whether we can identify a positive association of the probability that a birth is a twin with woman-specific time-varying health indicators conditional on woman fixed effects that sweep out genetic influences. Second, we leverage biomedical research showing that monozygotic (MZ) twins are randomly allocated across mothers, although genetic predispositions may influence the chances of having dizygotic (DZ) twins (Meulemans et al., 1996). Ideally, we would restrict the sample to MZ twins, but MZ versus DZ are not identified in the data. Instead, on the premise that MZ twins are necessarily same sex and about half of all DZ twins are same sex, we investigate our hypothesis restricting the sample to include only same-sex twins. If our results were driven by genetic predispositions, then we should find weaker associations in the same-sex sample. The methods and data used to conduct the robustness checks are discussed with the results. The rest of this section elaborates the specification used in the two main approaches to testing for twin randomness.

For ease of exposition, we maintain the subscript $y$ for the woman's age at birth, but most of the health indicators are measured before pregnancy to avoid the potential concern of reverse causality—that twin births cause greater depletion of the mother's health than singleton births or encourage women to adopt different behaviors. These include prepregnancy measures of smoking, diabetes, hypertension, obesity, height, kidney disease, and asthma. Measures of prenatal or medical care are constructed as community-level measures of availability. In a specific case we discuss below, we use an exogenous measure of environmental stress in pregnancy. We also show results for some variables measured in pregnancy—smoking, alcohol, drugs, diet—and for one measure (BMI in developing country data) measured after birth. We flag these variables so that their coefficients can be interpreted with this caveat in mind.⁵ Importantly, if we dropped all of the flagged variables, we would still have a fairly compelling breadth of evidence. We add controls for education and, where available, wealth to allow for the fact that education may motivate and wealth may facilitate health-seeking behaviors (Kenkel, 1991; Lleras-Muney & Cutler, 2010). This will confirm that the indicators in Health are not simply proxying for socioeconomic status. As discussed above, we will present additional specifications including woman fixed effects in the model and restricting to same-sex twins.

Not all birth records contain indices of maternal health or health-related behaviors. To estimate equation (1), we sought data that did and that were representative and, given the relative rarity of twins, large. Data sets fulfilling these criteria include administrative birth data from the United States, Spain, and Sweden and household survey data from Chile, the United Kingdom, and 68 developing countries (the DHS) for different sets of years. (Details of temporal and geographic coverage, and summary statistics for each data set are provided in online data appendix B.2.) Together, these data sets include 17 million births from 1972 to 2013. We consistently restrict the sample to women aged 18 to 49 years old and exclude triplets and higher-order multiple births. We take advantage of U.S. Vital Statistics data from 2009 to 2013 that identify ART use by birth, removing the approximately 1.6% of births that were ART assisted.⁷ For the developing country sample, on the premise that ART was not available prior to 1990, we split the birth data into pre- and post-1990 samples.

Equation (2) is estimated using only the DHS and the U.S. Vital Statistics files. The DHS has the complete fertility history, including the survival status and birthweight of all children preceding each twin or singleton birth, and the U.S. birth certificate data allow us to infer earlier miscarriages for every mother as the difference between total reported births and live births. The miscarriage data are discussed further in section IVB.

In table 2 we present estimates of equation (1) for several countries using multiple indicators of maternal health. We find broadly consistent results across indicators and across samples. In online appendix C, we provide additional discussion of the stability of the general result across countries and levels of economic development. All independent variables in table 2 are standardized as $z$-scores so that the estimates can be cast as the effects of increasing by 1 standard deviation (SD) the independent variable of interest. Unstandardized results are presented in appendix table A1.

We find that the probability of twin birth is significantly positively influenced by the following indicators of maternal health included independently: not underweight, tall,⁸ more educated, having greater access to medical or antenatal care, not having smoked before pregnancy, not having any of a range of morbidities prior to conception (obesity, diabetes, hypertension, asthma, kidney disease), and averting risky behaviors in pregnancy (smoking, alcohol, drugs, unhealthy diet). The effects are sizable, with a 1 SD improvement in the indicator tending to increase the likelihood of twinning by 6% to 12% in most cases, relative to a mean of about 2.7% in the (global) sample. There are smaller effects from fresh fruit consumption and larger effects from height. We shall see when we present the pre-twin survival test results below that these effect sizes are broadly comparable to the difference in U.S. data of about 7% in rates of miscarriage of first births between mothers who go on to have twins at second birth and mothers who do not. This similarity of orders of magnitude contributes plausibility to our argument that miscarriage is a mechanism. We directly test this mechanism in section IVB.

