Table 1 Sociodemographic characteristics in every period

 Male

1057(51.6)

891(51.0)

765(51.7)

 Only child

1560(82.5)

1431(81.9)

1218(82.3)

 Father’s age (mean ± SD, range)

33.14 ± 4.14,21-54

34.09 ± 4.17,23-54

35.24 ± 4.19,24-54

 Mother’s age(mean ± SD, range)

29.89 ± 3.68,20-52

30.98 ± 3.85,21-50

32.10 ± 3.74,21-49

 Father’s education ≥ 12y

1568(77.9)

1378(78.9)

1162(78.5)

 Mother’s education ≥ 12y

1506(74.8)

1310(75.0)

1095(74.0)

Family

 Nuclear family

1154(56.9)

984(56.3)

855(57.8)

 Income per capita < 5000 CNY

960(46.9)

847(48.5)

669(45.2)