Abstract

Despite rising numbers of only children in China, little is known about their family dynamics and well-being in adulthood—for example, how often they marry other only children and whether those in siblingless families have worse or better health than others. Theoretical expectations produce opposing predictions: siblings might provide social and emotional support and reduce parental caregiving pressures, but only children might receive more support from parents and grandparents. Using the 2010 China Family Panel Study, we examine marital sorting on Chinese adults’ number of siblings and test whether sibling availability and sibling sorting are associated with subjective physical and mental health. Despite general perceptions that China has an exceedingly high prevalence of adults with no siblings that might produce very small families, results demonstrate a low prevalence of siblingless couples (i.e., both spouses are only children). Married adults with no siblings or siblings-in-law have better subjective physical health but similar levels of subjective mental health relative to their counterparts with siblings. The health advantages of siblingless marital unions are greater for rural and female adults. Declining sibling prevalence in China will shape future family demographic dynamics but appears less detrimental to population health than sometimes assumed.

Introduction

Chinese families are changing, beset by unprecedented declines in kin availability and rapid population aging (Eberstadt and Verdery 2023; Peng 2011; Verdery 2019; Zhou et al. 2019). China's population policies, such as the Later-Longer-Fewer campaign (1972–19781) and the so-called One-Child Policy (1979–2015) (Feng et al. 2014; Wang et al. 2013), have collided with rapid economic development, educational expansion, rising women's labor force participation, and escalating child-rearing costs to create a steep increase in families with only one child (Du 2013; Whyte et al. 2015). Estimates of the number of couples with only one child in China vary, but the trend is clear: in 1979, 6.1 million couples received the “One Child Honorary Certificate” acknowledging that they had one child and pledged not to bear any more, and some estimates indicate that the number of one-child families had increased to roughly 150 million by 2010 (Wang 2013; Wang et al. 2013). Such a rapid increase has spurred speculation about new multigenerational family structures in China, such as the emergence of “4-2-1” families—those that have four grandparents, two parents with no siblings, and an only child (Eberstadt 2019).

Scholars have expressed alarm about family trends in China (e.g., Flaherty et al. 2007; Li et al. 2018; Liu et al. 2020; Phillips and Feng 2015; Zhang et al. 2012) amid ongoing worries about the prospective population decline and rapid population aging in the world's largest country (Attané 2022; Eberstadt and Verdery 2021; Wang and Mason 2007). The concern is that declining kin availability will challenge China's economy and society in the coming decades because of a shortage of family caregivers and a massive need for old-age support (Eberstadt 2019; Eberstadt and Verdery 2023; Verdery 2019). China's rapid population aging heightens concerns about who will care for the country's increasing population of older adults. The number of mainland Chinese adults aged 50 or older more than doubled between 1990 and 2021 (from 178 to 486 million) and is expected to reach 683 million by 2050 (United Nations 2022). The oldest age group, those aged 80 years or older, will see even more staggering growth: from 7 million in 1990 to 27 million in 2020 and to 115 million by 2050. From a global perspective, one cannot understand aging families without some consideration of family dynamics in China, a country that included one quarter of the global over-50 population in 2020 and will include more than 20% of that population in 2050 (United Nations 2022).

Scholars have also considered only children's well-being. As the scope of China's fertility restrictions crystallized in the scholarly and public consciousness, deep concerns about only children's well-being arose (Wu 1986). Early newspaper reports fretted that with so many only children, China was raising “a generation of spoiled brats” and touched off early concerns about this cohort's tendencies toward selfishness and antisocial attitudes as their parents “demand—and put all their hopes into” their academic achievement, as well as proneness to overweight and obesity (Deane 1992). The pejorative names given to these children, “Little Emperors” and “Little Princesses,” have stuck around since. Subsequent studies investigated the attributes of these cohorts of Chinese children (in elementary school at the time) and found higher test scores among children without siblings than children with siblings, limited evidence for personality differences, and mixed evidence regarding physical attributes (Falbo and Poston 1993). As the cohort aged into adolescence and emerging adulthood, other work periodically reassessed these ideas. Chinese children without siblings in the 1990s were found to have better nutritional outcomes than children with siblings (Bredenkamp 2008). Short et al. (2015) found that this cohort of Chinese children without siblings did not differ from peers with siblings on several physical, nutritional, and health variables in their adolescence. If anything, children without siblings were advantaged. Likewise, evaluations have documented mixed results for personality differences between Chinese sole children and those with siblings in emerging adulthood (Cameron et al. 2013; Zhao et al. 2013). However, how such children fare as they grow older remains unclear. More generally, it is unclear whether only children from the One-Child Policy era differ from other cohorts of children, such as those born a decade earlier when the country's fertility control efforts began.