Using all available measures of health for each country, we also calculated a factor index of maternal health (as in Biroli, 2016; see appendix D). Mothers of twins consistently have a higher score than mothers of singletons, but as the variables available for each country are different, the scores are not comparable across countries. The statistical significance of these health indicators is robust to running regressions, which condition on all available indicators of the mother's health and, importantly, education (appendix tables A2–A3). Our results all hold after correcting test statistics for large sample sizes that increase the likelihood of rejecting a null, following Deaton, 1997; see appendix table A4). First, we elaborate our findings by country. Then we present results from alternative approaches and the robustness checks concerned with the role of genetic traits.

We pool all non-ART births in the United States from 2009 to 2013. We estimate that a 1 SD increase in rates of smoking before pregnancy is associated with a 0.11 percentage point (pp) lower chance of a twin birth, which is about 5.5% of the mean rate of twinning.⁹ Diabetes and hypertension prior to pregnancy have standardized effects of 0.2 to 0.3 pp, while being obese or underweight prior to pregnancy has smaller effects of 0.04 and 0.16 pp, respectively. Height and education have larger standardized effects, of 0.61 and 0.8 pp, respectively. In appendix table A6, we remove potential outliers from the sample of mothers when considering height and the results are nearly entirely unchanged. Estimates for women using ART are presented in table A7 and are, with the exception of being underweight, larger and statistically significant for every indicator, underlining the additional sensitivity of birth outcomes in this group.

Analysis of birth registers from Sweden for 1993 to 2012 indicates strikingly similar standardized effect sizes for smoking, diabetes, height, and being underweight to those for U.S. women. There are, however, some differences: the standardized coefficient on obesity in Sweden is about three times as large, while the coefficient on hypertension is only half as large. The Swedish data additionally record asthma prior to conception, which we estimate reduces the risk of twin births by 0.015 pp. Survey data from Avon County for 1991 to 1992 and Chile 2006 to 2009 again exhibit patterns similar to those identified for Sweden and the United States for anthropometric indicators of health, risky behaviors, and prepregnancy illnesses. For instance, for the United Kingdom, estimates for being underweight, obese, or smoking before pregnancy are all very similar to the corresponding estimates for the United States. However, the standardized impact of hypertension before pregnancy is twice as large, and the associations with diabetes, height, and education are smaller. The U.K. data contain unique information on eating healthily during pregnancy, and our estimates indicate that the standardized effect of this is a 0.54 pp increase in the likelihood of having twins, which is the single largest coefficient among variables available for the United Kingdom. The coefficients in the Chilean data for being underweight and for smoking, drugs, and alcohol consumption during pregnancy lie between 0.16 and 0.33 pp, broadly similar to the coefficients for other countries, and the coefficient on obesity is considerably larger (0.26). Chile is the only country in our sample for which we have information on drug use during pregnancy and the standardized effect for this is similar to that for (frequent) alcohol consumption in pregnancy.

In the sample that pools data for 68 developing countries for 1972 to 2012, we observe height, weight, body mass index, and local availability of prenatal care and access to medical professionals. Reproductive health service coverage is far from universal in low-income countries, although this is a leading global health priority.¹⁰ After adjusting for demographic covariates as for the other samples, we observe again that taller and heavier women are more likely to twin. This is true even in the pre-ART period (see table A8). The effects of height, underweight, and education are all smaller than in richer countries, while the effects of obesity are larger than in all countries other than Chile.¹¹ We estimate that a 1 SD increase in availability of doctors or nurses is associated with a 0.092 pp and 0.06 pp increase in the likelihood of twins, respectively.

