Up to 16 million Americans are thought to have IED, more than the total number for bipolar disorder and schizophrenia combined.
Toxoplasmosis is a common and generally harmless parasitic infection that is passed on through the feces of infected cats, contaminated water or undercooked meat.
It affects around 30% of all humans but is normally latent.
Research has revealed that the parasite is found in brain tissue, and it has been linked to a number of psychiatric conditions, including schizophrenia, bipolar disorder and suicidal behavior.
Researchers from the University of Chicago, led by Dr. Emil Coccaro, have been looking for more effective ways to diagnose and treat IED and impulsive aggression.
Findings showed that 22% of those with IED tested positive for toxoplasmosis exposure, compared with 9% of the healthy control group and 16% of the psychiatric control group.
The psychiatric group and the healthy group had similar scores for aggression and impulsivity, but the group with IED scored far higher on both counts than either of the other two groups.
An association emerged between toxoplasmosis and impulsivity. However, when the team adjusted for aggression scores, this association became non-significant, indicating a strong correlation between toxoplasmosis and aggression.