Hybrid marriages and phenotypic heterosis in offspring: Evidence from China

Highlights

• This paper uses the 1% sample of the 2000 Chinese population census data to test whether there is a heterosis effect in human populations.

• Offspring of genetically more diverse parents exhibit better biological/genetic fitness in educational attainment and body height.

• The estimated heterosis effect is stronger in males, likely resulting from sexual selection.

• Results are consistent with predictions of the “distance-performance” hypothesis.

Abstract

In genetics, heterosis refers to the phenomenon that cross-breeding within species leads to offspring that are genetically fitter than their parents and exhibit improved phenotypic characteristics. Based on the theory of heterosis and existing genetic evidence, offspring of “hybrid” marriages (spouses originating from different states/provinces/countries/areas), though relatively rare due to physical boundaries, may exhibit greater genetic fitness in terms of intelligence, height, or physical attractiveness (the “distance-performance” hypothesis). This study explores whether heterosis is a contributing factor to offspring’s educational attainment in China by applying a high-dimensional fixed effects (HDFE) modelling framework to the unique 0.1% micro-sample of the 2000 Chinese Population Census data. Concerning potential endogeneity of hybrid marriages, we conduct a series of robustness checks. Reassuringly, the estimated heterosis effect remains significantly positive across various measurements, after controlling for parental educational attainments/height, environmental influences, and over a thousand region and region-by-year fixed effects. The effects in male and higher-educated offspring are found to be stronger. Results are replicated when analyzing body height using data from the China Health and Nutrition Survey. Although endogeneity of “hybrid marriages” may not be completely ruled out, the current study sheds light on the potentially beneficial effects of interprovincial migration on population-level human capital accumulation, and we hope that this paper can intrigue future studies that further address endogeneity. The implied heterosis effect could, therefore, be profound for Homo sapiens as a species from an evolutionary point of view. An additional important implication is that the overall genetic influences of parents on offspring’s performance may be further decomposed into a conventional heredity effect and a heterosis effect that has been neglected previously.

Introduction

Introduced by George Shull in 1914, heterosis (also known as hybrid vigour or heterozygote advantage) is a genetic phenomenon in which cross-breeding within species leads to offspring that are genetically fitter than their parents and exhibit improved phenotypic characteristics (Chen, 2013; Lippman and Zamir, 2007). Although biologists have documented a long list of heterosis in crossbred plants and animals, evidence for heterosis among human beings beyond the molecular level remains lacking. Yet given the substantial evidence of a positive correlation between geographic and genetic distances among different human populations (Manica et al., 2005; Jorde and Wooding, 2004), offspring of “hybrid” marriages (in which spouses originated from different states/provinces/countries/areas) may exhibit better genetic fitness in terms of intelligence, height, or physical attractiveness (Prokosch et al., 2005; Lewis, 2010), suggesting a potential heterosis effect among human populations.

A further belief in a general heterosis effect in human populations stems from three separate lines of biological evidence. First, there is an overwhelming amount of evidence on the heterozygote advantage for animals and plants found in both laboratory and wild environments (Chen, 2013; Comings and MacMurray, 2000). Second, the frequent evidence of inbreeding depression on human health indirectly suggests the existence of a heterosis effect among humans (Bittles, 2002; Polasek, 2009). Third, there are accumulating examples of molecular heterosis in humans (Comings and MacMurray, 2000; Aidoo et al., 2002). Despite these lines of evidence, however, only a limited number of studies have attempted to investigate heterosis in humans at a macro level. For example, Lewis (2010) proposed that the attractiveness of mixed-race individuals can be explained by heterosis. Mingroni (2007) argued that the tendency for IQ scores to increase in populations is a result of heterosis. And Kozieł et al. (2011) found that marital distance is a key predictor of offspring height. Inspiring as they are, a common limitation of these studies is that they were mostly based on small-scale samples and failed to control for a large number of possible confounding factors.

