The occurrence of twin births is widely regarded as occurring randomly across women and, under this premise, economists have used it to identify causal effects of fertility on parental investments in children and on women’s labour supply. Using data from 72 countries, including the US, the UK, Spain and a number of poorer countries, Bhalotra and Clarke (2016) demonstrate that twin births are not random. They conduct a bounding exercise on commonly used IV estimates to illustrate that – in contrast to the conclusions of recent high profile research- there are tradeoffs between fertility and child quality (and the same will hold for a tradeoff between fertility and women’s labour supply). This recent work is rapidly being cited.
