The occurrence of twin births has been widely used as a natural experiment. With a focus upon the use of twin births for identification of causal effects in economics, this chapter provides a critical review of methods and results.

This paper discusses research on the relationship between fertility and women's labour force participation. It surveys methods used to obtain causal identification, and provides an overview of the evidence of causal effects in both directions. We highlight a few themes that we regard as...

Twin births are often construed as a natural experiment in the social and natural sciences on the premise that their occurrence is quasi-random. We present new population-level evidence challenging this premise. Using data on about 18 million births in 72 countries, we find that maternal...

Leveraging close elections to generate quasi-random variation in the religious identity of state legislators in India, we find lower rates of female foeticide in districts with Muslim legislators, which we argue reflects a greater (religious) aversion to abortion among Muslims. These districts...

In this study we examine the passage of a reform to in-vitro fertilization (IVF) procedures in Sweden in 2003. Following publication of medical evidence showing that pregnancy success rates could be maintained using single rather than multiple embryo transfers, the single embryo transfer (SET)...

IVF allows women to delay birth and pursue careers, but IVF massively increases the risk of twin birth. There is limited evidence of how having twins influences women's post-birth careers. We investigate this, leveraging a single embryo transfer (SET) mandate implemented in Sweden in 2003,...

While land reforms are typically pursued in order to raise productivity and reduce inequality across households, an unintended consequence may be increased within-household gender inequality. We analyse a tenancy registration programme in West Bengal, and find that it increased child survival...

The introduction of prenatal sex-detection technologies in India has led to a phenomenal increase in abortion of female fetuses. We investigate their impact on son-biased fertility stopping behavior, parental investments in girls relative to boys, and the relative chances of girls surviving...

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 17 million births in 72 countries, we...

Twin births are often used to instrument fertility to address (negative) selection of women into fertility. However recent work shows positive selection of women into twin birth. Thus, while OLS estimates will tend to be downward biased, twin-IV estimates will tend to be upward biased. This is...