Further, despite ample research on old-age support and caregiving in China and the attention to only children in childhood and adolescence, an unaddressed but important issue is the phenomenon of individuals without siblings marrying each other in adulthood (Feng et al. 2014)—the “2s” in the purported 4-2-1 families. As only children enter adulthood, they may marry another only child. The implications of such marriages are potentially profound: this family structure influences the extended kin network of their children and parents, which might affect their own health and well-being. For adults in midlife, siblings are an important source of social and emotional support when navigating adverse life events and providing intensive care to aging parents (Cicirelli 1995; Connidis 1989, 1994b). On the other hand, couples with no siblings might receive more social and instrumental support from their parents and grandparents, including undivided attention from up to four grandparental caregivers when their children are small, which may lessen stress and improve well-being in adulthood. Given the potential increase in couples comprising two only children in China, it is surprising that no study has documented the sibling networks of Chinese married couples or tested the well-being of couples with different sibling availabilities. With China's large share of the global population, increased demographic attention to the dynamics of its one-child families is particularly important.

In this article, we pursue three goals. First, we examine adult sibling networks in contemporary China. We look at Chinese adults overall and then focus on married adults. For married Chinese adults, we specifically examine the sibling availability of each spouse (i.e., couples that have no siblings, couples with only one partner having siblings, and couples with both partners having siblings), a phenomenon we refer to as “sibling sorting.” Attention to these basic demographic issues highlights the scope of the potential problems arising from China's presumed preponderance of one-child families, which is often assumed but not documented. Indeed, no reliable estimates of the extent of sibling sorting in China are available, despite some attention given to the idea. Second, we test whether sibling availability and sibling sorting are associated with poorer health for Chinese adults and couples. We examine whether Chinese adults and couples with different sibling availabilities report different levels of subjective physical and mental health. These tests expand our understanding of how one-child families might influence population health for a country composing at least one sixth of the world's population (in 2020, 18% of the total global population, 20% of those aged 15 or older, and even larger shares of older groups; United Nations 2022). Third, we examine whether these associations differ by demographic characteristics of rural–urban residence, sex, and birth cohort. These results illuminate the country's vast diversity, which might be masked by focusing on national averages.

The Emergence of One-Child Families in China

The Chinese population has undergone unprecedented demographic and social changes since the 1960s, driven largely by the government's population policies and the country's rapid economic growth. In the early 1970s, China began encouraging later marriage, longer birth intervals, and fewer births (the Later-Longer-Fewer campaign), efforts that it enforced with restrictions on age at marriage and caps on a couple's number of children in certain regions (Whyte et al. 2015). Of course, in a country as large and heterogeneous as China, it is important not to conflate edicts with the strictness of their enforcement: several studies have shown frequent gaps between the two (Chen and Huang 2020; Merli 1998; Merli and Raftery 2000; Wei and Zhang 2014). In 1979, the coercive components of these campaigns became national policy, excluding only select ethnic minorities (Attané 2002; Poston et al. 2006). Under the post-1979 policy, couples were permitted only one child and were subject to severe penalties if they exceeded the quota (Whyte et al. 2015), although enforcement varied and was plagued with corruption and misreporting (Merli 1998). This policy came to be known as the One-Child Policy, although that characterization masks the complexity of the policy's myriad exceptions by region, parent's attested ethnic minority status, child's sex and disability status, and other considerations (Gu et al. 2007). Although the One-Child Policy is a misnomer in terms of its actual policy prescriptions, there is little doubt that since its implementation, China has experienced a massive increase in the number and percentage of families with only one child. However, the extent of this increase and the extent to which China's fertility control policies or other factors, such as economic development, were responsible for it are hotly debated topics (Gietel-Basten et al. 2019; Goodkind 2017, 2018, 2019; Greenhalgh 2018; Wang et al. 2018; Wei and Zhang 2014; Zhang and Sobotka 2021; Zhao and Zhang 2018).

China's implementation of fertility control policies coincided with the start of its decades of unprecedented economic growth. The concurrence of these events and their self-reinforcement culminated in some of the quickest demographic changes ever seen in a large country (Wang et al. 2016). China's total fertility rate dipped below the long-run replacement level of 2.1 births per woman in the 1990s (Morgan et al. 2009) and has remained low since, officially registered at 1.3 in 2020 (National Bureau of Statistics of China 2021). The One-Child Policy's strictures and increased access to prenatal ultrasounds also resulted in rising levels of sex-selective abortion in the country beginning in the late 1980s, although fertility misreporting precludes accurately ascertaining the true extent of these practices (Chu 2001; Merli and Raftery 2000). By 2020, according to the seventh Chinese census, the official sex ratio at birth was 111 males for every 100 females (National Bureau of Statistics of China 2021), well above the global norm of 103 to 107 but slightly lower than the peaks of the preceding two decades. The imbalanced sex ratio, coupled with preferences for men to marry women younger than themselves in a country with successively shrinking birth cohorts, is causing a marriage squeeze in China, with millions of men unable to find a spouse (Guilmoto 2012; Jiang et al. 2014). Such patterns portend a substantial divergence in the family formation of men relative to women, with a large share of men expected to age without spouses or children but very few women doing so (Verdery 2019).

