Sex Ratio at Birth by Parity

V. Results

The reported estimates in this section are from linear probability re- gressions in which the dependent variable is an indicator variable that equals 1 if the birth is male. Linear probability models are useful to this study because the fi tted prob- abilities are close to 50 percent. Probit estimation produces almost identical results. Standard errors are adjusted for serial correlation by clustering at the county level.

A. Sex Ratio at Birth by Parity

We fi rst explore how the probability of male births differs across birth parity. 21 Only the birth- order indicators with fi rst births as the omitted category are included as the explanatory variables in the fi rst column of Table 2. The next column includes a set of mother- and pregnancy- specifi c covariates, which could potentially affect the likeli- hood of a male birth. Mother- level controls include the mother’s ethnicity Han versus ethnic minorities, education, and a quadratic of maternal age at conception. Pregnancy characteristics include gestation length and indicators for the timing of initial prenatal care visits. Additional controls, including the year of conception effects and county fi xed effects, are added sequentially in Columns 3–4. Column 5 presents the results from a regression that controls for provincial characteristics, including per- capita GDP, per- capita fi scal expenditure, the number of hospital beds and doctors, and the number of teachers all in logs. The fi nal column shows the specifi cation with county- specifi c linear time trends. The coeffi cients of the birth- order indicators are interpreted as the difference between the probability of male higher- order births and male fi rst births. The regression results reported in Table 2 reveal that sex ratios increase with birth order. The estimates of birth- order effects are positive and statistically signifi cant at the 1 percent level, and neither the point estimates nor the standard errors are consid- erably affected by the inclusion of additional controls. In the preferred specifi cation, which contains the richest set of controls fi nal column, Table 2, the estimates imply that second births are 2.0 percentage points more likely to be male than fi rst births, whereas third- and higher- order births are 4.3 percentage points more likely to be male compared with fi rst- order births. Thus far, our results are consistent with the empirical regularity found in censuses and other fertility surveys, which indicate that the sex ratio in China tends to increase with birth order. These empirical fi ndings are suggestive of prenatal sex selection. This issue is explored more directly in the fol- lowing subsection.

B. Ultrasound, Birth Order, and Sex Ratio