For example, Dika and Singh ( Reference Dika and Singh2002 ) stated that social capital is positively associated with educational attainment such as reducing dropout rates and increasing college enrolment, and academic achievement such as increasing standardized test scores. More involved parenting and spending time with children (for example reading, playing, and talking) is positively correlated to children’s test scores and cognitive development (Thomsen, Reference Thomsen2015 ; Von Otter & Stenberg, Reference Von Otter and Stenberg2015 ). Plenty of this research is observational, but Price and Kalil ( Reference Price and Kalil2019 ), using an instrumental approach, found that parental time allocation had a positive effect on children’s cognitive test scores.
Two ways have been proposed as to how TWH functions. First, parental investments may be physiological (biological) and influence the sex ratio of offspring at birth. Second, it may be psychological and bias parental behavior toward the opposite-sex offspring according to the condition of the parent after the birth of the children (Hopcroft, Reference Hopcroft2005 ; Trivers & Willard, Reference Trivers and Willard1973 ). This study focuses on the second interpretation and observes whether family conditions explain educational outcomes of the sons and daughters within families. The same interpretation of the TWH is used in various previous studies that have investigated the investments of the parents in children’s education (see e.g., Hopcroft, Reference Hopcroft2005 ; Pink, Schaman, & Fieder, Reference Pink, Schaman and Fieder2017 ).
Although some scholars have proposed that this mechanism can be consider to function only in pre-modern societies such as tribe societies where the assumptions of the hypothesis are still valid, recent studies have shown that in modern societies, men with high education have higher probability to reproduce than women with high education or men with low education (Jalovaara et al., Reference Jalovaara, Neyer, Andersson, Dahlberg, Dommermuth, Fallesen and Lappegård2019 ). Additionally, it has shown a higher SES and educated men have more children compared to their lower SES brothers (Nisén et al., Reference Nisén, Martikainen, Myrskylä and Silventoinen2018 ).
The Trivers-Willard hypothesis (henceforth TWH), which is based on evolution theory by natural selection, states that in a stratified society, high-status parents will invest more in their male offspring, and low-status parents will invest more in female offspring (Trivers, Reference Trivers1972 ; Trivers & Willard, Reference Trivers and Willard1973 ). This investment strategy will maximize the reproductive success of the parents because the variance of reproduction success is higher for men than women (Bateman, Reference Bateman1948 ; Cronk, Reference Cronk2007 ). The higher variation for men than for women in reproductive success is based on sexual selection: males in good condition have a greater probability to reproduce because they are more likely to win dominance contest in sexual selection compared to males in poor conditions (Cronk, Reference Cronk2007 ; Hopcroft, Reference Hopcroft2005 ) Thus, the son of mother in good condition has a higher probability to reproduce than his sister, although the sister may have a higher probability to reproduce compared to other females. However, the son of the mother in poor condition has lower probability to reproduce than his sister. Although TWH is mainly applied to animals other than humans, the hypothesis can be also extended to humans. Trivers and Willard ( Reference Trivers and Willard1973 ) stated that “the model can be applied to humans differentiated on a socioeconomic scale, as long as the reproductive success of a male at the upper end of the scale exceeds his sister’s, whereas that of a female at the lower end of the scale exceeds her brother’s.”
Previous studies on Trivers-Willard hypothesis on human capital outcomes
In the U.S., the sons of high-status fathers attain more years of education and higher degrees than the daughters, whereas the daughters of low-status fathers attain more years of education and higher degrees than the sons (Hopcroft, Reference Hopcroft2005; Hopcroft & Martin, Reference Hopcroft and Martin2014). Subsequently, Hopcroft and Martin (Reference Hopcroft and Martin2016) found that the sons of high-status men are more likely than daughters to be sent to private school, are less likely to be employed extensively in high school, and are less likely to save money from their job for college; on the other hand, the daughters of low status men are more likely than the sons to be sent to private schools, less likely to be employed extensively in high school, and are less likely to save money from their job for college. These parental investments advance boys when the sex gap in GPAs between boys and girl is narrower (still favoring girls) for children of high status than low status fathers. Additionally, in the U.S., Pink, Schaman, and Fieder (Reference Pink, Schaman and Fieder2017) found that sons profited more from parental income and education in terms of their own income than daughters, and fathers with a high SES invest more in their sons’ education in terms of completed years of education and the parents’ support for college. In contrast, daughters of low SEI fathers completed more years of education and received more financial support than sons of low SEI fathers.
All these studies have used the father’s status as a measure of family status. Hopcroft (Reference Hopcroft2005) justified the usage of the father’s status rather than the mother’s because the former’s occupational status has traditionally been higher than the latter’s and is thus more important in determining the status of the family. Additionally, Hopcroft and Martin (Reference Hopcroft and Martin2014) argued that the father’s occupational status remains the single best measure of familial status because most men are employed full-time, whereas this is less true of women.