Despite the ample research on China's One-Child Policy and its implications for Chinese society, there are surprisingly few estimates of the number of only children and one-child families in the country. Studies have used different types of estimates (cross-sectional observation and population projections) and different sources of population parameters (e.g., Chinese census data, 1% National Population Sample Survey, and World Population Prospects) to evaluate the size of China's one-child population (National Health and Family Planning Commission 2005; Song 2005; Verdery 2019; Wang 2013). The estimates of only children range from roughly 90 million in 2007 to 145 million in 2010 (a period when the population grew by only 26 million; see United Nations 2022), showing substantive variation across studies. To our knowledge, there are no reliable estimates based on nationally representative family surveys (as opposed to administrative data like the census). For these reasons, our study's first aim is to use high-quality, nationally representative survey data to characterize the percentage of the Chinese adult population who are only children.

Sibling Sorting and the Marriage of Two Only Children

A clear ramification of the increase in only children in China is the potential production of small families in which two individuals with no siblings marry each other. As easy as it is to extrapolate from the idea that a country has many only children to the notion that the country should therefore have many only–only marriages, such armchair theorizing might substantially deviate from reality. One factor limiting the prospects of only–only marriages could be the number of only children in the local marriage market. Given the way China's one-child population policies were actually implemented, only children are more common in urban areas, among men, and among those born after 1979. Differential rates of change in such areas complicate a clear sense of how populations have changed in rural and urban areas: urban China has grown rapidly in recent decades, even in the face of exceedingly low urban birth rates, owing to high in-migration from rural areas. Another factor limiting only–only marriages might be only children's mate selection preferences. Only children might prefer to marry other only children because they have similar family-of-origin experiences and education levels or because of the ability to have two children together, given a policy loophole allowing two only-child adults to have two children, even in urban areas (Zhou 2019). Of course, although it seems less likely, the reverse could also be true: only children might prefer marrying those with siblings, providing counterbalance to such expectations. In either case, it is unclear how directly the country's growth in one-child families might translate into only–only marriages.

The extreme case of such a small family in China is known as a 4-2-1 family: four living grandparents, two parents who have no siblings, and only one child. Most studies consider 4-2-1 families as emblematic of China's aging challenges (Eberstadt and Verdery 2021), although few studies have comprehensively delineated precisely what 4-2-1 families entail. The most restrictive definition of the 4-2-1 family formation meets three requirements: (1) three generations coexist with paternal and maternal grandparents and parents all living; (2) neither the father nor the mother in the middle generation has siblings; and (3) the youngest generation is an only child (Song 2000). Yet, such a restrictive definition is not necessary for capturing the spirit of the 4-2-1 idea and its presumed challenges for Chinese families. For instance, we might say that an adult has something equivalent to a 4-2-1 experience if one of the four grandparents has died or a grandchild is not yet born. Similar notions have been debated regarding the history of nuclear and stem families in Europe and North America (Ruggles 2010). A less restrictive definition of 4-2-1 could focus on the minimal criteria that might generate similar pressures on families—namely, that two people without siblings marry each other and potentially care for two sets of aging parents—without restricting the living status of all four aging parents and their own (future) child(ren). Because of these definitional uncertainties and substantial speculation about their emergence, there are no reliable estimates of the prevalence of 4-2-1 families in China. Thus, in addition to our first aim of documenting sibling availability in Chinese adult population, we use nationally representative data to examine the prevalence of potential 4-2-1 families by investigating sibling sorting status among married couples. Such results can help us to understand the scope of 4-2-1 families with more restrictive and less restrictive definitions.

Sibling Availability and Health in Adulthood

Despite increasing marriages between two adult only children in China, research has not yet empirically examined whether sibling availability in such a context shapes adulthood health and well-being. Family scholars have devoted considerable attention to questions about the associations between siblings and well-being, but this literature focuses primarily on the natal household environment and neglects the role of siblings in adulthood. It is also an international literature with uncertain application to China. For instance, most research on siblings examines associations between sibship size and educational attainment (Steelman et al. 2002) or the development of social skills (Downey and Condron 2004; Gilligan et al. 2015; Kramer 2010), and these studies are primarily from the United States. Clearly, sibship size can influence longer term life course outcomes: educational attainment and social skills are strong predictors of such outcomes, after all. However, such influences will also operate through mechanisms beyond the natal environment, including social support. Research using Swedish population register data suggests that children without siblings experience a mortality disadvantage but that having more siblings does not negatively impact midlife physical and mental health (Baranowska-Rataj et al. 2017; Baranowska-Rataj et al. 2016). Drawing on this literature and a long tradition of family research examining the association between the number of siblings and later life outcomes, we present two competing sets of hypotheses.

