Editing Natural experiment (section)

===Family size===
An aim of a study Angrist and Evans (1998)<ref name=angrist:00>{{cite journal |last1=Angrist |first1=J. |author-link=Joshua Angrist |first2=W. |last2=Evans |year=1998 |title=Children and Their Parents' Labor Supply: Evidence from Exogenous Variation in Family Size |journal=[[American Economic Review]] |volume=88 |issue=3 |pages=450–477 |jstor=116844 }}</ref> was to estimate the effect of family size on the labor market outcomes of the mother. For at least two reasons, the correlations between family size and various outcomes (e.g., earnings) do not inform us about how family size causally affects labor market outcomes. First, both labor market outcomes and family size may be affected by unobserved "third" variables (e.g., personal preferences). Second, labor market outcomes themselves may affect family size (called "reverse causality"). For example, a woman may defer having a child if she gets a raise at work. The authors observed that two-child families with either two boys or two girls are substantially more likely to have a third child than two-child families with one boy and one girl. The sex of the first two children, then, constitutes a kind of natural experiment: it is as if an experimenter had randomly assigned some families to have two children and others to have three. The authors were then able to credibly estimate the causal effect of having a third child on labor market outcomes. Angrist and Evans found that childbearing had a greater impact on poor and less educated women than on highly educated women although the earnings impact of having a third child tended to disappear by that child's 13th birthday. They also found that having a third child had little impact on husbands' earnings.<ref name=angrist:00/>