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First, the cross-sectional studies:uppose for the sake of argument that high-crime countries are the ones that most frequently adopt the most stringent gun control laws.

Suppose further, for the sake of argument, that gun control indeed lowers crime, but not by enough to reduce rates to the same low levels prevailing in the majority of countries that did not adopt the laws. Looking across countries, it would then falsely appear that stricter gun control resulted in higher crime. Economists refer to this as an “endogeniety” problem. The adoption of the policy is a reaction to other events (that is, “endogenous”), in this case crime. To resolve this, one must examine how the high-crime areas that chose to adopt the controls changed over time —not only relative to their own past levels but also relative to areas that did not institute such controls.