On the one hand, resource dilution theory suggests that because parents have finite resources to distribute between offspring, siblings are competitors (Blake 1981; Downey 2001). Thus, growing up in a smaller family is associated with better economic status, health, and other life chances (Conley and Glauber 2006; Fletcher and Kim 2019; Hatton 2017). Thinking beyond the individual, couples with fewer siblings might benefit from more social and material resources inherited from parents (Baranowska-Rataj et al. 2016). This perspective suggests that in China, adults without siblings might be advantaged in terms of subjective health. Perhaps one's parents are more available to provide care to their grandchildren if they have only one set of grandchildren, potentially easing childcare burdens for couples in which at least one is an only child. Such influences are also likely to be gendered. Women might be especially likely to gain from being an only child because of potential gender inequality in multichild natal environments, and these gains are likely to persist into adulthood and influence their health (Zeng and Hesketh 2016).

On the other hand, siblings can be a source of support and companionship throughout the life course (Cicirelli 1995; Connidis 1989, 1994b). In adulthood, siblings are key sources of support for managing aging parents (Connidis 1994a; Connidis and Kemp 2008). Adults with siblings acquire better social skills to navigate interpersonal relations (Bobbitt-Zeher et al. 2016; Merry et al. 2020), which is also important for subjective well-being. In couples, these considerations multiply. A couple with no siblings might suffer greater stress from caregiving for two sets of parents without siblings. Beyond parental care responsibilities, they experience a high level of parenting stress without any cousins, aunts, or uncles for their children (Zhan 2002). Therefore, adults who themselves have siblings or whose spouse has siblings might have better health than only-child adults in siblingless families.

It remains an open question whether having no or fewer siblings will have negative implications for adult health in China or whether smaller, siblingless families are beneficial for married couples. An added layer of complexity in China, where the international literature on the topic might not offer particularly compelling insights, is the distribution of siblings and the sheer magnitude of only children. Limited research on the association between sibship size and educational attainment in China treats sibship size continuously and does not consider only children as a distinct category (Lu and Treiman 2008; Shen 2017). As a result, we examine these competing perspectives regarding how sibling availability impacts subjective physical and mental health among Chinese married adults.

Social Context, Sibling Availability, and Subjective Health

Examining the associations between sibling availability and adult health requires considering the formation of different family structures and how social contexts shape the prospects of marrying an only child versus someone with siblings. For instance, in places where there are more only children, the odds of marrying an only child increase, as do the combinatorial odds of two only children marrying each other, regardless of individual preferences for this union type. A similar process has been observed for peer effects in adolescent educational attainment, leveraging the fact that certain types of friendship ties are determined by “induced homophily” or the likelihood of some tie types forming as a function of group sizes (Fletcher et al. 2020). This is an old idea (e.g., Coleman 1958; Jones 1929).

Because of the implementation of the One-Child Policy, only children are more common in China's urban areas, among men, and among cohorts born after 1972 and especially after 1979. Urban adults are more likely to have been born into one-child families because of the strictly implemented penalties for above-quota births in urban areas (Gu et al. 2007; Zhang 2017) and the 1980s policy adjustments allowing rural couples to have a second child if the first child was female. Rural misreporting of the number of registered children might also have been higher (Merli 1998), creating the appearance of greater parity in the number of children between urban and rural areas than existed in practice. Therefore, adults living in urban areas who have no siblings are more likely to marry other only children (regardless of whether preferential selection is the cause) than those in rural areas, where the odds of meeting other only children are lower. Similarly, because of son preferences and the policy that rural couples with a firstborn girl could have a second child, Chinese men are less likely to have a sibling than women. As a result, men are more likely than women to marry a spouse with siblings, and only-child women are more likely to marry only-child men.

The changing policy environment, economic development, and women's changing status also shape selection into one-child families in China over time. Although we cannot parcel out how much of the change is due to policy environment versus economic development, those born after the 1972 implementation of Later-Longer-Fewer and especially those born after 1979 and the stricter One-Child Policy are much more likely to have been born into small families. Children born before 1958 and without siblings were likely differentially selected as a result of very high infant and child mortality rates. However, the directionality of this expectation is unclear. We might expect them to be in better health owing to lower frailty and their having survived health-selective pressures, but we may just as well expect them to be in worse health owing to their greater exposure to the adverse events that produced such health selection (Ferraro and Farmer 1996; Haas 2007). To evaluate such considerations, we explore whether the implications of sibling availability for subjective health vary by rural–urban residence, sex, and birth cohort.

