For the study, which has also been published by The American Journal of Psychiatry, researchers examined the court psychiatry records and other medical evidence for 2,000 people found guilty of murder, attempted murder, manslaughter or attempted manslaughter between 1998 and 2001 in Sweden.
The certified psychiatric illnesses include schizophrenia, personality disorders, anxiety, post-traumatic stress and depression. Many of the convicted people had two or more of these, and half had a history of substance abuse.
This percentage is higher than in other studies in comparable Western European countries, possibly because the Swedish researchers had access to more complete evidence, Forskning och Framsteg said.
But research authors Martin Grann and Seena Fazel at the Karolinska Institute claimed that their findings could even be on the low side, given that many killers commit suicide and therefore never undergo psychiatric tests.
The Swedish situation is different from that in countries where organized crime, drug trade and easy access to weapons result in a higher percentage of murders committed by people who are not certifiably ill than in Western Europe, the study’s authors acknowledged.
They cited as examples the United States, Bolivia, South Africa and the Baltic countries.