TABLE 1.

Definitions and measurements of the study variables.

Variable	Definition and Measurement
Mother–child relationship	How is the general relationship between the child and his/her mothers, ordinal variable (0 = not close; 1 = moderate; 2 = close)
Father–child relationship	How is the general relationship between the child and his/her fathers, ordinal variable (0 = not close; 1 = moderate; 2 = close)
Sibship size	Sibship size of the child, 3-category [0 = no sibling (only child), 1 = 1 sibling, 2 = 2 + siblings]
Birth order	Birth order of the child, 4-category [0 = only child, 1 = firstborn (of the multiple-child family), 2 = middleborn (of the multiple-child family), 3 = lastborn (of the multiple-child family)]
Younger bothers	Whether the child has younger brother(s), 4-category [0 = only child, 1 = without younger brother(s) (for child having siblings), 2 = with younger brother(s) (for child having siblings), 3 = younger brothers missing]
Younger sisters	Whether the child has younger sister(s), 4-category [0 = only child, 1 = without younger sister(s) (for child having siblings), 2 = with younger sister(s) (for child having siblings), 3 = younger sisters missing]
Older brothers	Whether the child has older brother(s), 4-category [0 = only child, 1 = without older brother(s) (for child having siblings), 2 = with older brother(s) (for child having siblings), 3 = older brothers missing]
Older sisters	Whether the child has older sister(s), 4-category [0 = only child, 1 = without older sister(s) (for child having siblings), 2 = with older sister(s) (for child having siblings), 3 = older sisters missing]
Total number of children in the family	Total number of children in the family, continuous variable (1–6); those having more than six children were coded as 6 due to the low proportion (<0.4%)
Children’s gender	Gender of the child, dummy variable (0 = son; 1 = daughter)
Grade	Grade of the child, dummy variable (0 = grade 7, 1 = grade 9)
Ethnicity	Whether the child is ethnic minority, dummy variable (0 = Han ethnicity, 1 = ethnic minority)
Cognitive score	The standard score of children’s cognitive abilities, continuous variable (from –2.03 to 2.71)
Academic performance	Self-rated academic performance of the child, continuous variable (from 0 to 4); the higher the score, the better of the performance
Boarding	Whether the child attends a boarding school, dummy variable, (0 = no, 1 = yes)
Parental co-residence	Parental residential status in the family, 3-category (0 = the child living with both parents, 1 = the child living with only one parent, 2 = the child living without parents)
Marital quality of parents	Whether the child’s parents are in good marital relationship (0 = no, 1 = yes)
Family economic condition	Family economic condition reported by parents, 3-category (0 = low income, 1 = medium income, 2 = high income)
Maternal education	Maternal education, 3-category (0 = lower than junior high, 1 = junior high, 2 = higher than junior high)
Paternal education	Paternal education, 3-category (0 = lower than junior high, 1 = junior high, 2 = higher than junior high)
Hukou type	The household registration type of the child, dummy variable (0 = rural registration, 1 = urban registration)
Parental age at birth	The age of mother/father at birth of the respondent child, 5 category (0 = 18–24, 1 = 25–29, 2 = 30–34, 3 = 35, and above, 4 = age missing)