Methods

Data and Sample

We use data from the China Family Panel Study (CFPS) 2010 baseline survey, a nationally representative study of Chinese families and residents (the dataset is available at https://www.icpsr.umich.edu/web/ICPSR/studies/36524; Xie and Hu 2014). The CFPS is a multistage probability sample that includes 33,600 adults aged 16 or older from 14,960 households in 25 provinces in its 2010 wave (response rate =  81%). The survey's approach judiciously sampled relevant regions of China, stratified by province size and socioeconomic development, and used multistage sampling to obtain hierarchically representative results and weighted subnational estimates valid for provincial strata as well as other groupings, such as urban versus rural distinctions (Xie and Lu 2015). The baseline survey interviewed respondents and their coresiding spouses about their families. We use the 2010 wave because more recent waves of the CFPS data did not include questions about sibling status in the adult questionnaire. We are unaware of other nationally representative surveys of the entire adult population of China (i.e., that are not age-restricted) that enumerate respondents’ coresident and noncoresident siblings. In general, too few surveys collect extrahousehold kinship data in China.

We use the CFPS full adult sample (N = 33,601), adjusted using person-level sample weights. When analyzing adults’ sibling networks, we exclude 416 adults who had missing information about their own sibling status or their spouse's sibling status (N = 33,185 in Table 1 for sample description by sibling status) and 25 adults with missing marital status information (N = 33,160 in Table 2 for sample description by sibling status and marital status). When we focus on each spouse's sibling availability, we further exclude 6,739 adults who were unmarried at the time of the interview and 3,944 respondents who were married but did not have a spouse participating in the survey (N = 22,477). We have complete data on most analytical variables except individual income. Most of the 1,634 (4.86% of the full adult sample) respondents with missing income values (n = 1,628) are unmarried and are thus excluded from the analysis. We exclude individuals with missing values in regression models, and the analytical sample sizes vary by model.

Measures

Sibling Availability

Respondents and their spouses reported the total number of siblings they ever had. For our analysis at the individual level, we examine whether respondents have siblings or are only children. For married respondents, we examine three couple combinations based on each spouse's sibling availability: (1) neither spouse has siblings, (2) only one spouse has sibling(s), and (3) both spouses have sibling(s). In sensitivity analyses, we differentiate the second group according to whether the only child is the focal respondent or the spouse (see the Results section).

Subjective Health

We examine two outcomes. First, subjective physical health is rated from 1 = healthy to 5 = very unhealthy and is reverse-coded, with a higher score indicating better subjective physical health. This subjective physical health measure has been well validated across contexts (Jylhä et al. 1998; Pan et al. 2022). Second, self-rated mental health is the sum of scores for six depressive symptoms rated from 1 = almost every day to 5 = never, with a higher score indicating a better subjective assessment of having fewer depressive symptoms. This coding strategy facilitates comparisons with subjective physical health. The items assessing depressive symptoms measure the prior-month frequency of feeling depressed, stressed, upset, hopeless, everything is difficult, and life is meaningless (α = .86). This measure of depressive symptoms has been validated for Chinese adults (Xu et al. 2021).

Control Variables and Stratifying Dimensions

We also examine several sociodemographic variables as statistical controls or key stratifying dimensions when examining the associations between sibling networks and adult subjective health. These include residency (rural or urban), year-of-birth categories defined around key cut points in China's demographic and family planning policy history (born before 1958, 1958–1972, 1972–1979, or after 1979),2 sex (female or male), ethnicity (Han or non-Han), education (high school or above vs. less than high school), employment status (working or not working), and individual income (in yuan and divided into quartiles). Of these, we also consider stratified models by urbanicity, year-of-birth category, and sex—factors that we think better isolate the associations between sibling availability and health net of sociodemographic and economic differences.

Analytic Strategy

We first examine the prevalence of sibling availability among Chinese adults (Table 1) and sibling sorting by marital status (Table 2). We then conduct multivariable linear regression models for adult subjective health by sibling availability. For the full adult sample (Table 3), we estimate an unadjusted model (Model 1) for the association between individual sibling availability and subjective physical health and then add sociodemographic control variables into the adjusted model (Model 2). To better gauge the importance of demographic issues in sibling networks, we stratify our analyses by rural–urban residence (Models 3–4), sex (Models 5–6), and birth cohort (Models 7–10). We then estimate the same models for the association between individual sibling availability and subjective mental health (Table 4). For married adults, we conduct similar analyses (Tables 5 and 6 for subjective physical and mental health, respectively), with the key variable being couples’ sibling status (i.e., both couples have no sibling vs. one spouse has at least one sibling and both spouses have at least one sibling).

