The Twin Instrument

107 Pages Posted: 19 Dec 2016 Last revised: 16 Apr 2023

See all articles by Sonia R. Bhalotra

Sonia R. Bhalotra

IZA Institute of Labor Economics

Damian Clarke

University of Oxford - Department of Economics; Market Imperfections and Public Policy (MiPP); University of Exeter; University of Chile


Twin births are often construed as a natural experiment in the social and natural sciences on the premise that the occurrence of twins is quasi-random. We present new population-level evidence that challenges this premise. Using individual data for more than 18 million births (more than 500,000 of which are twins) in 72 countries, we demonstrate that indicators of the mother's health and health-related behaviours and exposures are systematically positively associated with the probability of a twin birth. The estimated associations are sizeable, evident in richer and poorer countries, and evident even in a sample of women who do not use IVF.The positive selection of women into twinning implies that estimates of impacts of fertility on parental investments and on women's labour supply that use twin births to instrument fertility will tend to be downward biased. This is pertinent given the emerging consensus that these relationships are weak. Using two large samples, one for developing countries and one for the United States, and focusing upon twin-instrumented estimates of the quantity–quality trade-off, we demonstrate the nature of the bias and estimate bounds on the true parameter.

Keywords: quantity-quality trade-off, twins, bounds, miscarriage, maternal health, fertility, parental investment

JEL Classification: J12, J13, C13, D13, I12

Suggested Citation

Bhalotra, Sonia R. and Clarke, Damian, The Twin Instrument. IZA Discussion Paper No. 10405, Available at SSRN: or

Sonia R. Bhalotra (Contact Author)

IZA Institute of Labor Economics ( email )

P.O. Box 7240
Bonn, D-53072

Damian Clarke

University of Oxford - Department of Economics ( email )

Manor Road Building
Manor Road
Oxford, OX1 3BJ
United Kingdom

Market Imperfections and Public Policy (MiPP) ( email )

University of Exeter ( email )

Northcote House
The Queen's Drive
Exeter, Devon EX4 4QJ
United Kingdom

University of Chile ( email )

Pío Nono Nº1, Providencia
Santiago, R. Metropolitana 7520421

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