Although plenty of TWH studies are conducted in the United States, there are some from other regions as well. In China, Luo, Wei, and Weng (Reference Luo, Zhao and Weng2016) found that family heads whose fathers had a higher-class identity assigned by the Chinese Communist Party in the early 1950s tended to have a family with a higher socioeconomic status. Their data showed that as family heads’ current family status increased, the education of sons rose to a larger extent than that of daughters. Additionally, in Hungary, Bereczkei and Dunbar (Reference Bereczkei and Dunbar1997) found that the average number of years of education completed by children was higher for low SES Roma people girls than that completed by low SES boys, whereas the number of years of education completed by higher SES Hungarian boys was higher than that completed by higher SES Hungarian girls.
There are also studies where the hypothesis has not received support. Hopcroft and Martin (Reference Hopcroft and Martin2014) found no support for TWH in whether a child last attended a public or private junior or high school, the amount of tuition paid at the child’s last college, and the amount of total educational loans obtained by the child. Additionally, Freese and Powell (Reference Freese and Powell1999) found no sex difference in investments in adolescents in the form of measures such as saving money for college, enrolling children in private school, having educational objects in the home, talking to children about school, or monitoring children’s behavior. However, Kanazawa (Reference Kanazawa2001) and Cronk (Reference Cronk2007) criticized this study. Cronk argued that formal education is an evolutionarily new phenomenon and therefore a weak measure for the TWH. Additionally, Kanazawa claimed that parental investment measures related to education in the study of Freese & Powell particularly benefit only males’ reproduction success and not females’. He also stated that in the current environment, education is equally important for the sons’ and daughters’ career opportunities and thus parents should not discriminate in their investments according to the child’s sex. However, this argument fails because it has been shown that higher educated men have higher income and more children compared to low educated men and highly educated women (Lappegård & Rønsen, Reference Lappegård and Rønsen2013; Nisén et al., Reference Nisén, Martikainen, Silventoinen and Myrskylä2014). Women have lower variability in reproduction than men according to status or income. TWH is based on the variability of reproduction success according to status. The study of Freese and Powell (Reference Freese and Powell1999) can be criticised for the reason that their study relies on self-reports by parents that are subject to bias and that they are taken when children are 13 or 14 and also, they do not measure child’s final educational attainment that are crucial for their income and status attainment. Kanazawa (Reference Kanazawa2001) used parental activity for children, that is, helping with reading or homework, leisure activities with a child, etc., as dependent variables. His results were supportive of TWH. Neither Kanazawa nor Freese and Powell considered that children’s endowments can affect parental investments, because parents may be more likely to invest in the child who has, for example, higher cognitive abilities and is thus better at school. In addition to studies of direct measure of investments, Keller, Nesse, and Hofferth (Reference Keller, Nesse and Hofferth2001) did not find sex differences in investments in babies and young children in terms of hours per week spent with children, self-reported warmth, and the months they were breast-fed. However, this study is also challenged because it relies heavily on self-reports (Cronk, Reference Cronk2007). Lynch, Wasielewski, and Cronk (Reference Lynch, Wasielewski and Cronk2018) found only limited support for TWH and the link between socioeconomic status and biased investment, namely, males who grew up in poverty and males with lower perceived SES were more likely to choose to adopt girls, according to surveys. There was no link between socioeconomic status and parental activity as donations to either sex. This study fails due to the scarcity of data and variables not suitable testing for TWH, that is, lack of biological offspring. Further, the study used internet surveys that are prone to sample bias.
Reviews of the previous studies show that the empirical results of postnatal TWH studies among humans are mixed, and both data and measured variables have varied between the studies. For example, Pink, Schaman, and Fieder (Reference Pink, Schaman and Fieder2017) used income at adulthood as a dependent variable. However, income at adulthood is not a good measurement criterion because of structural restrictions for women, that is, childbearing. Women who have taken time off from a career to have children are less likely to be in high-status occupations (Hopcroft, Reference Hopcroft2005), which also leads to a lower salary. Moreover, the income of this study was measured when respondents’ mean age was 53 years. Therefore, it can be argued that at this age, women may have had several maternity leaves and time off from their careers. Additionally, the sample of high school students itself may be biased. Pink, Schaman, and Fieder (Reference Pink, Schaman and Fieder2017) themselves noted that the sample was one of the limitations of their study.
Finally, all the previous studies have been conducted using surveys that may contain measurement errors due to misreporting (see Engzel & Jonsson, Reference Engzel and Jonsson2015). The errors may be easily caused when children report information about their parents. This type of misinformation is avoidable when using register-based data. The interaction effect between children’s sex and parental status may also need a sample size that is large enough to reliably test TWH at least if the effect is rather small in the population. Previous studies have been conducted with analyses across families not within families which is how the hypothesis in this study has been approached. Trivers (Reference Trivers1972) himself addressed that the hypothesis should be studied comparing siblings of the opposite sex within the families. Hence, this study considers within family sibling comparison for the first time.