General population studies have found that the positive symptoms of psychosis are prevalent in the general population. The majority of the individuals experiencing these “symptoms” are not in need of care. However, longitudinal studies indicate that they may nevertheless have an increased risk of developing a clinical disorder. The aim of this study is to elucidate the mechanisms that mediate transition from having one or two psychotic symptoms to becoming a patient with a psychotic disorder.

Data from two large longitudinal general population studies (the NEMESIS study and the EDSP study) were analysed in order to investigate the risk-increasing effect of three important environmental risk factors, namely cannabis, urbanicity, and childhood trauma, as well as their interaction with pre-existing liability to psychosis.

Based on these studies, cannabis not only survives as a risk factor for psychosis, but the evidence is showing concrete synergistic effects between cannabis and pre-existing psychosis liability. The urban environment is, in terms of attributable risk, the most important proxy environmental risk factor and there is emerging evidence that it interacts with genetic risk. Early trauma is another important aspect of the environment that can be linked prospectively to psychosis, particularly for those with a pre-existing liability.

The mechanism by which the environment is likely to impact on risk is through cognitive and emotional pathways on the one hand, and biological pathways, possibly involving dopamine sensitisation, on the other.