For all regression models, we use ordinary least squares with survey weights. Some debate exists about the merits of treating Likert-style items, such as our measure of subjective physical health, in a least-squares framework (O'Connell 2006). Alternate approaches often applied in such situations (e.g., ordinal logistic regression or dichotomizing the measure) have assumptions that are often violated and either typically do not produce substantively meaningful differences from the least-squares results (Pohlmann and Leitner 2003) or limit inference to differences surrounding the particular cut point the researcher chooses for dichotomization (Scott et al. 1997). Further, such approaches can complicate cross-model comparisons for parsing the contribution of different subgroups to the overall associations examined, especially when using complex survey weights (Mood 2010; Mize et al. 2019). In addition, we also prefer the least-squares approach because the subjective mental health measure we use is a Likert-style scale that sums the responses to six items with five response categories, which differs from the subjective physical health measure based on a single five-response category Likert-style item. In such a situation, using ordinal logistic regression for the subjective mental health measure would be inappropriate. Thus, to enhance the comparability of the results, we apply the least-squares framework. In such situations, least-squares approaches are generally recognized to provide unbiased inferences about the direction of the association (e.g., see O’Connell 2006:41), which is our principal quantity of interest. To verify the robustness of our results, we created a binary measure of subjective physical health (1 = healthy and 0 = fair, relatively unhealthy, unhealthy, or very unhealthy) and estimated logistic regression models. The results from sensitivity analyses (see Tables A1 and A2, online appendix) are similar to those from ordinary least-squares models.

Results

Table 1 presents weighted estimates of the sibling availability of Chinese adults by their key sociodemographic characteristics. Overall, 67.5% had two or more siblings, 21.7% had one sibling, and 10.9% never had any siblings. Having no siblings was more likely among respondents born after 1979, males, individuals of Han ethnicity, those residing in urban areas, and those with higher education and income. These results are unsurprising, but they set the stage for our remaining analyses.

Table 2 describes the family and sociodemographic characteristics of Chinese adults by their marital status and sibling availability. As shown in the table, 23.9% were unmarried (left column), and 11.6% were married but their spouse did not participate in the survey. Most respondents (64.4%) were married with their spouse in the survey. Only 1.3% of respondents were in marriages in which both spouses were only children. Next, 6.4% of respondents were in a couple in which one was an only child and one had at least one sibling. Married respondents in which both spouses had siblings composed the largest category (56.7% of respondents).

As shown in Table 2, the characteristics of couples differ greatly by sibling sorting status. The couples of two only children were much more likely to live in urban areas, to have been born after 1979, and to have Han ethnicity, higher education, greater labor force participation, and higher income than couples with siblings.

Table 3 presents results from regression models assessing the association between individual-level sibling availability and subjective physical health among Chinese adults. The results for unadjusted Model 1 show that adults with no siblings had better subjective physical health than their peers with siblings. However, after we control for sociodemographic characteristics in the model (Model 2), the association reverses such that respondents without siblings perceive their physical health to be worse than do those with siblings; however, the coefficient in this model is not statistically significant at conventional thresholds. This reversal is due to the variation in the association between sibling status and subjective physical health by demographic subgroups. To determine what produces this pattern, we estimate regression models stratified by rural–urban residence, sex, and birth cohort. The results show that for urban residents (Model 4), males (Model 6), and adults born after 1979 (Model 10), health is worse among those with no siblings. However, for rural residents (Model 3), females (Model 5), and adults born in older cohorts (Model 8), subjective health does not differ by sibling status. The relative prevalence of these groups in the population and their respective associations between sibling status and subjective physical health, especially the strong positive association among respondents born between 1958 and 1972, combine to produce differential associations between the unadjusted and adjusted models.

Table 4 displays results for subjective mental health. Chinese adults with and without siblings generally had similar levels of depressive symptoms in the unadjusted model (Model 1). However, the adjusted model (Model 2) reveals a negative association, suggesting that those without siblings have worse mental health than those with siblings. The stratified models reveal that this overall association is produced by the poorer mental health of men without siblings relative to men with siblings (Model 6).

Turning to couples, Table 5 presents regression model results on the association between the sibling status of married adults and respondents’ subjective physical health. Results from unadjusted and adjusted models consistently show that respondents in only-child couples had better physical health than those in couples in which only one spouse had siblings (Models 1 and 2) and those in couples in which both spouses had siblings (Model 1). The stratified analysis reveals the health benefits of being in an only-child couple for rural (Model 3) but not for urban residents (Model 4), female (Model 5) but not for male (Model 6) adults, and adults born in 1958–1972 (Model 8) but not for younger cohorts (Models 9 and 10). For the outcome of subjective mental health by respondents’ marriage-level sibling status (Table 6), we found no differences in depressive symptoms in the overall sample or across stratified subgroups.

To collate the key insights of the sibling sorting models, Figure 1 visualizes the main results. Panel a displays the predicted subjective physical health values by couples’ sibling status and demographic dimensions of rural–urban residence and sex from Table 7, whereas panel b contains the predicted subjective mental health values by the same dimensions. These results further highlight the impact of siblingless marriages on adulthood subjective physical health and the variations by rural–urban residence and sex.

Sensitivity Analysis

Our stratified sample analyses show that the associations between sibling availability and subjective health vary across demographic subgroups. To determine whether the differences were statistically significant, we estimated models with interaction terms between sibling availability and each demographic characteristic (rural–urban residence, sex, and birth cohort). Significant interaction terms from the sensitivity analysis results echoed the differential associations between sibling status and subjective health in different populations. Table 7 presents the predicted values for subjective physical and mental health based on the results.

