Research has shown that the low‐activity MAOA genotype in conjunction with a history of childhood maltreatment increases the likelihood of violent behaviors. This genetic–environment (G × E) interaction has been introduced as mitigation during the sentencing phase of capital trials, yet there is scant data on its effectiveness. This study addressed that issue. In a factorial design that varied mitigating evidence offered by the defense [environmental (i.e., childhood maltreatment), genetic, G × E, or none] and the likelihood of the defendant's future dangerousness (low or high), 600 mock jurors read sentencing phase evidence in a capital murder trial, rendered individual verdicts, and half deliberated as members of a jury to decide a sentence of death or life imprisonment. The G × E evidence had little mitigating effect on sentencing preferences: participants who received the G × E evidence were no less likely to sentence the defendant to death than those who received evidence of childhood maltreatment or a control group that received neither genetic nor maltreatment evidence. Participants with evidence of a G × E interaction were more likely to sentence the defendant to death when there was a high risk of future dangerousness than when there was a low risk. Sentencing preferences were more lenient after deliberation than before. We discuss limitations and future directions.