"Compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis [i.e., who commenced use when around 15 years or younger] were twice as likely to develop a non-affective psychosis and were four times as likely to have high scores on the Peters et al Delusions Inventory [a measure of delusion]," wrote Dr. John McGrath, of the Queensland Centre for Mental Health Research, Park Centre for Mental Health in Wacol, and colleagues. "There was a 'dose-response' relationship between the variables of interest: the longer the duration since the first cannabis use, the higher the risk of psychosis-related outcomes."
About 3,100 young adults, with average age of 20 years, were surveyed. The news article does not indicate the proportion of males in the study group and does not distinguish survey findings by gender. The overwhelming percentage of marijuana users are young males, other studies have shown.