Using the methodology and data described in Quintana-Domeque and Ródenas-Serrano (2017), we estimated the impact of bombing by ETA, a Basque terrorist group, as a plausibly exogenous negative intrauterine shock that may cause fetal stress, a proxy for maternal health in pregnancy. We find that an additional bomb casualty in the province of residence of a pregnant woman decreases the likelihood that she will have a twin birth by 0.01% and 0.012% (see table 3). This effect is larger and statistically significant only during the second and third trimesters, similar to the effects of smoking by trimester documented in table 2.¹²

Here we discuss the alternative test of the quasi-randomness of twin births. Estimates of equation (2) are in table 4. In the developing country sample, mothers who went on to have third- and fourth-born twins had an infant mortality rate 1.3 to 1.7 pp lower among their prior births than women who had singletons at the same birth order. This is a natural measure of maternal health, capturing a woman's ability to produce surviving children, which is exactly what we hypothesize is challenged by carrying twins. We used birth size as a measure of child health that is less extreme than mortality. We used the DHS again, as it allows us to observe all children ordered within mother, and we find that earlier births of women who later have a twin birth are less likely to be small at birth than the corresponding births of women who have only singleton children (see appendix table A9).

Similarly, in the U.S. population, we observe that women who have twins are less likely to have suffered a miscarriage prior to the twin birth. Mothers who give birth to twins at second birth are 0.7 pp less likely to have suffered a miscarriage of their first conception, which is 6.7% of the baseline rate for this group. The rate of miscarriages in the population of all women who gave birth was approximately 10%. Parity-specific estimates and means are in table 4.

So as to control for any genetic characteristics of the mother, we sought data that follow women over time, recording multiple births per woman as well as time-varying measures of maternal health. Such data are scarce, but the National Longitudinal Survey of Young Women (NLSY) meets these requirements. A sample of 5,159 mothers aged between 14 and 24 in 1968 is followed until 1999, when the youngest are aged 45. The health variables measured consistently through this period are whether the mother has any physical limitation that restricts her ability to work, whether she smoked prior to the pregnancy, and whether she has had a prior cancer diagnosis. (More information on the data structure and summary statistics is in appendix E.) We estimate the probability that a birth is a twin as a function of these indicators of maternal health conditional on mother fixed effects and controlling also for a quadratic in family income, mother's age, birth order, and year of birth fixed effects. (Results are in appendix table A17.) We find large statistically significant negative effects of smoking and cancer on the probability of having a twin birth, and no significant impact of health limiting work.

The risk of giving birth to dizygotic twins (DZ) is elevated among women with high levels of the follicle-stimulating hormone (FSH), which is often more prevalent among taller and heavier women (Li et al., 2003; Hall, 2003; Hoekstra et al., 2008). Since dizygotic twins constitute about two-thirds of all twins, this could in principle contribute to explaining the associations we document with height and BMI (note that the biomedical literature has not documented these associations in any population-level data, let alone across countries, time, and indicators). Although, as discussed, genetic predispositions cannot explain our finding that health behaviors or aspects of the health environment (stress or prenatal care availability) predict twinning, we investigate this further by exploiting the fact that MZ twins are necessarily same sex (and about half of DZ twins are same sex) and repeat the analysis removing mixed-sex twins from the data.¹³ Results are in appendix table A10. We continue to find significant associations between proxies for maternal health and the chances of a twin versus a singleton birth, and the coefficients are not significantly different from those that obtain in the full sample.

We consider three alternative hypotheses for why maternal health may influence the probability of twinning, which relate to conception, gestation, and maternal survival. First, healthier mothers may be more likely to conceive twins on account of an underlying genetic or biological process. Second, conditional upon conceiving twins, healthier mothers may be more likely to take them to term. Third, conditional on conceiving twins and taking them to term, healthier mothers may be more likely to survive the birth, and hence appear in the available data.

Either of the first two processes is sufficient to violate the “as good as random” assumption insofar as they imply that observing twins will depend on possibly unmeasured maternal behaviors and characteristics. Since taller and heavier women and active smokers have higher levels of the FSH hormone associated with multiple births (Li et al., 2003; Hall, 2003; Hoekstra et al., 2008; Cramer et al., 1994), conception of twins may not be random. We cannot directly test the conception hypothesis since the required data are unavailable, but we now provide tests of the other two hypotheses and indicate the manner in which nonrandom conception will influence interpretation of our results.

