But the alcohol group displayed increased aggression while under the influence as compared to the control group, while the marijuana group displayed lower levels of aggression than the control group. While the findings show a definite decrease in aggressive tendencies while under the influence of marijuana, another study published in the journal Psychopharmacology in 1999 found that during the withdrawal phase of marijuana, aggression levels spike. “The results in the present study support the hypothesis that acute alcohol intoxication increases feelings of aggression and that acute cannabis intoxication reduces feelings of aggression following aggression exposure,” the study concludes.