approximately 40 to 50 percent of the increase in sex ratio at birth can be explained by local access to ultrasound technology. This number tends to be interpreted as a con-
servative estimate of the overall effect of the introduction of ultrasound technology on the sex imbalance in China. This is because individuals with a very strong preference
for male offspring may cross county borders to obtain ultrasound scanning for sex selection.
C. Previous Children and Sex Selection
The results from estimating Equation 3 show that the effect of ultrasound technology on sex ratio also varies with the sex composition of previous children. Table 4 presents
the results for the sample of second births, regardless of whether the second birth is the terminal birth. In Column 1, the regression includes an indicator that equals 1 if the
fi rst birth is female, with the availability of ultrasound technology and its interaction term as explanatory variables. The regression shows that the gender of the second birth
depends on that of the fi rstborn. If the fi rst child is a girl, the second birth is 3.6 per- centage points more likely to be a boy. More importantly, the coeffi cient of the interac-
tion term between ultrasound technology and the fi rstborn- girl indicator is positive and highly signifi cant. Meanwhile, the point estimate implies that if the fi rst- born child is a
girl, local access to ultrasound technology raises the probability that the second child will be a boy by 4.7 percentage points. The main effect of ultrasound technology is
negative but insignifi cant at conventional levels, suggesting that ultrasound technol- ogy does not affect second births if the fi rst birth was male.
To test the robustness of these results, a set of individual control variables, county fi xed effects, year effects, provincial controls, and county- specifi c linear time trends
are gradually added in Columns 2–6. The coeffi cients of the previous- gender indicator and its interaction with the availability of ultrasound technology are extremely robust
to the addition of these covariates.
The results for the third births regardless of whether the third birth is the terminal birth, reported in Table 5, confi rm those for the second births. Estimates show that in
general, the third child is more likely to be a boy if the fi rst two children are female. For mothers with two daughters, the probability that the third birth will be a boy is
roughly 8.3 percentage points higher compared with mothers who already have at least one son. Adjusting for observable characteristics does not substantially alter the
estimated effects. For all specifi cations, no consistent effect of ultrasound technology on the probability of male children at parity three is observed for mothers who already
have at least one son. However, the interaction effect between ultrasound technology and two girls is positive and statistically signifi cant. The point estimates indicate that
for mothers with two girls, local access to ultrasound technology increases the prob- ability that the third birth will be male by approximately 6.8 percentage points.
In summary, the results presented in Tables 4 and 5 show that the male- biased sex ratios at birth at higher parity appear to be due solely to the biased sex ratio of births to
mothers who have older daughters. This demonstrates that local access to ultrasound technology raises the probability of subsequent male births if no older brothers have
been born previously. These fi ndings indicate that the male- biased sex ratio of births at higher parities is largely due to prenatal sex selection motivated by son preference.
T he
J ourna
l of H um
an Re sourc
es 56
Table 4 Effect of ultrasound, sex composition of the fi rst child, and their interactions on male probability births 1975–92: linear probability
model results from the second births
Dependent variable: child is male 1
2 3
4 5
6 First child is female
0.036 0.035
0.036 0.036
0.037 0.038
0.006 0.006
0.006 0.006
0.006 0.006
Ultrasound –0.004
–0.009 –0.004
–0.015 –0.014
–0.010 0.005
0.005 0.006
0.008 0.008
0.009 First child is female ×
0.047 0.047
0.048 0.048
0.047 0.046
Ultrasound 0.008
0.008 0.008
0.008 0.008
0.008 Individual controls
No Yes
Yes Yes
Yes Yes
County fi xed effects No
No Yes
Yes Yes
Yes Year of conception fi xed effects
No No
No Yes
Yes Yes
Provincial controls No
No No
No Yes
Yes County- specifi c linear time trends
No No
No No
No Yes
Observations 86,356
86,356 86,356
86,356 85,016
85,016 R
- squared 0.0042
0.0049 0.0108
0.0114 0.0115
0.0168
Note: Individual controls include mother’s ethnicity, education, maternal age at conception and its squared term, gestation length and indicators for the timing of initial prenatal care visits. County fi xed effects are separate indicator variables for each county. Year fi xed effects are indicators that allow for unrestricted differences in year- to-
year changes. Reported in parentheses are standard errors clustered by county. denotes statistical signifi cance at the 10 percent level, at the 5 percent level, at the 1 percent level.
Che n, L
i, a nd M
eng 57
Table 5 Effect of ultrasound, sex composition of the fi rst two children, and their interactions on male probability births 1975–92: linear
probability model results from the third births
Dependent variable: child is male 1
2 3
4 5
6 First two children are
0.084 0.081
0.084 0.083
0.083 0.086
both female
0.011 0.011
0.011 0.011
0.011 0.011
Ultrasound 0.003
–0.001 0.005
–0.012 –0.013
–0.010 0.008
0.008 0.009
0.013 0.013
0.014 First two children are
0.064 0.065
0.068 0.068
0.067 0.061
both female × ultrasound 0.014
0.014 0.014
0.014 0.014
0.014 Individual controls
No Yes
Yes Yes
Yes Yes
County fi xed effects No
No Yes
Yes Yes
Yes Year of conception fi xed effects
No No
No Yes
Yes Yes
Provincial controls No
No No
No Yes
Yes County- specifi c linear time trends
No No
No No
No Yes
Observations 26,958
26,958 26,958
26,958 26,895
26,895 R
- squared 0.0144
0.0166 0.0311
0.0316 0.0318
0.0460
Note: Individual controls include mother’s ethnicity, education, maternal age at conception and its squared term, gestation length and indicators for the timing of initial prenatal care visits. County fi xed effects are separate indicator variables for each county. Year fi xed effects are indicators that allow for unrestricted differences in year- to-
year changes. Reported in parentheses are standard errors clustered by county. denotes statistical signifi cance at the 10 percent level, at the 5 percent level, at the 1 percent level.
VI. Robustness Checks
