Enter scientists from the University of Kuopio, Finland. Citing the lack of information available on free L-tryptophan levels in violent offenders, researchers tested tryptophan levels in the blood plasma of individuals who had been convicted of offenses such as homicide, sexual abuse, or aggravated assault and compared them with levels from a group of healthy controls (Psychopharmacology 2001, DOI: 10.1007/s002130100842). Because tryptophan levels vary geographically, only subjects within 50 miles of the testing facility in Kuopio were used. To control for seasonal variations, all were tested several times during the same year-long period with results compared seasonally.

Researchers found that free L-tryptophan levels were higher on average throughout the year in violent offenders than in controls. However, in the month of January, offenders showed a marked increase over the rest of the year—up to 160% higher free L-tryptophan plasma levels than controls. It is not surprising, then, that out of the 19 offenders tested for the study, 3 had committed violent crimes during this month.

These results indicate that decreased serotonin levels in violent personalities may result from disturbed tryptophan metabolism, causing ineffective turnover and subsequent upregulation. The researchers have several theories on the exact nature of L-tryptophan’s behavior in these offenders, but more studies are necessary to determine the possible faulty process. Discovering which mechanism is correct can bring scientists one step closer to possibly preventing violent tendencies in susceptible individuals.