Instrumental Variables Methods in Experimental Criminological Research: What, Why, and How?

Quantitative criminology focuses on straightforward causal questions that are ideally addressed with randomized experiments. In practice, however, traditional randomized trials are difficult to implement in the untidy world of criminal justice. Even when randomized trials are implemented, not everyone is treated as intended and some control subjects may obtain experimental services. Treatments may also be more complicated than a simple yes/no coding can capture. This paper argues that the instrumental variables methods (IV) used by economists to solve omitted variables bias problems in observational studies also solve the major statistical problems that arise in imperfect criminological experiments. In general, IV methods estimate the causal effect of treatment on subjects that are induced to comply with a treatment by virtue of the random assignment of intended treatment. The use of IV in criminology is illustrated through a re-analysis of the Minneapolis Domestic Violence Experiment.

Treatment Effect Heterogeneity in Theory and Practice

Instrumental Variables (IV) methods identify internally valid causal effects for individuals whose treatment status is manipulable by the instrument at hand. Inference for other populations requires some sort of homogeneity assumption. This paper outlines a theoretical framework that nests all possible homogeneity assumptions for a causal treatment-effects model with a binary instrument. The framework suggests strategies for using IV estimates for extrapolation, while making it clear that efforts to go from local average treatment effects (LATE) to population average treatment effects are inherently speculative. These ideas are illustrated in an application using sibling-sex composition to estimate the effect of child-bearing on economic and marital outcomes for mothers with two or more children. The application is motivated by welfare reform, which penalizes further childbearing by welfare mothers on the grounds that more children make continued poverty and welfare receipt more likely. The empirical results generally support the notion of reduced labor supply and increased poverty rates as a consequence of additional childbearing, but evidence on the impact of childbearing on marital stability and welfare use is more tenuous. Another interesting finding is that for the sample of teen mothers, LATE is essentially equal to the population average treatment effect.

The "misnorming" of the U.S. military entrance examination and its effect on minority enlistments

The score a prospective recruit must earn on the military's entrance examination was raised in 1980 in response to an error discovered in the score scale previously used. Raising this score led to a reduction in enlistments, especially among minorities. Recent plans to reduce the military have also had an adverse impact on service opportunities for minority applicants. This paper considers empirical and theoretical aspects of the relationship between entrance standards and minority representation in the military, focusing on racial differences in the proportion of qualified applicants who enlist. The results suggest that increases in test score standards have a large impact on minority enlistment not only because minorities have lower scores, but also because qualified minority applicants are far more likely than other qualified applicants to enlist.

Long-Term Economic Consequences of Vietnam-Era Conscription: Schooling, Experience and Earnings

Military service reduces civilian labor market experience but subsidizes higher education through the GI Bill. Both of these channels are likely to affect civilian earnings. New estimates of the effects of military service using Vietnam-era draft-lottery instruments show post-service earnings effects close to zero in 2000, in contrast with earlier results showing substantial earnings losses for white Vietnam veterans in the 1970s and 1980s. The recent estimates also point to a marked increase in post-secondary schooling that appears to be attributable to the Vietnam-era GI Bill. Seen through the lens of a Mincer wage equation, the wage effects observed in 2000 data can be explained by a flattening of the experience profile in middle age and a modest return to the additional schooling funded by the GI Bill. In particular, IV estimates of the returns to GI Bill-funded schooling are well below OLS estimates. Wage equations that allow for nonlinearities in the returns to schooling and a possible negative effect of military service on health, leave the main findings unchanged.veterans, returns to schooling, instrumental variables

Instrumental Variables and the Search for Identification: From Supply and Demand to Natural Experiments

instrumental variables, natural experiment

Protective or Counter-Productive? European Labor Market Institutions and the Effect of Immigrants on EU Natives

We estimate the effect of immigrant flows on native employment in Western Europe, and then ask whether the employment consequences of immigration vary with institutions that affect labor market flexibility. Reduced flexibility may protect natives from immigrant competition in the near term, but our theoretical framework suggests that reduced flexibility is likely to increase the negative impact of immigration on equilibrium employment. In models without interactions, OLS estimates for a panel of European countries in the 1980s and 1990s show small, mostly negative immigration effects. To reduce bias from the possible endogeneity of immigration flows, we use the fact that many immigrants arriving after 1991 were refugees from the Balkan wars. An IV strategy based on variation in the number of immigrants from former Yugoslavia generates larger though mostly insignificant negative estimates. We then estimate models allowing interactions between the employment response to immigration and institutional characteristics including business entry costs. These results, limited to the sample of native men, generally suggest that reduced flexibility increases the negative impact of immigration. Many of the estimated interaction terms are significant, and imply a significant negative effect on employment in countries with restrictive institutions.

Sources of identifying information in evaluation models

Estimation

How Large are the Social Returns to Education? Evidence from Compulsory Schooling Laws

Average schooling in US states is highly correlated with state wage levels, even after controlling for the direct effect of schooling on individual wages. We use an instrumental variables strategy to determine whether this relationship is driven by social returns to education. The instrumentals for average schooling are derived from information on the child labor laws and compulsory attendance laws that affected men in our Census samples, while quarter of birth is used as an instrument for individual schooling. This results in precisely estimated private returns to education of about seven percent, and small social returns, typically less than one percent, that are not significantly different from zero.

Average causal response with variable treatment intensity

Evaluation;Treatment