Because couples in which only one spouse has at least one sibling include two types of marriage unions (the husband or the wife is the only child, and only one of them is the reporting respondent), we conducted additional analysis to determine whether the sex of the reporting respondent impacts the conclusions. We estimated interaction terms between sibling availability and respondent's sex and found nonsignificant interaction terms, suggesting that regardless of which one in the couple was the only child, husbands and wives had similar subjective health levels relative to other types of unions.

In other sensitivity analyses, we conducted regression analysis and adjusted using propensity score weights. Because couples’ sibling availability is not random, we explicitly modeled selection into different couple combinations with a propensity score approach (Guo and Fraser 2014). These results did not substantively differ from our linear regression results.

Discussion

The rapid demographic, social, and economic change that produced and accompanied China's swift fertility decline over the last 60 years has led to concerns about the mass emergence of novel and small family structures, such as the 4-2-1 family, in the world's most populous nation. Given the ample attention to the potentially disastrous consequences of shrinking kin networks in China (Eberstadt and Verdery 2021; Song 2000; Zhan 2002), it is quite surprising that there are no reliable estimates of the prevalence of marriages between two only children. We documented a fairly low prevalence of couples comprising two only children in contemporary China (by 2010). We found that only 1.3% of Chinese adults overall and roughly 1.8% of Chinese adults born after 1979 are in such unions. These numbers are surprisingly low and seem misaligned with the attention paid to the issue in contemporary theorizing about China. Although families with one child have become the new norm in China, and millions of families have only one child in their household, the phenomenon of only children marrying each other is still at an early stage (Feng et al. 2014). Whether such changes presage the mass emergence of only-child (siblingless) marriages remains to be seen; we are not there yet.

Our results reveal the importance of considering family formation processes and demographic characteristics when examining the role of sibship in adult health. Prior work on sibship in China suggested potential health advantages associated with not having siblings, at least during childhood and adolescence among those born during the 1980s (Falbo and Poston 1993; Short et al. 2015), but this topic has not been explored for all Chinese adults. We found that groups in which only children were in better health (rural residents, females, and older cohorts) were less likely to be without siblings, whereas groups who might have benefited from having siblings (urban residents, males, and younger cohorts) were more likely to be without siblings. The resource dilution theory and sibling support perspective might explain each finding conditioning on potential resources. For groups with potentially limited resources and opportunities (rural residents, females, and older cohorts) relative to their counterparts (urban residents, males, and younger cohorts), competing for parental resources might be more important for personal development and adult well-being; thus, a smaller sibling network or being an only child is related to better subjective health. However, urban, male, and younger adults who have more resources but also face higher caregiving burdens (raising a child or caring for aging parents, which is culturally expected of sons in China) might have better subjective health if they have a sibling to help provide support.

We also examined the implications of sibling sorting in marriages for physical and mental health. Our findings suggest that adults in a couple of two only children are not worse off than married couples in which one or both have siblings. In fact, we found positive implications of being in a siblingless couple for subjective physical health in adulthood (see Figure 1).

Our results provide some evidence for the resource dilution theory concerning the advantages of smaller sibling networks (Blake 1981; Downey 2001). Relative to children with siblings, children in Chinese one-child households have enjoyed better childhood nutrition status, as evidenced by higher height-for-age (Bredenkamp 2008) and more nutrition intake (Liang and Gibson 2018). Moreover, Chinese parents of only children tend to have higher expectations for children's academic outcomes and invest more resources in their children's education (Lee 2012). As a result, the cumulative advantages of childhood nutrition and education attainments for only children in China might contribute to their better adulthood physical health. In the context of a marriage union, an only child might face an added burden of providing assistance to siblings-in-law if they marry someone with siblings. Such a burden could cause conflicts and marital problems, potentially undermining subjective health.

The beneficial physical health effects of being in a marriage of two only children were especially notable for rural residents, women, and older cohorts. The advantages of having fewer siblings include more resources inherited from parents, better education and career opportunities, and the utilization of social and government services for children and aging parents. Such advantages are related to better subjective health for adults. However, these opportunities might be more limited and might therefore be more important for adult health in rural China.

Women's advantages from one-child families are consistent with prior literature documenting increased gender equity, women's empowerment, and parental education investments after the implementation of the One-Child Policy (Fong 2002; Lee 2012; Zeng and Hesketh 2016). Relative to peers with siblings, daughters without siblings have less pressure regarding gender bias and son preferences in the family. Although both daughters and sons with no siblings have better education than their peers with siblings, being an only child provides greater advantages to daughters than to sons in China (Lee 2012). For women without siblings, parental support and better education opportunities increase their chances of having well-paying jobs and career advancements (Zeng and Hesketh 2016), perhaps ultimately improving their subjective health in adulthood.

