An agent-based model of sex ratio at birth distortions

Across a number of countries in Asia and the Caucasus, fertility decline in recent decades has been accompanied by an unprecedented and anomalous rise in the sex ratio at birth (SRB). Although the micro-level factors—persistent son preference within a context of fertility decline and growing access to pre-natal sex determination technology—are known, their specific levels, trends and interactions in explaining macro-level SRB trajectories are hard to discern with existing data and approaches. We present an agent-based model (ABM) that examines the contribution of each of these micro-level factors to the emergence of distorted SRBs at the macro-level. Calibrating our model to the South Korean and Indian scenarios, we show that even as son preference was declining in both settings SRB distortions emerged due to the diffusion of technology along with increases in probabilities to sex-selectively abort at lower parities as norms shifted towards smaller families. In South Korea, we find that SRBs peaked at 114 at relatively low levels of son preference of ∼ 30 % wanting a son. The SRB increase was due to the joint effects of technology diffusion combined with steady increases in the readiness to abort, including small increases at parity 0, that is, before the transition to first birth. In India, our model suggests that the SRB rise was less steep than South Korea’s as the readiness to abort was not as high as in South Korea, due to higher fertility levels when SRBs rose and slower technology diffusion.