ANN ARBOR—Nearly 40 percent of incarcerated youths carried hidden weapons at least once a day in the year before they were jailed, a new study says.

A history of trauma, being victims themselves, gang involvement and drug use greatly increased the likelihood of the 13-17 year olds arming themselves, according to the study by a University of Michigan professor and other colleagues.

They looked at self-protecting behavior and overt delinquent acts among 723 incarcerated youths. The results provide social policy implications in that strategies to reduce gang involvement and substance abuse may interrupt the cycle of violence by weapons, said Matthew Howard, professor in the U-M School of Social Work.

Questionnaires asked respondents in youth service facilities how often they carried a hidden weapon in the year before incarceration. Questions also inquired about substance use