The association between sibling availability and subjective health was strongest among the oldest cohorts and much weaker among younger cohorts. This finding appears to implicate selection because two only children marrying one another was a more selective event in past decades than it is today; indeed, being an only child was more selective, or at least it was rarer. However, younger cohorts might not have reached the ages at which small families exert particular pressures on adults, when they must simultaneously care for two sets of aging parents and young children or (more likely in the case of China) a young child. If so, we might expect to observe widening disadvantages to being in couples with no siblings in the future. Still, perhaps the larger ratio of grandparents to grandchildren could offset these burdens.

Limitations and Future Directions for Research

Despite its strengths, our study has some limitations. First, there may be unmeasured variables impacting the likelihood of having siblings or siblings-in-law in the marriage union, such as the influences from family and social network members (Chen and Austin 2017). These unmeasured variables might confound the observed associations between health and sibling status and sibling sorting. A more macro-level question is the extent to which China's surfeit of siblingless adults results from its fertility control policies relative to other factors, including rapid economic development. The answer to this question touches on hotly debated questions that have roiled the field (Gietel-Basten et al. 2019; Goodkind 2017, 2018, 2019; Greenhalgh 2018; Wang et al. 2018; Zhao and Zhang 2018). A firm answer would better contextualize China's potential uniqueness in the number of only children and how those only children are faring in adulthood. We suggest future comparative work examining other low-fertility contexts and those that experienced long periods of skewed sex ratios at birth. A comparison with South Korea would be especially useful.

Next, our analysis is cross-sectional. As the cohort with many only children ages, it will be important to explore the association of their family structure with their physical and mental health. Future research can examine the role of sibling availability over adulthood as siblings die or how families shift in other ways through remarriage and other processes. Future research should use panel data to examine the impacts of various family forms on health and well-being outcomes over time in China, which will be possible with the continued fielding of the CFPS. However, note that we used the only fielded wave of the CFPS that measured sibling distributions. Other surveys suffer similar limitations. Future work would benefit from collecting more extensive kinship rosters or otherwise gathering robust data on nonhousehold kin in China and elsewhere. The detailed kinship information would enable greater research into the inequality-inducing role of kin availability, kin transfers, and access to specific types of kin (e.g., educated relatives; Park et al. 2019).

Last, as the survey respondents’ parents advance in age and require more care, we should examine whether sex differences emerge in the relationship between sibling availability and subjective mental health. China is known for gendered norms regarding intergenerational transfers: sons are thought to be responsible for providing financial care, and daughters-in-law are often the providers of instrumental care. As the large cohort of only children begins to care for older relatives, new gendered patterns could emerge. Also worth exploring are the other generational members of purported 4-2-1 families. Although our results suggest few negative physical and mental health associations with sibling status or being in an only-child–only-child marriage for those in the middle generation (the “2s” in 4-2-1), they do not provide evidence for these experiences among the current children generation (the “1s” in 4-2-1). There are also evolving population health issues among Chinse older adults, which could change as the current “2s” age from middle adulthood to late adulthood, as already discussed. Likewise, despite the relative rarity of the phenomenon of 4-2-1 families observed to date, meaningful questions remain about its possible emergence in the coming decades that demographic scholarship would be well positioned to document.

Conclusion

Using nationally representative data from the China Family Panel Study, this article examined sibling networks in China and tested whether sibling availability among individuals as well as sibling sorting in couples were associated with subjective health. Our results demonstrated a very low prevalence of Chinese couples in which both spouses came from one-child families, at least in the contemporary era. Moreover, Chinese adults in couples with no siblings had better subjective physical health than their counterparts with siblings. The results of this study revealed the complexity of family structure in contemporary China in the wake of its last half century of change, including its rapid economic growth and unique fertility control policies collectively referred to as the One-Child Policy. With the declining fertility rate in many countries, the number of children growing up without siblings in the household is increasing with declining family sizes. As cohorts with increasing numbers of only children age, the odds of marriages in which neither partner has siblings will increase as well. This trend is global, although China is on the leading edge of such changes. This examination of adulthood well-being among only children and individuals in only-child marriages in China helps establish a knowledge base on family, kinship networks, and health in China and other aging societies.

Acknowledgments

The authors have no conflicts of interest to disclose. The Syracuse University Aging Studies Institute provided support for this research and its publication. The authors acknowledge support from the National Institute on Aging (1R01AG060949) and from the Pennsylvania State University Population Research Institute, which is supported by an infrastructure grant by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (P2C-HD041025). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health or other funding sources.

Notes

1

The precise start date of the Later-Longer-Fewer campaign is difficult to decipher. It is thought to have begun between the late 1960s, when family planning groups were established in different provinces, and 1973 (Chen and Huang 2020). We thank an anonymous reviewer for highlighting this fact.

2

These years correspond to the onset of the Great Leap Forward (1958) and its associated disruptions, such as the Great Chinese Famine, the onset of the family planning Later-Longer-Fewer campaign (1972), and the onset of the stricter fertility control policies collectively considered part of the One-Child Policy (1979).

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