Sex ratio at last birth: an indicator of son preference

Son preference is a widespread phenomenon observed in many countries, including India and other South Asian nations. A key demographic indicator reflecting this behavior is the Sex Ratio at Last Birth (SRLB), defined as the probability of a male birth being the final birth in a family. Despite its widespread use, existing studies largely focus on observed SRLB shifts, lacking precise quantitative frameworks. The objective of the present paper is to derive mathematical expressions for the SRLB under different stopping behavior by the females. We applied developed mathematical expressions to empirical data from India’s National Family Health Survey (NFHS), estimating SRLB values and assessing the evidence of son preference at the population level. The results reveal elevated SRLB values across parities, consistent with stronger stopping behavior following the birth of a son. Measures confirm that the methodology-based values are robust enough to provide reliable estimates of son preference at the state level. There is evidence of son preference. Our approach offers both theoretical contributions through explicit models of SRLB under different fertility decision rules and practical utility for demographic research and policy, enabling robust country-wise comparisons and improved evaluation of family planning outcomes amidst son preference.