In this study, we use the unique 0.1% sample of the 2000 Chinese Population Census Data to explore whether phenotypic heterosis exists among human population groups. China provides a good case study to achieve our research objective for three reasons. First, China contains a large geographical span and physical diversity. For example, the north-south distance of China is approximately 5500 kilometres and the east-west distance 5200 kilometres, which serve as the foundation for geographic isolation and genetic differentiation among populations (Manica et al., 2005). Second, there are 56 identified ethnic groups in China, and the existing literature has documented substantial evidence of distinct ancestors and genetic distances among different subpopulations¹ in the country (Chen et al., 2009; Du et al., 1997; Chu et al., 1998; Zhang et al., 2007, Zhang et al., 2010; Wen et al., 2004). For example, the Han ethnic group can be divided into two differentiated subgroups, northern Han and southern Han, who are approximately separated in geographical territory by the Yangtze River (Chen et al., 2009; Wen et al., 2004). Third, as a politically unified nation for decades, China provides a context in which one can disentangle the potential heterosis effect relatively more easily from certain socioeconomic, political and cultural factors than in the context of, say, Europe.

Existing evidence in genetics and experimental observations offer three important predictions about the heterosis effect in humans – the “distance-performance” hypothesis. First, the offspring of individuals from separate populations may show heterosis and perform better in a number of quantitative traits, such as intelligence, height and physical attractiveness (Emmrich et al., 2015; Prokosch et al., 2005; Lewis, 2010). Second, the heterosis effect may rise with increasing genetic differences of the parents (Bikard et al., 2009; Moll et al., 1965). Third, given the substantial evidence that geographic and genetic distances among different populations are positively correlated (Manica et al., 2005; Jorde and Wooding, 2004), offspring of “hybrid” marriages (with spouses originating from different states/provinces/countries/areas) may exhibit better genetic fitness and greater phenotypic heterosis.

The major phenotype analyzed in this study is the educational attainment (EA) of offspring. It is selected on the basis of a broad agreement that the educational achievement is at least moderately associated with intelligence levels (Plug and Vijverberg, 2003; Deary et al., 2007), and thus can reflect a general biological/genetic fitness to some extent (Beauchamp, 2016; Prokosch et al., 2005). To reduce selection bias and the risk of confounding environmental factors, we employ the high-dimensional fixed effects (HDFE) model proposed by Guimaraes and Portugal (2010), which allows us to include a large number of fixed effects (FE) in estimation. In addition, we exploit exogenous migration decisions of respondents’ parents and province-year level average marital distance as instrumental variables (IV) for measures of genetic diversity to estimate high-dimensional fixed effects-two stage least squares (HDFE-2SLS) models. We further replicate the results by analyzing body height, an alternative indicator of genetic fitness that is considered less likely to be affected by socioeconomic environments (Kozieł et al., 2011; Mascie-Taylor, 1991), using data from the China Health and Nutrition Survey.

Our empirical results provide strong evidence of the existence of a heterosis effect among human beings. Consistent with predictions of the “distance-performance” hypothesis, a 1000-kilometre increase in geographic distance between parents’ birthplaces would increase the offspring’s total years of schooling and height by 0.21 years and 0.94 centimetres, respectively. The estimated heterosis effect is robust across various measurements of genetic diversity and remains similar after we simultaneously control for parental educational attainments/height, environmental influences, and over a thousand province and province-by-year FEs. And the impacts in male and higher-educated offspring are found to be larger in magnitude. Although endogeneity of “hybrid marriages” may not be completely ruled out, our study serves among the first major attempts toward understanding heterosis in humans beyond the molecular level, and we hope that this paper can intrigue future studies that further address endogeneity issues.

The remainder of this paper is structured as follows. Section 2 summarizes the data and describes variables used in the analysis. Section 3 details our empirical framework, discusses potential sources of endogeneity problems, and develops strategies to solve these problems. Section 4 presents estimation results and supplemental robustness checks. Section 5 concludes and points out a number of directions for future research.