Birth registers often do not include maternal health indicators, and if they do, it is unusual that they also also include information on fetal deaths, but the U.S. Vital Statistics data do.¹⁴ We pooled all births and fetal deaths recorded from 1999 to 2002. We stopped in this year because from 2003, a considerable redefinition of birth certificate data meant that fetal death and birth data did not share similar controls. Prior to 2002, however, we are able to observe for all states whether a mother smokes or drinks during pregnancy, whether she suffered from anemia prior to pregnancy, and her educational level. The results, using the U.S. birth certificate and fetal death data, are in table 5. In panel A, we document the difference in the risk of fetal death for twin relative to singleton conceptions. The evidence confirms previous research showing that the spontaneous abortion rate among twins (at one in eight conceptions), is about three times that among singletons (Boklage, 1990). In panel B, we test how maternal health indicators modify this differential risk. We can consistently reject that the interaction term $γ3$ is 0. In other words, twin fetal survival is more sensitive to mother's health than singleton survival. For example, a 1 SD increase in rates of smoking while carrying a singleton elevates the risk of miscarriage by 1.39 fetal deaths per 1,000 live births. The corresponding risk elevation among mothers pregnant with twins is an increase of 2.55 fetal deaths, almost twice the risk. Alcohol consumption is similarly almost twice as risky for women carrying twins, and the risks associated with anemia are about three times as high. We also see that a college education, which is a predictor of healthy behavior, modifies the difference in miscarriage probabilities more than three times as much when the mother is carrying twins than when she is carrying a singleton. Now it may be that one of two twins miscarries. In such cases, if the survivor is recorded as a singleton birth, then we tend to underestimate the importance of maternal condition. In other words, our contention holds a fortiori.

Overall, these results establish a plausible mechanism for the associations that we document in tables 2 to 4. Here we have modeled miscarriage conditional on the conception being twin or singleton. If in fact maternal health also raises the chances of a twin conception, then this will reinforce our contention. If, instead, maternal health is for some undocumented reason negatively associated with twin conception, then our findings hold despite this and are conservative.

Trivers and Willard (1973) made an argument similar to ours but pertaining to the distribution of sons across women (Almond & Edlund, 2007). They observed that since the male fetus is more vulnerable to adverse health conditions (Waldron, 1983), sons are more likely to be born of healthy mothers. As for twins, so for sons, selective miscarriage is the suggested mechanism. Intersecting our hypothesis with theirs, we investigated whether males are underrepresented among twins, other things equal. We used the large data sets in table 2 (data from the United States, Sweden, and developing countries). We find that twin births are approximately 0.1 to 0.3 pp more likely to be female (⁠$p<0.001$⁠). This affords a further test of our hypothesis and a validation of the Trivers-Willard hypothesis (refer to appendix table A12).¹⁵

Our findings suggest that twin birth is a marker of fetal health. Our findings, which range across indicators and countries, highlight the relevance of maternal health to fetal health. Recent research demonstrating long-run socioeconomic returns to investing in fetal and infant health, improving the preschool environment, and raising parenting quality has stimulated policy interventions across the world that are motivated to enhance the potential for nurture to lift up the trajectories of children, especially when born into disadvantaged circumstances (Heckman et al., 2010; Almond & Currie, 2011; Carneiro, Løken, & Salvanes, 2015). Our results point to the significance of, for instance, nutrition, stress, and prenatal care for mothers in achieving these goals.

A potential concern is that the less healthy women among those who delivered twins died in childbirth, and data sets like the DHS that obtain birth histories from mothers will not contain those women. In such cases, our findings could arise from selective maternal survival. This concern does not apply to the administrative U.S. and Swedish data where all births are recorded and where we see clear associations of twinning and maternal health, so it cannot be the only explanation of those findings. Similarly, in the U.K. and Chile data sets, the survey design ensures that representative coverage is not affected by maternal death.¹⁶ The lifetime risk of a maternal death is 1 in 41 in low-income countries as compared with 1 in 3,300 in high-income countries. If twins only spuriously appear to be born of healthier mothers due to selective maternal death, then as mothers become more likely to survive childbirth (i.e., as maternal mortality declines), the associations should dissipate. The fact that they do not also undermines the relevance of selection.

