In 2008, Oregon expanded its Medicaid program using a lottery, creating a rare opportunity to study the effects of Medicaid coverage using a randomized controlled design (Oregon Health Insurance Experiment). Analysis showed that Medicaid coverage lowered the risk of depression. However, this effect may vary between individuals, and the identification of individuals likely to benefit the most has the potential to improve the effectiveness and efficiency of the Medicaid program. By applying the machine learning causal forest to data from this experiment, we found substantial heterogeneity in the effect of Medicaid coverage on depression; individuals with high predicted benefit were older and had more physical or mental health conditions at baseline. Expanding coverage to individuals with high predicted benefit generated greater reduction in depression prevalence than expanding to all eligible individuals (21.5 vs 8.8 percentage-point reduction; adjusted difference = +12.7 [95% CI, +4.6 to +20.8]; P = 0.003), at substantially lower cost per case prevented ($16 627 vs $36 048; adjusted difference = -$18 598 [95% CI, -156 953 to -3120]; P = 0.04). Medicaid coverage reduces depression substantially more in a subset of the population than others, in ways that are predictable in advance. Targeting coverage on those most likely to benefit could improve the effectiveness and efficiency of insurance expansion. This article is part of a Special Collection on Mental Health.

Punishment serves as a balancing force that dissuades people from acting selfishly, which complements cooperation as an essential characteristic for the prosperity of human societies. Past studies using economic games with two options (cooperation and defection) reported that cooperation decisions are generally faster than defection decisions and that time pressure possibly induces human players to be more intuitive and thus cooperative. However, it is unclear where punishment decisions sit on this time spectrum. Therefore, we recruited human players and implemented two series of online network games with cooperation, defection, and punishment options. First, we find that punishment decisions are slower than cooperation or defection decisions across both game series. Second, we find that imposing experimental time pressure on in-game decisions neither reduces nor increases the frequency of punishment decisions, suggesting that time pressure may not directly interact with the mechanisms that drive players to choose to punish.