We assess the magnitude of selection bias in our DHS estimates, following Alderman, Lokshin, and Radyakin (2011). We simulate the presence of the women who died and test whether correcting for maternal survival selection causes the association of twin births and maternal health to disappear.

A data challenge is that we do not observe the health of women who died in childbirth; indeed, the original problem is that we do not observe these women at all. We address this by using the maternal mortality status of all sisters of every female respondent.¹⁷ We assume that the respondent's health (indicated by height and BMI) proxies the health of her sisters and validate this (figure A1).¹⁸ We put our results to the harshest test by assuming that less healthy women who died in childbirth were all carrying twins, and healthier women who died in childbirth were not carrying twins, and the results stand up to this (see table 6). We test the sensitivity of the adjusted estimates to a range of different binary distinctions of healthy versus less healthy. Overall, these results establish that maternal mortality selection does not drive the DHS results.

Twin births are not random. We show that mothers who have twin births are healthier prior to the occurrence of the twin birth. The findings in this paper have implications for identification strategies in economics and a number of other fields of research, and they extend the existing social science and biomedical literature on twinning. Here we delineate these contributions, using a list format for clarity.

1. The biomedical literature has identified an association of twinning with the height, weight, and smoking status of the mother and attributed this to hormonal variation. This is the first study to demonstrate that these associations hold in representative population-level data in several richer and poorer countries and across several years. We also show that these associations hold conditional not only on age and parity (known predictors) but also on the mother's socioeconomic status and a range of other indicators of her health.

2. This is the first paper to demonstrate associations of twinning with other indicators of maternal health. These include a range of (prepregnancy and pregnancy) morbidities, the health of her lower-order births, the mother's health-related behaviors (before and during pregnancy), availability of reproductive health services, and indicators of the mother's exposure to environmental stress in pregnancy. The last three are clearly not genetic or hormonal associations. We nevertheless show associations of maternal health and twinning conditional on woman fixed effects that purge genetic differences between women.

3. Since it is known that twins are more likely among ART-assisted births and that ART users tend to be more educated, we show that associations of maternal health with twinning hold in ART-purged and pre-ART data samples. We also show the first systematic evidence that the education of the mother is positively associated with twinning in these samples, consistent with educated women being more likely to engage in health-seeking behaviors.

4. Our findings indicate no clear tendency for the association of maternal health with twinning to dissipate with economic development. Although intrinsic maternal health and access to public health services tend to improve with economic development, it is unclear that all relevant indicators (hypertension, obesity, diabetes) improve, and differences between rich and poor countries in age, parity, and race will also modify this relationship.

5. We are able to demonstrate that maternal health determines fetal selection conditional on conception of twins. The biomedical literature has discussed hormonal (FSH) predictors; our hypothesis that it is selectively healthy women who are able to mount the challenge of carrying twins to birth is new.

6. In the economics literature, the validity of several studies investigating the hypothesis that fertility has a causal effect on investments in children, or on women's labor supply, rests on the assumption that twin births are random (at least conditional on age, parity, and education). Twin births are used as an instrument because OLS estimates tend to be biased upward on account of negative selection of women into fertility. Our findings suggest that twin-IV estimates will tend to be biased downward on account of positive selection into twin birth. This is important because recent prominent studies cited in section I find that the trade-off is frequently not statistically different from 0, and in principle, this could be explained by a downward bias in the estimates.¹⁹

Educational attainment has risen considerably, while completed and desired fertility have fallen sharply over the past fifty years (see Hanushek, 1992). It is of considerable relevance to researchers and policymakers to determine whether these trends contain a causal component. Similarly, the fertility-work trade-off for women is topical again as educational attainments of women are overtaking those of men and transforming the work-family balance, with consequences for women's autonomy, marital stability, and child outcomes (Newman & Olivetti, 2016; Lundberg et al., 2016).